bachelor thesis - kitgeothermiewiki.iai.kit.edu/images/a/a5/minev.pdf · bachelor thesis validation...

Karlsruhe Institute of Technology (KIT)Institute for Applied Computer Science

Supervisor : Dr. J. Isele

Bachelor Thesis

Validation Of A Super Insulation For A Geothermal Borehole

Probe

Dionis Minev

December 15, 2011

KIT � University of the State of Baden-Wuerttemberg and National Laboratory of the Helmholtz Association

Validation Of A Super Insulation For A Geothermal Borehole Probe | i

A�davit

I hereby declare that all of the information presented in this thesis is my own. All othersources and aids used to complete the thesis are clearly marked as not my own. Thisthesis has not been received by any examination board, neither in this nor in a similarform.

Karlsruhe, December 15, 2011

Signature:

Dionis Minev

Dionis Minev

Validation Of A Super Insulation For A Geothermal Borehole Probe | ii

Acknowledgements

I would like to thank Prof. Dr. Georg Bretthauer for allowing me to complete my Bach-elors thesis at the Institute of Applied Computer Science. Furthermore, I would like tothank my supervisor Dr. Jörg Isele for the ongoing support and guidance during thecompletion of my thesis.

Karlsruhe, December 15, 2011

Dionis Minev

Validation Of A Super Insulation For A Geothermal Borehole Probe | iii

Contents

1 Introduction 1

1.1 General project overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Bachelor thesis overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Objectives and requirements . . . . . . . . . . . . . . . . . . . . . . . . . 21.4 Current project status . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Probe concept 4

2.1 Theoretical concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.2 Borehole probe concept . . . . . . . . . . . . . . . . . . . . . . . . 62.1.3 Numerical simulation . . . . . . . . . . . . . . . . . . . . . . . . . 62.1.4 Super insulation concept . . . . . . . . . . . . . . . . . . . . . . . 62.1.5 Heat transfer theory . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1.5.1 Heat conduction . . . . . . . . . . . . . . . . . . . . . . 62.1.5.2 Thermal radiation . . . . . . . . . . . . . . . . . . . . . 6

2.1.6 Vacuum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.1.7 Multi layer insulation . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2 Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Manufactured Probe 10

3.1 Assembled Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.1.2 Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.1.3 Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 Equipment 13

4.1 Temperature measuring equipment . . . . . . . . . . . . . . . . . . . . . 144.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.1.2 Test set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.1.3 Thermocouple theory . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.1.3.1 Type K . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.1.4 Test set up and hardware . . . . . . . . . . . . . . . . . . . . . . 164.1.5 PicoLog recorder Software . . . . . . . . . . . . . . . . . . . . . . 164.1.6 Test recording process . . . . . . . . . . . . . . . . . . . . . . . . 174.1.7 Test data evaluation . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.2 Vacuum pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Contents Dionis Minev

Contents | iv

4.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.2.2 Vacuum theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.2.2.1 Gas �ow . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.2.2.2 Viscous �ow . . . . . . . . . . . . . . . . . . . . . . . . . 214.2.2.3 Molecular �ow . . . . . . . . . . . . . . . . . . . . . . . 21

4.2.3 Pumps theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224.2.3.1 Rotary vane pump . . . . . . . . . . . . . . . . . . . . . 224.2.3.2 Diaphragm pump . . . . . . . . . . . . . . . . . . . . . . 224.2.3.3 Turbo molecular pump . . . . . . . . . . . . . . . . . . . 22

4.3 Heating equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244.3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244.3.2 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.4 Pressure measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264.4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264.4.2 Pirani sensor theory . . . . . . . . . . . . . . . . . . . . . . . . . 264.4.3 Pirani Pfei�er Vacuum sensor . . . . . . . . . . . . . . . . . . . . 284.4.4 PV ActiveLine software . . . . . . . . . . . . . . . . . . . . . . . 28

5 Experiment set up 29

5.1 Vacuum experiment set up . . . . . . . . . . . . . . . . . . . . . . . . . . 295.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295.1.2 Original set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5.1.2.1 Problems of the original set up . . . . . . . . . . . . . . 305.1.3 Probe alteration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305.1.4 Current set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5.2 Temperature experiment set up . . . . . . . . . . . . . . . . . . . . . . . 345.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345.2.2 Set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

6 Data evaluation 36

6.1 General data overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366.2 Cold vacuum pressure experiments . . . . . . . . . . . . . . . . . . . . . 37

6.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376.2.2 Pressure drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376.2.3 Pressure rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

6.3 Hot vacuum pressure experiments . . . . . . . . . . . . . . . . . . . . . . 416.3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416.3.2 Pressure drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416.3.3 Pressure rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

6.4 Heat transfer tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446.4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446.4.2 Tests with ∼200 ◦C . . . . . . . . . . . . . . . . . . . . . . . . . . 446.4.3 Heat capacity calculation . . . . . . . . . . . . . . . . . . . . . . . 456.4.4 Calculation for probe with cooling and internal electrical load . . 466.4.5 Test with ∼165 ◦C . . . . . . . . . . . . . . . . . . . . . . . . . . 516.4.6 Strubels calculations . . . . . . . . . . . . . . . . . . . . . . . . . 51


Contents | v

7 Analysing Problems 53

7.1 Problem overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537.2 Possible problem factors veri�cation . . . . . . . . . . . . . . . . . . . . . 54

7.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 547.2.2 Components and valves . . . . . . . . . . . . . . . . . . . . . . . . 547.2.3 MLI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 587.2.4 Thread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 587.2.5 Leak test with mass spectrometry . . . . . . . . . . . . . . . . . . 60

7.2.5.1 Leak detector theory . . . . . . . . . . . . . . . . . . . . 607.3 Experimentally not veri�able problem factors . . . . . . . . . . . . . . . 63

7.3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 637.3.2 Desorption, Permeation and Leaks . . . . . . . . . . . . . . . . . 63

7.3.2.1 Permeation . . . . . . . . . . . . . . . . . . . . . . . . . 637.3.3 Mass �ow permeability and gas release rate calculation . . . . . . 64

7.3.3.1 Desorption . . . . . . . . . . . . . . . . . . . . . . . . . 657.3.3.2 Leak rate . . . . . . . . . . . . . . . . . . . . . . . . . . 66

7.3.4 Leak rate calculation . . . . . . . . . . . . . . . . . . . . . . . . . 677.3.5 Venting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

8 Alternative solutions 70

8.1 Solution overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 708.2 Getter material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

8.2.1 Getter activation instructions . . . . . . . . . . . . . . . . . . . . 708.2.2 Getter test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

8.3 Re-evacuation valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 738.3.1 CAD construction . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

9 Final conclusion and outlook 75

9.1 Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 759.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76


Validation Of A Super Insulation For A Geothermal Borehole Probe | 1

Chapter 1

Introduction

Contents

1.1 General project overview . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Bachelor thesis overview . . . . . . . . . . . . . . . . . . . . . . 1

1.3 Objectives and requirements . . . . . . . . . . . . . . . . . . . . 2

1.4 Current project status . . . . . . . . . . . . . . . . . . . . . . . 2

1.1 General project overview

The Karlsruhe Institute of Technology as a National Laboratory of the Helmholtz Associ-ation is working with the geothermal research group at the Institute of Applied ComputerScience on the representation and the acquisition of data regarding geothermal sites. Onegoal is the development of a construction kit, which will ease the design and the realisa-tion of geothermal probes. These probes have to withstand pressures of approximately600 bars in a depth of 5000 m and temperatures of up to 200 ◦C.

An important component of this kit is a cooling module, which dissipates heat of sen-sitive devices. Cédric Strubel investigated this problem in his Master Thesis [11]. Hedeveloped a simulation algorithm that describes the heat transfer of the high tempera-ture borehole environment into the probes inside. The mantle of the probe containing asuper insulation that reduces heat conduction and thermal radiation. He also designed aprototype which was manufactured in order to evaluate the correctness of the simulation,the manufacturing and the welding processes.

1.2 Bachelor thesis overview

In the scope of this Bachelor thesis the geothermal probe prototype is being put intooperation under real boundary conditions in order to acquire and evaluate data for vali-dation of the super insulation. Before the experimental tests for the prototype can begin,all of the necessary hard- and software required for the measurements has to be set upand operational. A vacuum is then established in the outer mantle of the probe. The

Chapter 1. Introduction Dionis Minev

1.3. Objectives and requirements | 2

prototype is then put into a heating jacket simulating the temperature in 5000 m depth.The gathered information after the tests will be used to validate or improve the superinsulation concept. All experimental results and change recommendations in the conceptwill be displayed and documented in this thesis.

1.3 Objectives and requirements

The main objective is to ensure that the inside temperature of the prototype stays un-der 75 ◦C during the operational hours, to guarantee no heat damage to the sensitiveelectronic equipment inside the prototype.

1.4 Current project status

The geothermal probe project is in development since 2008. Since then, di�erent aspectsof the project have been completed or are still in the process of completion.

• Diane Van Dorsselaer was working on a system that will improve the visibilityconditions of the borehole

• Lucie Malaurent was working on a camera system that will enable the capturing ofimages while in the borehole

• Aaron Frueh did work on a communication device that will enable communicationwith the earths surface

• Andreas Eberle advance developed a thermoelectric cooling module based on thePeltier e�ect as well as dimensioning the probe and an insulation concept

• Chris Bauer developed a pressure compensation module that keeps the inside andoutside pressure equal, in order to ease the construction of the gaskets. He iscurrently working on a high temperature circuit board with a micro controller anda high temperature motor controller

• Roland Lohrer developed a mechanical clutch for the probe

• Cédric Strubel developed a passive cooling module for the borehole probe [11]

• Benedict Holbein developed and constructed an active cooling module for the probe

• Xavier Sanson calculated the stability of separate components of the probe, undermechanical stress with the �nite element method

• Jochen Antons developed and de�ned the range of functions of an algorithm usedin a temperature resistant motor controller for borehole applications

• Dipl.-Ing. Stefan Dietze supervises the software and electronic hardware develop-ment and is currently working on communication aspects


1.4. Current project status | 3

Figure 1.1: System components for the geothermal probe kit [3]

Fig.1.1 shows the overall system and all its main components that are still under develop-ment or completed. In the mid and long term perspective, the research team is workingon two prototype probes.

1. In cooperation with a borehole inspection company, a video probe will be developedfor temperatures of up to 165 ◦C and depths in a range of 4000 m.

2. Within the KIT network, a geothermal water sampler is constructed that will main-tain temperature and pressure of the samples.



