temperature measurement in high temperature mechanical...

High Temperature Mechanical Testing Committee

NPL Thermal Measurement Awareness Network

UKAS

Temperature Measurement in High Temperature Mechanical Testing

Wednesday 15th March 2006

Programme

09:00 Registration and coffee 09:55 Welcome - Peter Quested, NPL 10:00 The requirements of temperature measurement:

Fatigue, tensile, thermomechanical, limitations of temperature measurement, uncertainties of measurement Phil G Jones, ALSTOM Power Paul R McCarthy, Paul McCarthy Consulting

10.30 Meeting the requirements of temperature measurement – The role of UKAS Phil Endersby, UKAS

10:45 Thermocouples: How they work : Best practice : Are present uncertainties acceptable ?David Head, NPL

11:00 Practical considerations using thermocouples for measuring high temperatures during long duration mechanical testing Malcolm Loveday / Peter Quested, NPL

11:35 Discussion 12:00 Lunch 13:00 Radiation thermometry

Helen McEvoy, NPL 13:30 Recent developments in industrial non-contact thermometry at NPL, including

Laser Absorption Radiation Thermometry (LART) Andrew Levick, NPL

13:50 Dynamic temperature measurement and use of ribbon thermocouples Peter Haehner, DG-JRC

14:30 Comparison: Thermocouples and optical pyrometers Matt Brooks, NPL

14:50 Concluding discussion

15:10 Laboratory Tour

Temperature Measurement in High Temperature Mechanical Testing

Wednesday 15th March 2006 Abstracts

The requirements of temperature measurement Phil G Jones - ALSTOM Power

Paul R McCarthy - Paul McCarthy Consulting The need for accurate, reliable temperature measurement is key to the successful testing of materials at elevated temperatures. Currently, the bulk of temperature measurement is performed using thermocouples complemented, in limited cases, by pyrometry and PRT’s (Platinum Resistance Thermometers). This paper reviews the physical and operational characteristics of the various sensor types for isothermal and dynamic applications. It examines a range of inherent properties and considers the interactions between measurement sensors and the associated elements of the measurement system / electronics. The requirements of a number of testing procedures / standards are reviewed, producing a set of “ground rules” within which the temperature measurement sensors need to operate. The ability of the various sensors to meet these requirements is explored and some conclusions drawn as to suitability for use in isothermal or dynamic applications. The paper concludes with an assessment of the application of uncertainties of measurement of sensors and measurement systems to the permitted temperature ranges within national / international standards, technical consensus documents and codes of practice for elevated temperature testing.

Meeting the requirements of temperature measurement – the role of UKAS

Phil Endersby - Assessment Manager, UKAS UKAS assesses the competence of all aspects of high temperature mechanical testing, including appropriate calibration and use of temperature measuring equipment. This paper addresses some of the factors which need to be considered when specifying requirements for internal or external calibration of temperature measuring equipment. This is illustrated for the calibration of a base metal thermocouple. The schedule of accreditation of a specialist temperature calibration laboratory is used to illustrate how the characteristics of PRTs, noble metal thermocouples and base metal thermocouples affect the uncertainty of measurement that can be achieved in practice. Consideration is then given to the process of UKAS’ assessment of internal calibration of temperature measuring systems, to confirm that they are fit for purpose.

1

Thermocouples David Head - NPL

An introduction into how thermocouples work and the actual signal source will be given. Some example uncertainty budgets will be reviewed. There will be a discussion on how a lab comparison calibrates thermocouples. The presentation will conclude with some thoughts on how to set up "industrial" measurements to avoid some obvious pitfalls.

Practical considerations using Thermocouples for measuring high temperatures during long duration mechanical testing

Malcolm Loveday, Bryan Roebuck, Peter Quested - NPL The measurement of temperature during long duration mechanical tests is a key parameter and can contribute significant uncertainties in the measurements with consequences designing with the data, The thermocouple is the most frequently used temperature sensor because it is relatively cheap and small and if a few simple precautions are taken it is capable of achieving the required uncertainties of measurement. A list of factors, which need to be considered. For the design of the assembly:

• Wire types (use of type N and noble metal thermocouples). • Wire diameter (thin wires are more prone to surface reactions but thicker

wires act as heat shunts). • Sheath materials must be stable. (danger in reducing atmospheres of

reducing some oxide ceramics). Or operating conditions:

• Temperature range, which effects choice of wire materials. • Time. The longer the duration of the test, the more likely to degrade. • Environmental gas. (In oxidising environments Rh can migrate in noble

metal thermocouples and under reducing conditions need stable sheath materials).