Chapter 2

Probe concept

Contents

2.1 Theoretical concept . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1.2 Borehole probe concept . . . . . . . . . . . . . . . . . . . . . . 6

2.1.3 Numerical simulation . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1.4 Super insulation concept . . . . . . . . . . . . . . . . . . . . . . 6

2.1.5 Heat transfer theory . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1.5.1 Heat conduction . . . . . . . . . . . . . . . . . . . . . 6

2.1.5.2 Thermal radiation . . . . . . . . . . . . . . . . . . . . 6

2.1.6 Vacuum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1.7 Multi layer insulation . . . . . . . . . . . . . . . . . . . . . . . 8

2.2 Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1 Theoretical concept

2.1.1 Overview

Cédric Strubel developed a theoretical concept of the probe in his master thesis. Threekey components of his thesis are important factors in this bachelor thesis [11]. Firstof, the theoretical concept developed in CAD, the plans were used to manufacture theprobe. Secondly, an insulation concept to keep the inside temperature of the probe duringoperational hours on a possible minimum1 and thirdly a numerical simulation to evaluatethe probes theoretical behaviour in the borehole environment.

1The maximum temperature should not surpass 75 ◦C

Chapter 2. Probe concept Dionis Minev

2.1. Theoretical concept | 5

B-B ( 1 : 2 )

C ( 2 : 1 )

A-A ( 1 : 2 )

1

1

2

2

3

3

4

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A A

B B

C C

D D

1 A3

Flansch-AussenuntenStatus Änderungen Datum Name

Gezeichnet

Kontrolliert

Norm

Datum Name14.05.2010 cedric.strubel

B

B

C

A

A

128,00140,00

154,00

30°

10°

6x M10x1.5

- 6H6,00 H7 -26,00 TIEF

DIN 74 - 10,00 X 90°

12x M10x1.5 - 6H

R10,00R2

,00 20,00

60,00

82,0092,00

114,30

137,76

110,30

168,30

31,64( )

15,8280

,0040

,00

G 1/2 - 12,00 TIEF

8,00

-22

,00

TIEF

7,00 -23,00 T

IEF

139,76

- 0,80

0,60

-

25,00

20,00 25,00

Oberfläche - O-RingBezeichnung Ra Rmax

A 0,40 1,60C 3,20 12,50

Position 1

A

1,80 X

20°

C

15°

82,65

115,00

H

8 -

15,00

TIEF

20,00

1 Stück

6x 4,00

-6,00 TIE

F

DIN 74 -

10,00

X 90°

12,00 + 0,000,10+

64,00

(a) External grooves

A-A ( 1 : 2 )

B ( 1 : 1 )

1

1

2

2

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A A

B B

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Sonde-Detail-GewindeStatus Änderungen Datum Name

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Norm


A

A

B

(b) Detailed view of thread

B-B ( 1 : 2 ) C-C ( 1 : 2 ) D-D ( 1 : 2 )A-A ( 1 : 2 )

1

1

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A A

B B

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1 A3

Prinzip-VakuumpumpeStatus Änderungen Datum Name

Gezeichnet

Kontrolliert

Norm


BB CC DD

AA

6,00 H7

-37,00

2,00 TIE

F

-2-

Bohrung von 6H7, 37 Tief,

mit Vorbohrung (4)

-1-

Flansch-Aussenunten vor die Evakuierung

-3-

Der Passstift von ca. 16,5mm einfügenDann Raumevakuieren

-4-

Passtift komplett einfügenDeckel im Gewinde einfügen

Dann Deckel und Stift schweißen

16,50

(c) Vacuum pump connection and sealing mech-anism

(d) Welding plan

Figure 2.1: Construction and welding plans by Cédric Strubel [11]

Figure 2.2: CAD assembled probe



2.1.2 Borehole probe concept

Fig.3.3 shows the construction and welding plans developed in CAD. The plans were usedto manufacture the prototype at the KIT campus north production hall in 2010. Thecompletely assembled prototype in CAD is shown in Fig.2.2.

2.1.3 Numerical simulation

Another important component is the simulation Strubel programmed in Microsoft Excel.It includes calculations, that approximate the theoretical time the probe can stay inthe borehole, while the inside temperature stays under 75 ◦C. Besides the time andtemperature relation of the probe, Strubel considered the use of a phase change material,to cool the inside of the probe.

2.1.4 Super insulation concept

The heat transfer from the borehole environment2 inside the probe is reduced by twomain measures, a multi layer insulation and a vacuum. Both of these concepts are partof the super insulation in the mantle of the probe.The MLI is used to reduce the e�ects of thermal radiation. Heat conduction is minimizedby the establishment of a vacuum. For the experiments and calculations, in�uencesthrough convection are being neglected [11]. In Fig.2.2 the gray outer pipes enclose thevacuum used for the super insulation. The blue space is the inside of the probe that needsto stay under 75 ◦C during operations. Two Te�on blocks inserted on either side in axialdirection where no vacuum can be established, are used to reduce the heat conductionwith the hot environment.

2.1.5 Heat transfer theory

2.1.5.1 Heat conduction

Heat transfer through conduction is a transfer of thermal energy that takes place be-tween regions of matter with a temperature gradient. Heat always �ows from greatertemperature regions to lower temperature regions seeking to ultimately establish a ther-mal equilibrium. To distinguish conduction from convection, it should be stated thatconduction transfers heat through regions of matter itself. Whereas convection requires,bulk motion of the matter, which due to the vacuum can in our case be neglected. Heatconduction through static matter depends solely on the temperature gradient and mate-rial properties. Fig.2.3(a), shows the heat transfer from ϑ1 to ϑ2. [1] [21]

2.1.5.2 Thermal radiation

Thermal radiation is electromagnetic radiation generated by the thermal motion of chargedparticles in matter. All matter with a temperature greater than absolute zero emits ther-mal radiation [26]. In our case the environment of the probe will have a higher temper-ature than the probes inside, meaning that there will be thermal radiation. The probe

2around 200 ◦C



(a) Heat conduction [1] (b) Thermal radiation between two sur-faces [1]

Figure 2.3: Heat transfer

is assumed to be a gray body that only absorbs a certain amount of thermal radiation,while emitting the rest [11].


2.2. Summary and outlook | 8

2.1.6 Vacuum

The high vacuum in the outer mantle of the probe, signi�cantly reduces the thermalenergy that is being transferred through conduction. By increasing the mean free pathof the gas molecules in the outer mantle conduction is reduced. Calculated previously[11], the mean free path λ and the distance e between the two pipes forming the mantlehave a relationship for heat transfer. If λ < e conduction will mainly take place over themolecular interaction. If however λ > e the interaction between the walls of the mantleand the molecules will be the main reason for conduction.

2.1.7 Multi layer insulation

A layer of thin metallized synthetic foils that are separated by glass �bre to preventthermal contact is used as multi layer insulation. These sheets are used in aerospaceengineering, to insulate satellites from thermal radiation. In our case, the MLI will be�lling up the space between the inner and outer pipe (the mantle).This way, we can ensure that heat transferred through thermal radiation is minimized.In theory, according to [11] 35 layers are supposed to be wrapped around the inner pipe.Practice, however showed that the number of layers had to be reduced to 20, in order toaccurately assemble the probe. Fig.2.4(b) shows the MLI enclosing the inner pipe beforethe assembly and the welding process.

2.2 Summary and outlook

The theoretical calculations done by Strubel, are in the scope of this Bachelor thesis tobe tested. The vacuum in the mantle needs to be in a range of 10−5 − 10−6 bar in orderto have a large enough λ. Both Te�on blocks and all thermocouples are to be accuratelyinserted and assembled on either side of the probe.

The tests are to take place with a heating jacket, enfolded around the probe. The dataacquired is then to be compared with the theoretical concept and if needed the prototypehas to be adjusted in order to operate accurately. The concept also has to be put intoperspective with the future plan, of building a 15 m long probe.


2.2. Summary and outlook | 9

(a) MLI before assembly

(b) enfolded MLI

Figure 2.4: MLI



Chapter 3

Manufactured Probe

Contents

3.1 Assembled Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1.2 Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1.3 Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1 Assembled Probe

3.1.1 Overview

The plans developed by Strubel where sent to the KIT production hall in 2010. Aftercompletion of the manufacturing process, the probe was ready for the experimental phase.During the experiments problems arose, which later lead to an alteration of the probe toachieve better results in the measurements. All alteration to the probe will be shown inchapter 5.

3.1.2 Probe

The whole assembled probe after all manufacturing processes were completed is shownin Fig.3.1(b). A CAD half- and quarter-cut view with all important components labelledis shown in Fig.3.2.

3.1.3 Components

The inner and outer �ange components are compared with their CAD counterparts inFig.3.3(a), 3.3(b), 3.3(c) and 3.3(d). The Te�on block with the inner �ange can be seenin Fig.3.3(f) and both inner and outer �ange in an assembled state welded to the probeare displayed in Fig.3.3(e).

Chapter 3. Manufactured Probe Dionis Minev

3.1. Assembled Probe | 11

(a) Without the Te�on block ends (b) With the Te�on block ends

Figure 3.1: Assembled Probe

(a) Half-cut view in CAD [11]

(b) Quarter-cut view in CAD [11]

Figure 3.2: CAD view


3.1. Assembled Probe | 12

(a) CAD inner �ange (b) Inner �ange

(c) CAD outer �ange (d) Outer �ange

(e) Assembled inner and outer �ange (f) Te�on block with inner�ange

Figure 3.3: Components



Chapter 4

Equipment

Contents

4.1 Temperature measuring equipment . . . . . . . . . . . . . . . . 14

4.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.1.2 Test set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.1.3 Thermocouple theory . . . . . . . . . . . . . . . . . . . . . . . . 14

4.1.3.1 Type K . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.1.4 Test set up and hardware . . . . . . . . . . . . . . . . . . . . . 16

4.1.5 PicoLog recorder Software . . . . . . . . . . . . . . . . . . . . . 16

4.1.6 Test recording process . . . . . . . . . . . . . . . . . . . . . . . 17

4.1.7 Test data evaluation . . . . . . . . . . . . . . . . . . . . . . . . 18

4.2 Vacuum pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.2.2 Vacuum theory . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.2.2.1 Gas �ow . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.2.2.2 Viscous �ow . . . . . . . . . . . . . . . . . . . . . . . 21

4.2.2.3 Molecular �ow . . . . . . . . . . . . . . . . . . . . . . 21

4.2.3 Pumps theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.2.3.1 Rotary vane pump . . . . . . . . . . . . . . . . . . . . 22

4.2.3.2 Diaphragm pump . . . . . . . . . . . . . . . . . . . . 22

4.2.3.3 Turbo molecular pump . . . . . . . . . . . . . . . . . 22

4.3 Heating equipment . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.3.2 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.4 Pressure measurement . . . . . . . . . . . . . . . . . . . . . . . 26

4.4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.4.2 Pirani sensor theory . . . . . . . . . . . . . . . . . . . . . . . . 26

Chapter 4. Equipment Dionis Minev

4.1. Temperature measuring equipment | 14

4.4.3 Pirani Pfei�er Vacuum sensor . . . . . . . . . . . . . . . . . . . 28

4.4.4 PV ActiveLine software . . . . . . . . . . . . . . . . . . . . . . 28

4.1 Temperature measuring equipment

4.1.1 Overview

To measure the temperature on the probe and record it, 7 thermocouples combined witha USB data logger and software are used. The goal is to record the temperature trend inthe probe and project the theoretical calculations on the practical outcome.Before beginning with the actual measuring of temperature on the probe itself, the mea-suring equipment and software had to be tested. For this a preliminary set up was chosen,with the objective to measure the temperature in a room over a brief period of time. Thegoal of this test was to accurately operate the USB TC-08 thermocouple data logger withits software and to obtain a minimum of temperature scale �uctuation for each thermo-couple, in order to get a consistent output of the temperature data.The USB TC-08 has a temperature measuring accuracy1 of ± 0,5 ◦C. According to this,there should be a maximum variance of ± 0,5 ◦C in the measured temperature.

4.1.2 Test set up

For the set up a normal cardboard box had eight slits cut into it. Each slit has a distanceof approximately 2 cm from the next one. The thermocouples were then �xated in theslits, to ensure that they were stationary during the time of the measurements. The setup is displayed in Fig.4.1.All outside in�uences on the system2, which could a�ect the outcome of the data werereduced to a possible minimum3.

4.1.3 Thermocouple theory

The thermocouple consists of two di�erent metals that are joined at both ends. If thereis a temperature di�erence between the hot and cold junction (shown in Fig.4.2) of thethermocouple a small electric current is induced, this is called the Seebeck e�ect.The small voltage that is formed within the circuit is between 1 and 70 microvolts per de-gree Celsius (µV/◦C). The higher the temperature di�erence between the two junctionsthe higher the voltage. When the right materials are paired, this can be used to mea-sure the temperature. There are a variety of di�erent thermocouples used for di�erentapplications, based on their metal compound. The basic operating principle is depictedin Fig.4.2. The thermocouples used in our set up, are type K. [10] [27]

1According to the general speci�cations for temperature accuracy given by the manufacturer2The room in which the measurements are taking place3referring to closed windows, stationary measurements and no direct sunlight on the set up.



Figure 4.1: Test set up

Figure 4.2: Thermocouple operating principle [9]

4.1.3.1 Type K

In theory, any two di�erent conductors can be used to make a thermocouple. In practice,however only a few combinations of metals are used [6]. According to [10], the type Kthermocouple has a Chrome positive leg and an Aluminium negative leg. The temperaturerange for type K alloys is −200 to 1250 ◦C. Type K sensors are recommended for use inoxidizing or completely inert environments. Type K and type E should not be used insulphurous environments. The Thermocouples used for the measurements are additionallyembedded in a small high-grade steel pipe. All requirements are met by the probesexperimental set up.



Figure 4.3: Temperature measuring equipment

4.1.4 Test set up and hardware

The equipment consists of multiple components shown in Fig.4.3.

• USB TC-08 thermocouple data logger

• Eight type K thermocouples, of which only seven will be used on probe itself

• Cardboard box used to �xate the thermocouples

• Computer with the PicoLog recorder software

4.1.5 PicoLog recorder Software

The PicoLog recorder software is set out in a very simple layout, allowing easy operationand recording of the temperature. Fig.4.4 shows the graphic user interface of the software.The sampling intervals and di�erent parameters are arbitrarily adjustable for each of themeasurements. The recordings of each channel, over a given period of time can be plottedin a graph.For these simple tests, the measurements where set to continuous real time with di�erentsampling intervals in seconds. The test scans varied in length, with most of them beingin a range of around 500 seconds. The software allows the output of all recorded data ina text document, making it easy to integrate the data into Microsoft Excel.



Figure 4.4: PicoLog recorder Software (German)

4.1.6 Test recording process

The �rst three measurements made, where set to an arbitrary sampling intervals rangingaround 100 − 150 seconds to get accustomed to the new software and try out di�erentparameters and settings. The data in these measurements partially showed a variancethat exceeded the ± 0,5 ◦C range. The exceeded ranges were due to adjustments onthe set up made during the scoping process or testing the real time reaction of somethermocouples by touching them.All anomalies can be concluded to be e�ects from outside in�uences on the system.These measurements were mere trials of the soft- and hardware. Fig.4.5 showing a test ofthe real time response by touching the thermocouples during the temperature recording.Further two measurements showed a consistent and accurate recording of the temperature,where the given accuracy of the manufacturer was veri�ed. Both tests where set at ∼500 seconds. The two tests where set out to run in optimal conditions with minimalin�uences.



Figure 4.5: Thermocouple reaction tests

4.1.7 Test data evaluation

Each graph represents a test with the eight thermocouples, with similar sampling inter-vals. The graph is plotted over two variables, temperature and time. The time scale isshown on the abscissa and the temperature on the ordinate. Each channel is displayedwith either a di�erent color or shape allowing the surveyor to distinguish the recordeddata from each thermocouple.

Tests:

Fig.4.6 displays that the temperature �uctuation within the room over the recordinginterval is no more than ± 0,5 ◦C. The thermocouples show similar patterns while record-ing, with similar peaks and lows. Isolated anomalies between the thermocouples, wherethere is a variance in the pattern can be concluded to be in�uences of the environmenton the measurement set up.Temperature �uctuation in di�erent locations of the room could be the cause of numeroussources. Heat dissipated from electronic equipment, sunlight in�uences or temperaturegradients from air movements caused by human motion. The results of the evaluationdisplay a clear analogy to the manufacturers information.



(a) Test 1 ∼ 500 seconds

(b) Test 2 ∼ 500 seconds

Figure 4.6: Thermocouple Tests


4.2. Vacuum pumps | 20

(a) First set up (b) Second set up

Figure 4.7: Vacuum pump arrangement

4.2 Vacuum pumps

4.2.1 Overview

During the measurements, two di�erent pump set ups were installed. The �rst one shownin Fig.4.7(a), consisted of a rotary vane pump and a turbo molecular pump, connectedto one another and subsequently to the prototype. With the �rst set up all preliminaryand �rst regular tests were performed on the prototype. Due to the age and continuoususage of the pump, eventually the system broke and �rst repair works had to be started.The state of the damage turned out to be too advanced, for an optimal time and costlyrepair. A second pumping system was ordered at Pfei�er Vacuum in August 2011.The second system from Pfei�er Vacuum is an integrated solution with both, diaphragmbacking pump and turbo molecular pump in one apparatus. The second set up is thecurrent system used to evacuate the probe, it is displayed in Fig.4.7(b).

4.2.2 Vacuum theory

In order to establish a vacuum in the range of 10−5 − 10−6 bar two di�erent gas �owshave to be considered. According to [11] λ(10−6bar) ≈ 55mm, this is enough to ensurethe only way heat can transfer is through collisions of gas molecules and the wall.The backing pump, in our case the rotary vane or diaphragm pump, establishes a vacuumin the range of 10−2 − 10−3 mbar. The turbo molecular pump, can however reach muchlower pressures of up to 10−7 − 10−8 mbar, depending on the model.

4.2.2.1 Gas �ow

The evacuation process goes through two di�erent stages of gas �ow, depicted in Fig.4.8.Higher pressures are evacuated with the backing pumps and operate with viscous gas�ow. Where as, the turbo molecular pump operates with a low pressure molecular �ow.The backing pump normally establishes a pre-vacuum of ∼ 10−2 − 10−3 mbar, where a



medium-pressure range is characterized as a transitional �ow between viscous and molec-ular. In this transitional phase, there are equally as much collisions between the gasparticles as there are with the wall. Another term to describe this transitional �ow isthe Knudsen �ow. For a particular type of �ow to occur two main criteria can be char-acterized. The Knudsen number Kn, the ratio of the mean free path of the gas particlesbetween two particle-particle collisions λ and the characteristic geometrical dimension dof the tube's diameter.

Kn :=λ

d

The Knudsen number is inversely proportional to the pressure, thus meaning a highKnudsen number indicating low pressure and molecular �ow. Where as, a low Knudsennumber high pressure and viscous �ow suggests. Quantitative investigations show thefollowing conditions. [5]

Kn > 0,5 molecular �ow0,5 > Kn > 0,01 transitional �owKn < 0,01 viscous �ow

The second criteria is the velocity of �ow. According to [5], the velocity of a gas �ow isthe mean velocity component of the gas particles in the direction of the tube. Duringmolecular �ow, the individual gas particles travel back and forth between the walls of thetube with thermal velocity. The particles take on a zig-zag route, where the geometrydetermines the resulting velocity of �ow.

4.2.2.2 Viscous �ow

[5],states that under high pressure, the mean free path of gas particles is much lower thanthe cross dimensions of the tube. The particles experience frequent mutual collisions,thereby exchanging momentum and energy continuously. Even a small volume containsmany frequently colliding particles. Thus, the gas behaves as a continuum. A �ow is theresult of the local pressure gradient. This situation is referred to as a continuum �ow orviscous �ow. This �ow is used by the backing pumps.

4.2.2.3 Molecular �ow

The turbo molecular pump operates in the molecular �ow range. Given by [5], for su�-ciently low pressure, the mean free path of gas particles is high, compared to the crossdimension of the tube. Hardly any mutual particle collisions occur. Each gas parti-cle travels through the tube due to its thermal motion, independent of other particles.However, frequent collisions with the tube walls cause a zig-zag route. On average, thepath of many individual particles combine to form the macroscopic �ow behaviour. Thissituation is referred to as a single-particle motion or molecular �ow.