• Pressure levels. (special interface design. Pressure has little effect on output of thermocouples).

• Thermal cycling • Ionising radiation (transmutations of material change the thermoelectric

voltage characteristic). • Strong electromagnetic alternating fields. (induction heating). • Strong inhomogeneous magnetic fields along thermoelectric wires can

influence thermoelectric voltages. Despite its simplicity the thermocouple is a precision instrument which needs to be handled with care. It has been recognised for many years that the characteristics of the wire can be changed by subjecting it to mechanical work and this is the reason that the wires are annealed at high temperatures. Therefore care needs to be taken minimising the bending of the wires. Exit wires from the furnace with the associated high temperature gradients means that any damage in this area has a disproportionate effect on the output voltages. Good practice is not only to calibrate the thermocouple but also examine it for inhomogeneities by for example making sure that in the calibration furnace the depth of immersion of the

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thermocouples is identical to the testing set up or passing a temperature spike down the assembly maintaining both junctions at 0 °C monitoring any changes in the voltages.

Radiation thermometry Helen McEvoy - NPL

Radiation thermometry, or ‘pyrometry’, is the technique whereby the temperature of a surface is determined by measuring the amount of thermal energy or radiance emitted by that surface. Measurements are made using a detector-based instrument, situated remotely from the surface. The technique is non-contact, meaning that it can be used to measure temperatures in situations where a contact probe would be unsuitable or impractical; for example, if the target is very hot, moving, in an hostile environment or where contamination of a product needs to be avoided. Also, thermal imaging cameras allow many points over a large area to be measured simultaneously. However, care is required when using the method and there are issues that need to be taken into consideration to prevent large errors in the temperature reading; for example, there must be a clear line-of-sight between the target and the thermometer and the emissivity of the surface needs to be known or understood. The talk will give the principles behind radiation thermometry, describe the construction of a typical radiation thermometer and discuss some of the calibration issues, applications and advantages of using the technique. It will also cover the disadvantages and areas of potential error, and give examples of how these errors could be minimised.

Recent developments in industrial non-contact thermometry at NPL including Laser Absorption Radiation Thermometry (LART)

Andrew Levick - NPL Temperature is the most frequently measured parameter in process engineering, and affects the quality of the product and energy consumption of the process. Radiation pyrometry is a well-established non-contact thermometry technique used in a wide range of industrial processes. A radiation pyrometer collects and detects thermal radiation emitted from the target over a narrow band of wavelengths, and infers the temperature via the Planck relationship. However, the radiation pyrometry is prone to systematic errors if the emissivity of the target is uncertain or there is reflected background radiation present. This talk will describe recent developments in non-contact thermometry that are potentially immune to emissivity and background radiation, focusing on two methods:

The Laser Absorption Radiation Thermometry (LART) method is based on photothermal radiometry, and involves the detection of modulated thermal radiances from the target irradiated by modulated high power laser beams. The Radiation thermometry in High Ambient Temperatures (RTHAT) method involves separate direct measurements of the background and target radiances. A least squares analysis is utilised to obtain the surface emissivity

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and temperature. The technique relies on the extra information provided by fluctuating components in the background radiation that can be correlated with the reflected radiation from the target itself.

We will discuss the pros and cons of each method and possible industrial applications.

Dynamic Temperature Measurement and Use of Ribbon Thermocouples

Peter Haehner - DG-JRC

Comparison: Thermocouples and optical pyrometers Matt Brooks - NPL

Accurate temperature measurements via pyrometry require the characterisation of emissivity as a function of temperature for a given detection wavelength. Surface emissivity for a given material can be affected by dynamic surface conditions such as roughness, oxide layer and contamination. Such variations can be the cause of significant error in the temperature measured by a pyrometer. A small scale mechanical test system, the ETMT, has been used at NPL to investigate the effect of the time dependent oxide scaling process on the temperature measured by a two colour pyrometer when controlled by thermocouple. The time dependent variation of indicated pyrometer temperature was found in some instances be as large as 100°C when compared to the temperature of the controlling thermocouple.