Figure 4.8: Viscous and molecular gas �ow [16]

4.2.3 Pumps theory

4.2.3.1 Rotary vane pump

In Fig.4.9(d), the operating principle is shown where, 1. is the pump housing, 2. therotor, 3. the vanes and 4. the springs. A rotary vane pump is a positive displacementpump, with the vanes mounted to a rotor that circulates inside a cavity. The vanes areable to slide in and out of the rotor, sealing on all edges, subsequently creating vanechambers that do the pumping work. The pump can create pressures of up to 10−3 mbar.Fig.4.9(e), displays the rotary vane pump used in the experiments. [25] [5]

4.2.3.2 Diaphragm pump

The upward and downward motion of the diaphragm results in a periodic change of thesuction volume, creating an aspiration and compression phase. Fig.4.9(b) shows thatthe Gas is aspirated through the right valve during the upward motion and compressedand ejected through the left valve during the downward motion of the diaphragm. Thediaphragm pump used in the experiments is integrated in a Pfei�er Vacuum HiCubepumping station, shown in Fig.4.9(c). [5] [22]

4.2.3.3 Turbo molecular pump

The basic pumping principle of a turbo molecular pump is shown in Fig.4.9(a). A backingpump is always connected to the exhaust of the TMP, in our case either a rotary vaneor a diaphragm pump. The pumping e�ect relies on the arrangement of rotor and statorblades that transfer impulses onto the gas molecules while rapidly rotating. Gas moleculesthat collide with the rotating blades are adsorbed and remain on the blade for a certainamount of time before discharging again.During this process the blade speed is added onto the thermal molecular speed4, which is

4Gas molecules in a vessel move back and forth in di�erent directions and at di�erent speeds. Theirvelocity distribution corresponds to a bell curve having its peak at the most probable velocity [18]



(a) Turbo molecular pumpingprinciple [28]

(b) Diaphragm pumping principle [22]

(c) Diaphragm pump HiCube (d) Rotary vane pumping prin-ciple [25]

(e) Rotary vane pump

Figure 4.9: Used pumps and working principles

the natural velocity at which molecules travel in a certain environment. To be sure thatthe velocity transferred by the blades onto the molecules is not lost by collisions withother molecules, molecular �ow must prevail within the pump i.e. the mean free pathof the molecules must be greater than the blade spacing. The comparison between old(left) and new (right) turbo molecular pump can be seen in Fig.4.10. [5] [19]


4.3. Heating equipment | 24

(a) Old TMP (b) New TMP in HiCube

Figure 4.10: Turbo molecular pumps

4.3 Heating equipment

4.3.1 Overview

In order to simulate a high temperature environment, the probe is wrapped in a heatingjacket. The jacket can be heated up to high temperatures of ∼ 200◦C. It consists of fourbasic components, the main body, two caps for either side of the probe and a controllerunit for each device. The various components are shown in Fig.4.11.

4.3.2 Layout

The heating jacket heats up through a heating wire that is sewed into the fabric. Theoutside is covered by a re�ecting synthetic material with a metallic resemblance. On theinside of the jacket a scrim covers the heating wire. The heating wires are embedded in asoft insulation keeping the heat within the jacket. A Velcro fastening allows the jacket tobe securely attached around the probe during the heating process. The top and bottomheating elements have the same layout and are also attached by Velcro.


4.3. Heating equipment | 25

(a) Main body

(b) Head cap with opening for thermocouples

(c) Head cap

Figure 4.11: Heating equipment


4.4. Pressure measurement | 26

4.4 Pressure measurement

4.4.1 Overview

The main sensor used for all pressure measurements is the Pfei�er Pirani sensor inFig.4.13. The sensor is connected to a TPG5 controller that allows the sensors mea-surement to be recorded onto a computer. The software used to record the pressure isthe PV ActiveLine, also from Pfei�er Vacuum. Two older sensors are also used during themeasurements, with no digital output to record the pressure. Both sensors are connectedto a analogue indicator, showing the real time change in pressure.

4.4.2 Pirani sensor theory

NOTE: Only general Pirani gauge theory will be depicted in this segment, there is nospeci�c operational principle published by Pfei�er Vacuum about the sensor used in theexperiments.The Pirani sensor makes use of the pressure dependence on thermal conductivity. Usuallythe sensor is designed with cylinder symmetry, it consists of a thin wire (mostly Tungsten)of diameter d mounted along the axis of a cylinder with diameter D (d� D). The thinwire is heated up (between 110− 130◦C) with an electrical current, while the cylinder isat ambient temperature. Thus, a heat �ux develops from the wire through the gas in thecylinder and towards the cylinder. A section of a measuring tube is shown in Fig.4.12(b).The thermal conductivity in the molecular regime is dependent on the molecular numberdensity and therefore proportional to the pressure. When the temperature of the wire iskept constant the heat output �uctuation will be caused by the pressure [5].The thermal power transported by the gas, according to [5] is calculated by:

P = aE1 2 π r lT1 − T2

T2

f + 1

8c p, in the molecular regime

With: c Mean thermal speed; T1 Wire temperature; T2 Tube temperature; f Degree offreedom of a particle; p Pressure; r Wire radius; l Wire length; aE1 Energy-accommodationcoe�cient.

The equation above shows that heat transport in the molecular regime is dependenton the pressure. Another important factor is, once the pressure is below 10−4 mbar thegas has very little conductivity, which means it does not a�ect the transported heat. Thethermal power transferred by the wires ends is therefore greater than the transfer by thegas, making the measurement inaccurate.The thermal conductivity is measured by aWheatstone bridge network shown in Fig.4.12(a),which serves both to heat the wire and measure its resistance. The heating voltage at thebridge is calibrated so it keeps the wires temperature constant and independent from thethermal conductivity. However, the conductivity from wire to the gas is increased withhigher pressures, therefore the voltage can be converted to indicate the pressure.

5SingleGauge measurement equipment TPG 261 from Pfei�er Vacuum used to connect the sensor toa digital output



(a) Wheatstone bridge control circuit for the Pirani sensor [4]

(b) Section of measuring tube [5]

Figure 4.12: Pirani sensor theory



Figure 4.13: Pfei�er Pirani sensor

4.4.3 Pirani Pfei�er Vacuum sensor

The operation principle of the Pfei�er gauge is based on the theory above. Fig.4.13,shows the sensor. The reading range is limited to 5 ∗ 10−4 mbar.

4.4.4 PV ActiveLine software

The software allows in connection with the sensor and controller, an accurate recordingof the pressure. The sampling intervals are arbitrarily adjustable and are in all measure-ments set to one minute. The output in the graph takes place in real time. The recordeddata can be evaluated by integrating the data in Microsoft Excel.



Chapter 5

Experiment set up

Contents

5.1 Vacuum experiment set up . . . . . . . . . . . . . . . . . . . . . 29

5.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5.1.2 Original set up . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5.1.2.1 Problems of the original set up . . . . . . . . . . . . . 30

5.1.3 Probe alteration . . . . . . . . . . . . . . . . . . . . . . . . . . 30

5.1.4 Current set up . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5.2 Temperature experiment set up . . . . . . . . . . . . . . . . . . 34

5.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5.2.2 Set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5.1 Vacuum experiment set up

5.1.1 Overview

With the manufactured probe and the vacuum pumps, an appropriate set up was chosen.The experiment set up was changed during the measurements, concern arose that theprobe could not be accurately evacuated with the diameter of the screw connection inthe �ange being too small. Additionally the Pirani sensor was placed in an adverselyposition to record the pressure. A solution was designed in CAD and along with theprobe, sent to the production hall, where it was altered. Besides the change on the probeitself, a new vacuum pump system was ordered and replaced the old one.

5.1.2 Original set up

Fig.5.1(a), shows the complete structure. The red line indicates the �ow of the gasparticles that are being evacuated. A close view of the direct connection to the probe isdisplayed in Fig.5.1(b), again the red line indicates the gas �ow during evacuation.In Fig.5.1(c), a close view of the gas �ow in the valve and connection to the turbo

Chapter 5. Experiment set up Dionis Minev

5.1. Vacuum experiment set up | 30

molecular pump is shown. Fig.5.1(d) and 5.1(e) label all important components up tothe turbo molecular pump. The analogue sensor used to indicate the pressure closest tothe pumps can be seen in Fig.5.1(f). This sensor does not have a digital output and cantherefore not record the pressure.

5.1.2.1 Problems of the original set up

The old set up consisted of a screw connection directly in the outer �ange, with a smalldiameter for evacuation. While in laminar gas �ow the diameter does not matter as much,in molecular �ow a small diameter can be disadvantageous. The molecules have a verysmall opening to pass in order to be ejected, making the evacuation process more di�cult.If λ is much larger than the diameter of the opening, the probability of the moleculesexiting through the opening sinks. Fig.5.4(b) shows the original CAD construct [11] andthe real set up , for the vacuum connection in the �ange.Another problem of the old set up was, the unfavourable position of the pressure sensor.Not measuring the pressure directly inside the probes mantle, but between the valve andthe di�user. Thus, no accurate evaluation of the pressures behaviour inside the probesmantle could be made. The sensors old position is shown in Fig.5.1(b).

5.1.3 Probe alteration

After narrowing down the problems with the old set up, an alteration in CAD was con-structed. The two main problems were solved by replacing the vacuum connection in the�ange, with a DN 40 nominal width in the probes middle. The �ange was welded ontothe opening, as seen in Fig.5.3(b), with the gas �ow symbolized by the red lines.The alterations on the probe, where primarily made to establish a working vacuum.Thus, no consideration was given to the usability of the probe in the actual borehole1. Aconstruction plan created in CAD is shown in small in Fig.5.3(a), the large plan can befound in the appendix.

1A new connection for repeated evacuation and usability in the borehole is depicted in chapter 8.



(a) Complete set up (b) Close view on vacuum connection

(c) Close view on valve (d) Components connected to the valve

(e) Four way valve (f) Sensor

Figure 5.1: Original set up



(a) CAD vacuum connection in the outer �ange[11]

(b) Real Vacuum connection

Figure 5.2: Vacuum connection

(a) CAD modi�ed outer pipe (b) Real modi�ed outer pipe

Figure 5.3: Probe alteration



(a) Full view

(b) Vacuum connection 1

Figure 5.4: Current set up

5.1.4 Current set up

After the alteration on the probe the Pirani sensor was placed where the old vacuumconnection was and an accurate statement of the pressure in the corner of the mantlecould be made. Due to the symmetrical design of the probe, the pressure can be recordedon one side and the result can be projected on the opposite side. Allowing the precisebehaviour of the pressure in the regions furthest away from the vacuum pumps, to beevaluated. Furthermore the centred �ange enables a symmetrical evacuation of the probe.During every measurements cycle with the old set up, the observation was made, thatupon closing the valve the Pirani sensor reacted immediately. The same observation wasmade for initial pumping, where the pressure drop was immediately after turning thepumps on. This suggests that the small opening in Fig.5.2(a), is large enough to havean immediate impact on the creation of a vacuum. However, a larger diameter is stillpreferable for the evacuation process and was implemented after alteration. A full andlabelled view of the current set up is shown in Fig.5.4.


5.2. Temperature experiment set up | 34

Figure 5.5: Heating jacket with probe

(a) Side cap with thermocouple open-ing

(b) Side cap

Figure 5.6: Temperature measuring with heating jacket

5.2 Temperature experiment set up

5.2.1 Overview

To determine the heat transfer into the probe, the temperature had to be measured overa given period of time. A heating jacket was placed around the probe and heated up, theresulting heat transfer simulates the borehole environment. Fig.5.6, depicts the probeenclosed by the heating jacket and both side caps.

5.2.2 Set up

The thermocouples had to be placed in di�erent locations throughout the probe, todetermine the di�erent temperature gradients within the probe while in the borehole. Tocompare the recordings with the PicoLog software and the location of the thermocouples,


5.2. Temperature experiment set up | 35

(a) Thermocouple location and numbers

(b) Probe with thermocouples

Figure 5.7: Thermocouple set up

all thermocouples where labelled with numbers. The numbers correspond to the channelsin the recordings. Fig.5.7(a), shows the location and numbers of the thermocouples. Theactual set up is displayed in Fig.5.7(b). Channel 4 measures the outside temperature inthe laboratory, where as channel 6 the temperature between the probe and heating jacketmeasures.



Chapter 6

Data evaluation

Contents

6.1 General data overview . . . . . . . . . . . . . . . . . . . . . . . 36

6.2 Cold vacuum pressure experiments . . . . . . . . . . . . . . . . 37

6.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

6.2.2 Pressure drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

6.2.3 Pressure rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

6.3 Hot vacuum pressure experiments . . . . . . . . . . . . . . . . 41

6.3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

6.3.2 Pressure drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

6.3.3 Pressure rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

6.4 Heat transfer tests . . . . . . . . . . . . . . . . . . . . . . . . . . 44

6.4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

6.4.2 Tests with ∼200 ◦C . . . . . . . . . . . . . . . . . . . . . . . . 44

6.4.3 Heat capacity calculation . . . . . . . . . . . . . . . . . . . . . 45

6.4.4 Calculation for probe with cooling and internal electrical load . 46

6.4.5 Test with ∼165 ◦C . . . . . . . . . . . . . . . . . . . . . . . . . 51

6.4.6 Strubels calculations . . . . . . . . . . . . . . . . . . . . . . . . 51

6.1 General data overview

The data is split into three categories. The �rst two are the vacuum experiments withand without heating jacket, where the pressure drop during pumping is recorded andrespectively the pressure rise when the valve is closed. The third one depicts the heattransfer into the probe during a vacuum1 and with the heating jacket on.The tests are each labelled with a number, every line represents one experiment. Forthe evaluation of the data the most presentable data was chosen. During the tests a

1between 10−6 − 10−7 bar

Chapter 6. Data evaluation Dionis Minev

6.2. Cold vacuum pressure experiments | 37

number of technical di�culties in�uenced the system. The data gathered in these tests,was not considered because it had no actual value for the objective of the project. Mostexperiments where set at 24 or more hours, which ran over night. If during the recordingprocess a technical di�culty occurred, the data gathered the next day wasn't usable. Theresults are all presented with Microsoft Excel 2007.

NOTE: The sensors measuring range ends at 5 ∗ 10−4 mbar (In the graphs indicatedby a constant trend. Furthermore, the pressure range of interest is highlighted in allpressure experiments.

6.2 Cold vacuum pressure experiments

6.2.1 Overview

The cold vacuum experiments are split into two categories. The pressure drop duringoperating pumps and the pressure rise with sealed valve and switched o� pumps. The inFig.6.1 depicted tests are performed with the original set up and small diameter vacuumconnection. The pumps used, were the split set up, rotary vane and turbo molecularpump. Pressure rise was initiated by closing the valve.

6.2.2 Pressure drop

The graphs shown in Fig.6.1, describe the pressure drop while operating the pumps. If weconsider the blue line in Fig.6.1(a), we can see that the �rst part of the curve converges toa certain pressure, before reaching a breaking point and dropping again until it ultimatelyconverges again. These are characteristic attributes of the pumps. At lower pressure therotary vane pump operates very quickly, with the viscous gas �ow. Once the pressure islow enough and the gas particles are in a transitional �ow, the turbo molecular pump isturned on and the evacuation process speeds up again.It is clearly visible, that with repeated evacuation trials the breaking points and conver-gence of the graphs turn into a more linear trend. In retrospective the repeated evacuationwith no in between air venting of the probe, always resulted in a quicker pressure dropwhile pumping. In all cases a vacuum pressure below the desired end pressure between10−5 − 10−6 bar was achieved within less than 20 minutes of pumping.

NOTE: The tests in Fig.6.1 were reduced to the �rst 100 minutes, for a clearer viewof the pressure drop. The Tests where set at 1440 mins (24 h). After a certain amountof time all tests reached the end of the sensors measuring range and could no longerdisplayed relevant data.



(a) Tests 2,4 and 6

(b) Tests 9, 11 and 13

Figure 6.1: Cold vacuum pressure drop



Figure 6.2: Tests 3,5 and 7

6.2.3 Pressure rise

A very similar result is displayed in the pressure rise of the probe. Again, after repeatedevacuation the pressure rise is slower with a more gradual slope. Although a steady lowpressure in the ranges of 10−3−10−4 mbar could not be achieved for a long time, repeatedevacuation showed a trend towards slower pressure rise. Figs.6.3(a) and 6.2 shows thepressure rise in 6 tests. An interesting can be seen in test 15. The test was conductedafter a long evacuation cycle2 displayed in Fig.6.3(b). Here a clear analogy can be made,between time of evacuation and the pressure rise afterwards. Long evacuation causes slowpressure increase.

2The small pressure rise was caused by manually opening and closing the valve



(a) Tests 10, 12 and 15

(b) Long evacuation test

Figure 6.3: Cold vacuum pressure rise and long evacuation


6.3. Hot vacuum pressure experiments | 41

6.3 Hot vacuum pressure experiments

6.3.1 Overview

The experiments cycle was repeated with the heating jacket enfolded around the probe.The jackets control unit was set to 200 ◦C during all measurements. Again the probewas evacuated and the pressure drop was recorded. Subsequently after a certain time ofevacuation, the valve was closed and the pressure rise measured. These experiments wereconducted in order to evaluate the pressures behaviour under heat in�uence.

6.3.2 Pressure drop

Fig.6.4(a), shows Test 17 and 19 with the heating jacket. In both cases the pumpspressure drop is impeded by the pressure �uctuation. The most plausible explanation isthe increased di�usion of the gas under heat. Water that was trapped between the MLIand on the surface of the metal, is released in the form of vapour. Although, we can againsee with test 19 that the pressure eventually goes below the sensors measuring range andproves that repeated evacuation generates lower pressures.Test 21, 25 and 27 in Fig.6.4(b) display the pressure drop after the alteration on theprobe. While the probe was being altered, the vacuum chamber was not sealed o�,allowing humid air to enter and embed itself into the MLI. Another factor that played animportant in�uence in the pressures trend, was the failure of the turbo molecular pump.The tests in Fig.6.4(b), where conducted without a turbo molecular pump3. Hence therelatively high pressure �uctuation in the tests. These �uctuations were most likelyenhanced by water vapour and other gas di�usions.

6.3.3 Pressure rise

Following the pressure drop in Fig.6.4(a), is the pressure rise in Fig.6.5(a). Test 17 and19 had an long evacuation period, with test 19 just over 8000 mins (5,5 days). The risein Fig.6.5(a) is plotted over the �rst 400 mins, where the steepest slopes can be recorded.The di�erence between test 18 and 20 is clearly visible and suggests that not only a longerevacuation time (Test 19) but also a repeated evacuation, decelerate the pressure rise.In tests 22 and 26 depicted in Fig.6.5(b), the rise is very similar. If test 21 and 25 is con-sidered, it is clearly visible that both tests have a resembling pressure drop. Accordinglyin Fig.6.5(b), both test 22 and 26 have a similar rise.

3Technical di�culty with the turbo molecular pump, only left the rotary vane pump for evacuation



(a) Tests 17 and 19

(b) Tests 21, 25 and 27 without the use of a TMP

Figure 6.4: Hot vacuum pressure drop



(a) Tests 18 and 20

(b) Tests 22 and 26

Figure 6.5: Hot vacuum pressure drop


6.4. Heat transfer tests | 44

6.4 Heat transfer tests

6.4.1 Overview

The thermocouples were set to record the temperatures with the heating jacket on. Tosimulate an accurate super insulation, the vacuum pumps were turned on during the heattransfer tests. Thus, always having a pressure below 5 ∗ 10−4 mbar. The heating processof the jacket simulates the probes trip to the bottom of the borehole. Once the highesttemperature is reached, it is kept steady and each channels progress can be evaluated.A number of tests with di�erent lengths and a temperature of ∼200 ◦C were conducted.Only one test with a temperature of ∼165 ◦C was conducted. In all experiments is a 75 ◦Cconstant, showing the maximum temperature the probes inside can reach with sensitiveequipment. An additional theoretical calculation is done to get a estimated quantitativevalue of the heat capacities in the probe.

6.4.2 Tests with ∼200 ◦CIn order to see the inside temperature trend of the probe and verify the results withmultiple tests, three measuring lengths were chosen. The �rst two experiments, with oneat 17 h and the other at 24 h, can be seen in Fig.6.8. In both cases, the diagrams showthat is takes ∼1000 mins (∼16 h) to reach the constant 75 ◦C line. Small �uctuationsin that 1000 minute range are caused by a slightly di�erent outer surface temperature ofthe probe. The control unit of the heating jacket does not allow a precise temperaturesetting, therefore the surface temperature �uctuates between measurements. A long ex-periment is shown in Fig.6.9, with a 72 h heating cycle. An interesting trend can be seenwith channel 5, that has a much faster temperature rise at the start, reaching the 75 ◦Cmark quickly. However after passing it, the channel seems to reach a limit, where it stopsrising and drops behind the other channels with a lower temperature. This trend has todo with the non-existent super insulation on both ends of the probe. While the probe isheating up, heat transfer over both ends is accelerated due to the lack of insulation andbecause the heating caps were not turned on. After channel 5 is heated up on a highertemperature niveau surpassing the other thermocouple channels. The channels temper-ature drops o� again and rises slower compared to the other. This cooling e�ect is alsocaused due to the heat transfer over both ends. It allows both a more rapid heating andcooling process. Additionally the position of channel 5 has to be considered, it can beseen that it is the closest to the end of the probe, making it most vulnerable to the heatexchange. While pressure experiments where conducted, a cotton wool insulation wasplaced into the thermocouple heat cap to prevent too much heat transfer. This turnedout to have little, to no e�ect. This heat loss poses an error in our experiments makingthem not entirely accurate. Fig.6.9 shows that the channels temperature trend and thesurface temperature caused by the heating jacket seem to converge. Yet, special consid-eration has to be given to the heat loss over both probe ends, which in return makes itunlikely that in in�nite time both channels and surface temperature converge.Another important factor that has to be considered is the heat capacity of the probes in-ner pipe and air. The calculation can be seen in subsection 6.4.3. The calculation showsthat air has very small ability to capacitate heat, while water is with a factor of 639



greater. This suggests that the inside of the probe can be cooled with water, due to itshigh heat capacity. Although the probe in our experiments had no water inside, it took aconsiderable amount of time before it was heated up and the channels started measuringtemperature rises. To display that the inner pipe of the probe has a considerable heatcapacity, it was also calculated in 6.4.3. This knowledge needs to be in cooperated intothe probes theoretical calculations. Strubel however, did not consider this, his calcula-tions were solely based on the use of a PCM to capacitate the heat from the borehole. In

practise water (with cp = 4, 192kJ

kgK[2]) seems to be the best solution, because it is both

cheap and environmentally friendly. The inner pipe needs to be embedded into waterso it can increase the amount of heat that it can store. Ammonia could pose another

solution with a cp = 5, 042kJ

kgK[2], other substances barely pass the 2 point mark. If the

additional container required for the water reduces the free space too drastically on theinside, transformer oil could be used for the cooling. This however only has about half

of the waters cp (∼ 1, 9kJ

kgK) value [12].