4

March 2006March 2006

The Practicalities of Temperature Measurement

Mr Philip JonesMr Paul McCarthy

Date of last change Reference/Name of Presentation/SN 2

The Practicalities Of Temperature MeasurementHTMTC & TMAN, NPL, March 2006

Throughout the materials testing world, the need for accurate, reliable temperature measurement is a key component to successful testing of materials at elevated temperatures.

TensileCreepRuptureStress relaxationLow & high cycle fatigue

High Temperature Mechanical TestingHigh Temperature Mechanical Testing



RobustAccurate and easy to useFast responseCheapHeating equipment compatibilityAge insensitive

The Ideal Temperature Sensor



Thermocouples–Base–Noble

Pyrometers

Platinum Resistance Thermometers (PRT’s)

Other Novel Methods

The Traditional OptionsThe Traditional Options



GoodSpecial needs

GoodOKHeating compatibility

HighHighHighCheapPrice

OKGoodGoodGoodSpeed of response

GoodOK to Good

GoodOKAccuracy

PoorSpecial needs

GoodGoodHandling & Installation

PRTPyroNobleBase

Comparative PerformanceComparative Performance



T/C’s – well developed, extensive experience

Pyrometers – unusual, unless surface contact / contamination issues

PRT’s – rare, due to mounting issues. Excellent for long term, very stable sensor (v. low drift)

Static ApplicationsStatic Applications



T/C’s – attachment issues, also thermal mass and r.f. field considerationsPyrometers – non-contacting , able to access areas difficult for t/c’s, emissivity changes can be problematicPRT’s – thermal mass and contact issues

All are critically dependent upon the associated electronics

Dynamic ApplicationsDynamic Applications



T/C’s– bead size & wire path– strip t/c’s– r.f. interference

Pyrometers– Changes in surface emissivity ( pre-oxidise to

minimise impact)– Electronics - immunity to r.f. interference

RF Heating ConsiderationsRF Heating Considerations



The Critical Issue :- sampling rate vs. accuracy / discrimination

– Greater accuracy requires longer integration times

– For high rate testing this controls temperature uncertainty

• e.g. a test at 50ºC/s, reading at 8 integration's per sec, gives 6.25ºC “steps”

• More integration's possible, but lower discrimination.

Dynamic Testing Dynamic Testing -- ElectronicsElectronics



But we usually test to recognised standard

What do the testing standards call for?

– Test temperature accuracy

– Measuring system capability

– Calibration Frequency

– Test temperature stability

Complying with StandardsComplying with Standards



Document number Subject

EN10002-5 : 1992Tensile testing of metallic materials - Part 5: Method of test at elevated temperatures.

ASTM E 21 – 03aStandard test methods for Elevated Temperature Tension Tests of Metallic Materials

BS EN 10291 : 2000 Metallic materials – Uniaxial creep testing in tension - Methods of test

ASTM E 139 - 00є1Standard test methods for Conducting Creep, Creep-Rupture, and Stress- Rupture Tests of Metallic Materials

BS EN 10319-1 : 2003Metallic materials – Tensile stress relaxation testing - Part 1: Procedure for testing machines

ASTM E 328 - 02 Standard Test Methods for Stress Relaxation for Materials and StructuresECCC Recommendations - Volume 3 Part 1

Data acceptability criteria and data generation - Generic recommendations for creep, creep rupture, stress rupture and stress relaxation data.

ASTM E 606 – 92 (Re-approved 2004) Standard Practice for Strain-Controlled Fatigue TestingBS 3518 part 1 1993 Fatigue testing - Guide to general principles

BS 7270 : 1990 Constant amplitude strain controlled fatigue testing - Method for

Review of Testing StandardsReview of Testing Standards



Test temperature accuracy and deviations.

Thermal monitoring sensors at the specimen surface.

– Temperature sensor attachment good practice.