With a working vacuum the result of the heat transfer measurements display a good be-haviour. The probe is supposed to operate only a certain amount of time in the borehole,before being lifted back up again. The ∼16 h indicate a long enough operating window,without posing any threat to sensitive electronic equipment inside the probe.

NOTE: The exact pressure during the heat transfer tests could not be determined,due to the sensors measuring range. Fig.6.10 shows a standard pressure recording duringthe temperature experiments.

6.4.3 Heat capacity calculation

Heat capacity of air

Inner volume of the probe.

V = π ∗ r2 ∗ L =⇒ VAir = π ∗ 55, 152mm2 ∗ 800mm = 7, 644l

Speci�c heat capacity of air

cv = 0, 717kJ

kg ∗Kfor T and p variable and V = const.

The mass of the air inside the Volume VAir

mAir = ρAir ∗ VAir =⇒ mAir = 1, 2928kg

m3∗ 7, 644 ∗ 10−3m3 = 9, 8822g

The temperature di�erence, from starting point until reaching the maximum temper-ature for the inside of the probe.

4T = TMax − TStart =⇒4T = 75◦C − 20◦C = 55◦C



The absolute amount of heat, that the air can save for that temperature di�erence.

QAir = cv ∗mAir ∗ 4T =⇒ QAir = 0, 717kJ

kgK∗ 00098822kg ∗ 55K

↪→ QAir = 0, 3897kJ = 1, 0834 ∗ 10−4kWh = 1, 0834 ∗ 10−1Wh ≈ 0, 1Wh

In comparison to 1l of water.

QWater = 4, 182kJ

kgK∗ 1kg ∗ 55K =⇒ QWater = 230, 01kJ = 0, 039kWh = 63, 9Wh

which shows that water is with a factor of 639 greater.

The heat capacity of the 'cold' inner pipe can be calculated, as follows.

VInnerpipe = 800mm ∗ π ∗ (57, 152 − 55, 152) = 564481, 368mm3 = 0, 56448 l

mva = 7, 8kg

l∗ 0, 56448 l = 4, 403 kg

QInnerpipe = 0, 5kJ

kg K∗ 4, 403 kg ∗ 55K = 121, 0825 kJ = 33, 66Wh

So the sum (Inner pipe, air and water) of the heat capacity, calculates to:

QΣ = QWater +QAir +QInnerpipe = 97, 68Wh

6.4.4 Calculation for probe with cooling and internal electrical

load

Scenario 1, with no internal cooling medium (like water), shown in Fig.6.7(a).

If the outside heat �ow is calculated with the experimental data recorded an approachto the real heat �ow can be calculated. Fig.6.6 shows the marked time at 120 minutes,where it is assumed that the outer pipe has completely heated up to 200 ◦C. By doingthat, we can neglect the outer pipes heat capacity (which is already exhausted) and con-centrate on the inside. The next step is the time di�erence between the time the insidestarted heating up and the time it reaches 75 ◦C. The knowledge of the time di�erenceand the heat capacity of air and inner pipe is enough to calculate the heat �ow, as follows.

Q̇Heatflow =QAir +QInnerpipe

t1

with t1 = t(75◦C)− t(200◦C)

Q̇Heatflow =33, 66Wh+ 0, 1Wh

18 h− 2 h= 2, 11W

Fig.6.7(b) depicts scenario 2 of the probe with an electrical load of 20 Watts and a



Figure 6.6: Time di�erence

1 kg water cooling. Q̇Heatflow from the previous calculation can be used to solve theamount of time the probe can absorb the heat from the outside environment in the bore-hole and the internal electrical load.

tTotal =QAir +QInnerpipe +QWater

Q̇Heatflow + Q̇E−load

=0, 1Wh+ 33, 66Wh+ 63, 9Wh

2, 11W + 20W= 4, 417 h

This would allow the probe to absorb heat for 4,41 h.



(a) Scenario 1

(b) Scenario 2

Figure 6.7: Two scenarios for the probe cooling calculation



(a) ∼ 17 h

(b) ∼ 24 h

Figure 6.8: ∼200 ◦C Heat transfer tests



Figure 6.9: ∼ 72 h at ∼200 ◦C

Figure 6.10: Pressure recording during 72 h heat transfer experiment



Figure 6.11: ∼ 72 h at ∼165 ◦C

6.4.5 Test with ∼165 ◦COne experiment with 165 ◦C was conducted with the results shown in Fig.6.11. Thelength was set to 72 h. It took ∼1400 mins (23,3 h) to reach the 75 ◦C line.A lower surface temperature shows that the operational hours within the set temperaturemaximum are increased.

6.4.6 Strubels calculations

Strubel [11] simulated the theoretical behaviour of the probes inner temperature whilein the borehole. Unfortunately di�erent factors, like the camera glass4, neglection of theheat capacity in the pipes and the use of a PCM, were included in the probes simulation.Making it not similar enough to compare with the experimental data. These factors couldnot be implemented into the experiments. Due to the di�cult nature of the establishmentof a vacuum and the unforeseen errors in the probes concept as well as the di�erent factorsin�uencing theory and experiments, the results could not be compared.

4concept to allow recordings of the borehole



(a) Theoretical simulation (Diagram in German) [11]

(b) Practical experiment

Figure 6.12: Comparison between practical and theoretical diagram



Chapter 7

Analysing Problems

Contents

7.1 Problem overview . . . . . . . . . . . . . . . . . . . . . . . . . . 53

7.2 Possible problem factors veri�cation . . . . . . . . . . . . . . . 54

7.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

7.2.2 Components and valves . . . . . . . . . . . . . . . . . . . . . . 54

7.2.3 MLI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

7.2.4 Thread . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

7.2.5 Leak test with mass spectrometry . . . . . . . . . . . . . . . . . 60

7.2.5.1 Leak detector theory . . . . . . . . . . . . . . . . . . . 60

7.3 Experimentally not veri�able problem factors . . . . . . . . . 63

7.3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

7.3.2 Desorption, Permeation and Leaks . . . . . . . . . . . . . . . . 63

7.3.2.1 Permeation . . . . . . . . . . . . . . . . . . . . . . . . 63

7.3.3 Mass �ow permeability and gas release rate calculation . . . . . 64

7.3.3.1 Desorption . . . . . . . . . . . . . . . . . . . . . . . . 65

7.3.3.2 Leak rate . . . . . . . . . . . . . . . . . . . . . . . . . 66

7.3.4 Leak rate calculation . . . . . . . . . . . . . . . . . . . . . . . . 67

7.3.5 Venting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

7.1 Problem overview

A big problem that arose during the experiments on the probe, was the establishment ofa stable vacuum. The vacuum itself could be established in a very short amount of time,the actual problem was to keep the vacuum steady while the valve was closed.The idea behind the super insulation can not be realised if the vacuum in the probesmantle is not consistent over a longer period of time. The heat transfer experimentsshowed that with an accurately established vacuum, the probe is able to be operated

Chapter 7. Analysing Problems Dionis Minev

7.2. Possible problem factors veri�cation | 54

Figure 7.1: Test parts

in hot environments. By breaking down di�erent possible problem factors that mightin�uence the vacuum, the problem was narrowed down.

7.2 Possible problem factors veri�cation

7.2.1 Overview

To test di�erent materials and their e�ect on the vacuum, an appropriate set up had tobe assembled. Both MLI and the thread used to �xate the MLI was tested by taking asmall sample, evacuating it and recording the pressure rise with a closed valve. To be sureno leak was in the system, a leak test on the whole probe was conducted. Additionallythe new and old valve was tested and the pressure rise recorded and compared. For theMLI, thread and component tests, the corrugated tube, valve and T-pipe in Fig.7.1 wereused.

NOTE: For the thread tests only the rotary vane pump was used. The turbo molecularpump was sent in for repair works. All other tests were conducted with the new HiCubevacuum pump station.

7.2.2 Components and valves

The vacuum components tested contained the Pfei�er valve (New), old valve, Pfei�ergauge and a T-pipe. Fig.7.3(a) and 7.3(b) display the test set up. In order to test bothvalves and compare the results the same set up was chosen for both valves. In Fig.7.2(a)the comparison of the Pfei�er and old valve can be seen. The pressure rise is very rapidin both valves, however the Pfei�er valve seem to rise quicker. This small di�erence canbe caused by previous evacuations or venting of the system (while switching the valves),changing the initial conditions. In conclusion the pressure rise for both valves is to rapidand similar, for any suspicion of a serious leak in the valves.A more rigorous experiment was conducted with the components evacuating them 4 timesand respectively letting the pressure rise, is depicted in Fig.7.2(b). Test 1 was evacuatedthe longest (24 h) and shows a gradual slop with the lowest pressure. The 'reduced'simply indicates that the experiment was shortened and only the initial pressure rise was



depicted in the �gure. Test 2, 3 and 4 all show an increase in the pressure, despite thefact of repeated evacuation. All three of the tests were performed, by opening the valveventing the system and evacuating it (for ∼5 mins). The pressure rise can be a�liatedto the repeated venting, enabling water vapour in the air to accumulate on the innersurfaces. If the system had a serious leak, it would display itself by having a consistentpressure rise and not the low pressure rise e�ect after long evacuation (as seen in test 1).All of the measurements were conducted with the HiCube vacuum pump.



(a) Comparison between Pei�er and old valve

(b) Components tests with Pfei�er valve

Figure 7.2: Component and valve results



(a) Set up with Pfei�er valve

(b) Set up with old valve

(c) MLI test set up

(d) MLI

Figure 7.3: MLI and components experiments



7.2.3 MLI

Before the probe was assembled the MLI was exposed to air for a long period of time.During that time humidity in the form of water vapour could have stored itself in themany layers of the glass �bre separating the thin metal foils. Because of the very largesurface and the space in between the glass �bre, the MLI presents an extremely goodstorage ability for gas molecules to be accumulated. Once the valve is closed the trappedgas di�usion starts and the pressure inside the vacuum chamber immediately rises. Thiswas evaluated by taking a small piece of MLI and using a smaller vacuum chamber. Theset up can be seen in Fig.7.3(c), with the MLI enfolded around a screw and inserted intothe corrugated tube. The results are displayed in Fig.7.4(a). If the results are comparedto the component tests (shown in Fig.7.4(b), it can be seen that after long evacuationthe MLI has a similar trend to the empty components. One explanation could be thatafter long evacuation the main part of water vapour is removed, leaving behind a similarbehaviour to the empty components (both MLI test 4 and component test 1 are the longevacuation tests). However, short evacuation shows that the MLI has a much steeper

slope in the pressure rise and ends up1

10mbar higher than component test 4.

7.2.4 Thread

When the MLI was �xated around the inner pipe of the probe, the red thread in Fig.7.5(a)was used. For the test it was coiled around a screw that was inserted in the bottom endof the corrugated tube. To verify that the thread was not the cause of the rapid pressurerise two tests with the thread and two without the thread were conducted in a set upshown in Fig.7.5. In comparison, there was no greater di�erence, the pressure rise couldagain be perceived as gas accumulated on the inner surfaces. The results are depictedin Fig.7.6, where it is clear that the thread has very little in�uence on the vacuum. Alltests were conducted with only the rotary vane pump.



(a) MLI tests

(b) MLI and component test comparison

Figure 7.4: MLI and component tests comparison



(a) Thread (b) Set up

Figure 7.5: Thread and set up

7.2.5 Leak test with mass spectrometry

The helium leak detector was used on the original set up by Strubel [11]. After initial testsfailed and the the undesirable pressure rise was recorded a leak test was performed, tore-evaluate the seal tightness. Fig.7.7 shows the set up. The leak detector was connectedto the Pfei�er valve and established a vacuum in the probe, after that helium was sprayedaround the volatile areas suspected of leaks. Here two di�erent tests were performed, thelocal leak test while the probe had an established vacuum and the integral leak test wherea plastic sheet covered the probe and helium was sprayed into it. The integral leak testsums up all leaks, due to the large amount of helium trapped under the plastic cover.The helium then penetrates small openings and gets into the vacuum chamber, beforebeing evacuated through the leak detector and recognised with the mass spectrometer.The results stated that the probe had no major leak with the same leakage rate measuredafter production processes of the probe were completed [13]. After the assumption of amajor leak was proven to be false, a second 'virtual leak' theory was introduced. Which isthe combination of the water vapour desorption and the small leak1 that is inevitable inany manufacture device. In subsection 7.3.4, various leak rates with di�erent scenariosare calculated.

7.2.5.1 Leak detector theory

For the leak tests performed on the probe, an oerlikon leybold vacuum L200 leak detectorwas used. This leak detector consists of two elements, the mass spectrometer and thevacuum pump system. By establishing a high vacuum previous to the helium injection,the detector assures that no high mass �ows of particles in�uence the measurements.The mass spectrometer operates on the principle of a sector �eld spectrometer [13] [14].According to [17], natural gas molecules are ionised through an ion source with electronbombardment. These electrons are accelerated in the magnetic sector �eld by an electricalvoltage. The magnetic �eld is homogeneous in the area of the trajectories of the ions andis positioned perpendicular to the image plane. Helium ions with the mass of 4 can pass

1There is no completely sealed manufacturing or material available



(a) Tests without thread

(b) Tests with thread

Figure 7.6: Thread experiments



(a) Helium leak test on original set up (b) Vacuum connection test

(c) Helium leak detector (d) Helium gas injector

Figure 7.7: Leak detection


7.3. Experimentally not veri�able problem factors | 63

Figure 7.8: Sector �eld spectrometry [17]

the slot and reach the detector. Every other molecule is not able to pass the slot and issubsequently re-neutralised. The ion current measured for helium is proportional to thehelium partial pressure. Fig.7.8 displays the principle.

7.3 Experimentally not veri�able problem factors

7.3.1 Overview

Every test conducted to narrow down the problem failed to isolate the problem factorand target one particular element in the system that causes the rapid pressure rise. Inconclusion, after speaking to many di�erent vacuum experts, the main source of theproblem seems to be in the accumulation of water and other substance on the innersurfaces of the vacuum chamber. Pressure and temperature are linked to one anotherin a 2-phase-system (meaning water vapour, where gas and liquid co-exist) [8]. If thepressure drops and the Temperature rises, water will vaporise. Fig.7.9 shows the linkbetween pressure and temperature in a p,T diagram, the area of interest for the probe isin the red frame.

7.3.2 Desorption, Permeation and Leaks

7.3.2.1 Permeation

If the outer surface of the vacuum chamber is exposed to air and normal atmosphericpressure, while the inside is exposed to a vacuum, permeation can occur. Permeationfeatures three steps and is depicted in Fig.7.10. According to [5], this process usuallyoccurs in vacuum systems with pressures below 10−8 mbar, which makes it irrelevant forthe probe. However, a theoretical calculation with the permeability of the materials inthe probe is calculated below. [15]

• Adsorption of a molecule to the outer surface (atmospheric pressure side)

• Di�usion through bulk material

• Desorption from the interior surface (Vacuum pressure side)



Figure 7.9: p,T-Diagram [24]

7.3.3 Mass �ow permeability and gas release rate calculation

The mass �ow with a certain permeability is de�ned by the equation [5].

qm = qPerm (p1 − p2)A

d

with p1 = 1 bar = 105 N

m2= 0, 1

N

mm2,

d = 2mm,

A = 1000mm ∗ π ∗ 110mm = 3, 5 ∗ 105 mm2 = 0, 35m2,

p2 = 1 ∗ 10−7 bar = 10−2 N

m2= 10−8 N

mm2

and qPerm (N2 / Fe up to 200 ◦C) ≈ 3 ∗ 10−8 mbar l

s

mm

m2bar

For the inner pipe:

qm−I = 3 ∗ 10−8 ∗ (1− 1 ∗ 10−7) ∗ 0, 352

mbar l

s

mm

m2barbar

m2

mm=⇒ qm−I = 5 ∗ 10−9 mbar l

s

For the outer pipe:



Figure 7.10: Permeation [5]

qm−O = 3 ∗ 10−8 ∗ 0, 3514∗ (1− 10−7)

mbar l

s=⇒ qm−O = 0, 75 ∗ 10−9 mbar l

s

Sum of both:

qm−Σ = 5, 75 ∗ 10−9 mbar l

s

According to [5], the gas release rate after 10 hours of evacuation for metals is

∼ 10−9 mbar l

s cm2.

Not only the inner and outer pipe have to be considered, but also the metal foil inthe MLI. The MLI consists of 20 metal foils that have both a top and bottom side. Ad-ditionally, the two surfaces of the outer and inner pipe are included, making it a total of42 surfaces.

AΣ = 42∗0, 35m2 = 14, 7m2 = 1, 47∗105 cm2 =⇒ qGasrelease = 10−9 ∗1, 47∗105 mbar l

s=

1, 47 ∗ 10−4 mbar l

s

The result for the gas release rate in the metal is very similar to the leakage rate

calculated in subsection 7.3.4 qpV = 4, 92 ∗ 10−4 mbar l

s. Although [5] states that after

100 hours of evacuation time, this rate should be ∼ 10−10 mbar l

s. The pumping cycles

in numerous past experiments exceeded well over 100 hours making this statement alsovery vague.



Figure 7.11: (a) Leak rate only, (b) Water vapour desorption only, (c) Both leakageand desorption [5]

7.3.3.1 Desorption

Water vapour desorption from the inner surfaces is very noticeable below 0,1 mbar. Theprobes mantle is ideally evacuated in a pressure range of 10−3 − 10−4 mbar and hastherefore noticeable desorption. The diagram shown in Fig.7.11, has 3 trends all of themrepresenting di�erent pressure rise scenarios. The most likely and comparable trend is(b), it resembles the recorded pressure rises. It is very likely that the rapid pressure riseis caused by water vapour trapped in the vacuum chamber and due to the large surfaceof the MLI, removing it is very di�cult.

7.3.3.2 Leak rate

No vacuum system is completely sealed. So in order to calculate the pressure rise caused

by a leak, the pressure change over time has to be considered.dp

dtin a sealed volume V.

The increase in pressure caused by a leak is constant and can be expressed as the in�uxof gas over time. [5]

qpV (t) = V4p4t

= constant

Although (a) in Fig.7.11 and the leak detector test, show that the pressure rise in oursystem is not caused by a major leak in the probes vacuum chamber, the equation forthe leak rate is a linearisation of the �rst pressure rise in the chamber. [23] categorisesvacuum chambers according to how sealed they are.

• System very leakproof: qpV < 10−6mbar l

s

• System su�ciently leakproof: qpV < 10−5mbar l

s



• System not leakproof: qpV > 10−5mbar l

s

[7] on the other hand states, that a vacuum insulation concept should have a leak rate

qpV between 10−7 and 10−9 mbar l

s.

7.3.4 Leak rate calculation

First an experiment was conducted, where the pressure rise was recorded for a short pe-riod of time. The leak rate could then be calculated through the experimental data.

Fig.7.12(a). The data can be used to calculate the leak rate accordingly:

qpV =6 l ∗ (9, 8− 1, 6) ∗ 10−3 mbar

100 s=⇒ qpV (t) = 4.92 ∗ 10−4 mbar l

s=⇒ The System

is not leakproof.

Additionally, more data was recorded with 8 pressure rises from 10−3 mbar, shown inFig.7.12(b). The mean time it took for the rise was 11, 74 s

qpV =6 l ∗ 10−3 mbar

11, 74 s=⇒ qpV = 5, 11 ∗ 10−4 mbar l

s

=⇒ Again not leakproof.

If the pressure rise from 1 ∗ 10−3 → 2 ∗ 10−3 mbar over a period of one year (= 365 d ∗24 h ∗ 3600 s = 3, 153 ∗ 107 s) was to happen:

qpV would have to be qpV =6 l ∗ 10−3 mbar

3, 153 ∗ 107 s=⇒ qpV = 1, 902 ∗ 10−10

=⇒ Extremely leakproof. Which brings up the question, if this is only possible withadditional pumping elements inside. Like a getter.

If we consider that we have a very leakproof system (qpV = 10−6mbar l

s) and we are

not evacuating the probe over a whole year. With a starting pressure of 10−3 mbar Theend pressure would be accordingly:

p2 = p1 + qpV ∗t

V=⇒ p2 = 10−3 +

10−6 ∗ 3, 153 ∗ 107

6mbar = 5, 256mbar

=⇒ Making the vacuum insulation useless.

If we again consider that we have a very leakproof system, this time according to [7]

with qpV = 10−9mbar l

sand we allow a pressure rise of 1 ∗ 10−3 mbar. The time for

re-evacuation would be accordingly:



t = 4p VqpV

=⇒ t = 10−3 mbar6 l

10−9mbar l

s

= 6000000 s = 1666, 7 h = 69, 45 d

A system with qpV = 10−6mbar l

s, would come to a result of t = 1.67 h for re-evacuation.

7.3.5 Venting

According to [20], to prevent desorption of water vapour and undesired contamination inthe vacuum chamber, it should only be vented with dry nitrogen. The probe was onlyvented with air, which allowed water vapour to accumulate on the vacuum chamber walls.



(a) Pressure rise

(b) 8 Time tests

Figure 7.12: Pressure rise tests for the leak rate



Chapter 8

Alternative solutions

Contents

8.1 Solution overview . . . . . . . . . . . . . . . . . . . . . . . . . . 70

8.2 Getter material . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

8.2.1 Getter activation instructions . . . . . . . . . . . . . . . . . . . 70

8.2.2 Getter test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

8.3 Re-evacuation valve . . . . . . . . . . . . . . . . . . . . . . . . . 73

8.3.1 CAD construction . . . . . . . . . . . . . . . . . . . . . . . . . 73

8.1 Solution overview

The main problem that was encountered is the extremely rapid pressure rise once thevacuum seems to be established and the valve is closed. Two possible solutions couldbe an alternative valve that allows re-evacuation and the use of getter materials in theprobes mantle. A getter could increase the operational time with an acceptable vacuumpressure, while the valve would allow easy re-evacuation without permanently sealing thevacuum connection.

8.2 Getter material

8.2.1 Getter activation instructions

The getter material used is a ALVATUBEGETTER by Alvatec. According to the man-ufacturer, the activation should be performed below a vacuum level of 10−4 mbar, whichcan easily be provided. The components should only be touched with latex gloves. Fig.8.1displays the manufacturers description of an ideal activation, with a pressure �uctuationduring the heating process.

Chapter 8. Alternative solutions Dionis Minev

8.2. Getter material | 71

Figure 8.1: Getter activation description

8.2.2 Getter test

The getter material was placed in a pipe and �xated with a �exible piece of metal, asseen in Fig.8.2. The vacuum pump was turned on and a vacuum was established. Thegetter was then heated up with a heat gun to around 200 ◦C for 1 hour, during whichthe vacuum pump remained in operation. After the su�ciently long heating process thegetter material was activated. The activation �uctuation according to the manufacturercan be seen in Fig.8.3 in the �rst 60 minutes. The heat gun was then turned o� and thevalve closed. The getter was left for ∼4200 mins and the pressure rise was recorded, asseen in Fig.8.3. In Fig.8.2(b) the activated getter material can be seen, the protectivemetal coating was melted o� during the activation process.The test outcome was a stair like trend. One possible explanation for the trend couldbe the in�uence from the outside temperature. The test was started at 16:00 in theafternoon, with the �rst pressure rise beginning ∼24 h after. The same phenomena canbe seen after ∼48 h. The temperature di�erence during day and night was ranging duringthe measurements at ∼20 ◦C. This change in temperature could have caused water on theinner surfaces to vaporise during the higher temperatures, surpassing the getter materialssuction capabilities and increasing the pressure. While during lower temperature periodsthe vapour condensates and the getter material keeps the pressure steady again. Inconclusion the use of a getter material could increase the operational time of the probesigni�cantly. Compared to all previous tests with closed valves, the getter test showedthe longest period of time (∼ 1000 mins) with a pressure ranging between 10−3 − 10−4

mbar.


8.2. Getter material | 72

(a) Getter test set up (b) Activated Getter material

(c) Heating up the getter material

Figure 8.2: Getter test set up

Figure 8.3: Getter test


8.3. Re-evacuation valve | 73

8.3 Re-evacuation valve

8.3.1 CAD construction

The construction of an alternative evacuation valve allows the probe to be easily re-evacuated. When the test results are taken into consideration, it is clear that the probessuper insulation will not work with closed valve and no constant suction from the pumps.If however, the vacuum can be established for a certain amount of time until the pressure isabove the operational limit, a re-evacuation would lower the vacuum pressure again. Hereeasy access to the vacuum chamber is necessary, with no standing out geometries that willinhibit the probe to be lowered into the borehole. The easiest solution was to place the re-evacuation valve on the inside of the probe. In Fig.8.4(b) the inner pipe of the probe withthe re-evacuation valve can be seen. The construction consists of 8 separate elements, ofwhich 3 will be dismounted after the evacuation process is completed. Fig.8.4(f) showsthe components that are dismounted after the re-evacuation is complete. Fig.8.4(d) showsthe Pfei�er plans for the gate valve. Although, the integration of a re-evacuation valvefalsi�es the initial concept of a one time evacuation that would last a long time. Theconcept seems to be the best solution, compared to others. If a sensor was placed onthe inside, stating the real time pressure. It would too, take up a considerable amountof space, without the bene�t of a easy re-evacuation. The acceptance of a re-evacuationvalve might destroy the initial thought model of a 'vacuum �ask'1, but on the other hand,we don't know how well a �ask really works. Given these circumstances a valve on theinside might increase the di�culty of de- and assembling the electronic equipment insidethe probe, however it poses the most bene�ts for the success of the probes insulationconcept.

1These devices also hold the vacuum trapped for a long time


8.3. Re-evacuation valve | 74

(a) Side view (b) Complete side view

(c) Gate valve (d) Pfei�er gate valve plan

(e) Inner pipe (f) Evacuation set

(g) Clamping ring 1 (h) Clamping ring 2

Figure 8.4: CAD constructionChapter 8. Alternative solutions Dionis Minev


Chapter 9

Final conclusion and outlook

Contents

9.1 Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . 75

9.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

9.1 Summary and Conclusion

The initial problem task was to evaluate the simulation algorithm proposed by Strubel[11]. However, during the experimental set up the vacuum seemed to be composed ofa much larger problem. The focus was then shifted on the establishment of a stablevacuum. This task could not be completed, but the main problem factor were pinpointedand a number of alternative solutions were presented. Due to the shift of priorities theexperimental evaluation of a phase change material in the probe could not be assessed.Therefore no comparison between the simulation by Strubel and the experimental datawas done. It was still visible that the simulations initial proposal of 11 hours operationaltime was surpassed in the practical experiments without the use of a PCM and withcontinuously running vacuum pumps. The tests showed that an accurate super insulationwith a low enough pressure, meets all requirements. In conclusion this thesis focuses onthe establishment of a working vacuum with all possible problem factors veri�ed andthoroughly described. The completion of a vacuum seems as a very trivial problem atthe start but should not be underestimated, with many factors in�uencing the outcome.Although after narrowing down on di�erent factors, the most plausible cause for thepressure rise seems to be water vapour desorption inside the vacuum chamber. This isof course only a theoretical model, that will have to be veri�ed by further experimentalevaluation.

Chapter 9. Final conclusion and outlook Dionis Minev

9.2. Outlook | 76

9.2 Outlook

For the future outlook the probe concept has to be revised. A key point here is the superinsulations concept. For this a number of tasks have to be completed.Before further experiments are run, these steps should be completed.

1. Repeating the helium leak detection test on the heated up probe.

2. Integration of a sensor for lower pressure ranges1.

3. Installation of a nitrogen venting system.

4. Additional insulation for the probes ends in axial direction.

For a newly constructed probe prototype, following steps should be considered.

1. Thorough assortment of the materials used for the probe (possibly separating thepressure pipe from the vacuum chamber).

2. Careful surface treatment of the vacuum components.

3. Integration of getter materials.

4. Lose installation of the MLI instead of tightly packaging it on the inner pipe.

1below 5 ∗ 10−4 mbar



Appendix


9.2. Outlook | 78



List of Figures

1.1 System components for the geothermal probe kit [3] . . . . . . . . . . . . 3

2.1 Construction and welding plans by Cédric Strubel [11] . . . . . . . . . . 52.2 CAD assembled probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.3 Heat transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.4 MLI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1 Assembled Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.2 CAD view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.3 Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.1 Test set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.2 Thermocouple operating principle [9] . . . . . . . . . . . . . . . . . . . . 154.3 Temperature measuring equipment . . . . . . . . . . . . . . . . . . . . . 164.4 PicoLog recorder Software (German) . . . . . . . . . . . . . . . . . . . . 174.5 Thermocouple reaction tests . . . . . . . . . . . . . . . . . . . . . . . . . 184.6 Thermocouple Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.7 Vacuum pump arrangement . . . . . . . . . . . . . . . . . . . . . . . . . 204.8 Viscous and molecular gas �ow [16] . . . . . . . . . . . . . . . . . . . . . 224.9 Used pumps and working principles . . . . . . . . . . . . . . . . . . . . . 234.10 Turbo molecular pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . 244.11 Heating equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254.12 Pirani sensor theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.13 Pfei�er Pirani sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

5.1 Original set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315.2 Vacuum connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325.3 Probe alteration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325.4 Current set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335.5 Heating jacket with probe . . . . . . . . . . . . . . . . . . . . . . . . . . 345.6 Temperature measuring with heating jacket . . . . . . . . . . . . . . . . 345.7 Thermocouple set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

6.1 Cold vacuum pressure drop . . . . . . . . . . . . . . . . . . . . . . . . . 386.2 Tests 3,5 and 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396.3 Cold vacuum pressure rise and long evacuation . . . . . . . . . . . . . . . 406.4 Hot vacuum pressure drop . . . . . . . . . . . . . . . . . . . . . . . . . . 426.5 Hot vacuum pressure drop . . . . . . . . . . . . . . . . . . . . . . . . . . 43

List of Figures Dionis Minev

List of Figures | 80

6.6 Time di�erence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476.7 Two scenarios for the probe cooling calculation . . . . . . . . . . . . . . . 486.8 ∼200 ◦C Heat transfer tests . . . . . . . . . . . . . . . . . . . . . . . . . 496.9 ∼ 72 h at ∼200 ◦C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 506.10 Pressure recording during 72 h heat transfer experiment . . . . . . . . . . 506.11 ∼ 72 h at ∼165 ◦C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 516.12 Comparison between practical and theoretical diagram . . . . . . . . . . 52

7.1 Test parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 547.2 Component and valve results . . . . . . . . . . . . . . . . . . . . . . . . . 567.3 MLI and components experiments . . . . . . . . . . . . . . . . . . . . . . 577.4 MLI and component tests comparison . . . . . . . . . . . . . . . . . . . . 597.5 Thread and set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 607.6 Thread experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 617.7 Leak detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 627.8 Sector �eld spectrometry [17] . . . . . . . . . . . . . . . . . . . . . . . . 637.9 p,T-Diagram [24] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 647.10 Permeation [5] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 657.11 (a) Leak rate only, (b) Water vapour desorption only, (c) Both leakage and

desorption [5] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 667.12 Pressure rise tests for the leak rate . . . . . . . . . . . . . . . . . . . . . 68

8.1 Getter activation description . . . . . . . . . . . . . . . . . . . . . . . . . 718.2 Getter test set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 728.3 Getter test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 728.4 CAD construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

List of Figures Dionis Minev


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