Measuring equipment accuracy, resolution and stability.

Calibration of the complete monitoring system.

Taking account of uncertainties within the whole system.

Items of Key ImportanceItems of Key Importance



Test type Procedure Limit type < 600 °C

Max specimen gradient <

600°C 601 – 800

°C

Max specimen

gradient 601 – 800 °C

801 – 1000 °C

Max specimen


1001 – 1100 °C

1101 – 1200 °C

1201 – 1350 °C

Max specimen gradient >

1000 °C

ET Tensile EN10002:5 Deviations +/- 3Not

specified +/- 4 Not specified +/- 5Not

specifiedNot

specified

Creep EN 10291 Deviations +/- 3 3 +/- 4 4 +/- 5 5By

agreement

Creep / RuptureASTM E139 Deviations +/- 2

Not specified +/- 2 Not specified +/- 2

Not specified +/- 3 +/- 3 +/- 3

Not specified

Stress Relax EN 10319 Deviations +/- 3 2 +/- 4 3 +/- 5 3By

agreement

Creep & Rupture ECCCMax total deviation +/- 3 +/- 3 +/- 4 +/- 4 +/- 5 +/- 5 +/- 6 +/- 7 +/- 8

Not specified

Uniaxial Stress Relax ECCC

Max total deviation +/- 3 +/- 3 +/- 4 +/- 4 +/- 5 +/- 5

Not specified

Strain-Controlled Fatigue

ASTM E606 – 92 Reapp 2004 Deviations

+/- 2 or reported if greater

+/- 2 or 1% if greater





Not specified

Fatigue testingBS 3518 pt1 Deviations +/- 2 2 +/- 2 2 +/- 2 2 +/- 2 +/- 2 +/- 2 2


By agreement

By agreement

By agreement

Not specified

This table indicates a number of common accuracy and deviation levels

Test temperature accuracy and deviations



Test type Procedure Limit type < 600 °C

Max specimen gradient <

600°C 601 – 800

°C

Max specimen


801 – 1000 °C

Max specimen


1001 – 1100 °C

1101 – 1200 °C

1201 – 1350 °C

Max specimen gradient >

1000 °C

ET Tensile EN10002:5 Deviations +/- 3Not

specified +/- 4 Not specified +/- 5Not

specifiedNot

specified

Creep EN 10291 Deviations +/- 3 3 +/- 4 4 +/- 5 5By

agreement

Creep / RuptureASTM E139 Deviations +/- 2

Not specified +/- 2 Not specified +/- 2

Not specified +/- 3 +/- 3 +/- 3

Not specified

Stress Relax EN 10319 Deviations +/- 3 2 +/- 4 3 +/- 5 3By

agreement

Creep & Rupture ECCCMax total deviation +/- 3 +/- 3 +/- 4 +/- 4 +/- 5 +/- 5 +/- 6 +/- 7 +/- 8

Not specified

Uniaxial Stress Relax ECCC

Max total deviation +/- 3 +/- 3 +/- 4 +/- 4 +/- 5 +/- 5

Not specified


ASTM E606 – 92 Reapp 2004 Deviations







Not specified

Fatigue testingBS 3518 pt1 Deviations +/- 2 2 +/- 2 2 +/- 2 2 +/- 2 +/- 2 +/- 2 2


By agreement

By agreement

By agreement

Not specified

Conclusion - for both accuracy and gradient requirements, there are typically two levels specified, the most common (3,4,5°C) or a tighter level.




Testing laboratory applied control levels, distilled from the various standards

– Temperature deviation limit <600°C: +/- 3°C– Gradient along specimen at <600°C: +/- 2°C

– Temperature deviation limit 601-800°C: +/- 4°C– Gradient along specimen at 601-800°C: +/- 2°C

– Temperature deviation limit 801-1200°C: +/- 5°C– Gradient along specimen at 801-1200°C: +/- 3°C










Thermal monitoring sensors at the specimen surface.Thermal monitoring sensors at the specimen surface.



Test type ProcedurePosition for GL < 50mm

Position for GL > 50mm Attachment to specimen

Shielding of thermocouple

T/C wire hot zone insulation

Elevated Temperature Tensile EN10002:5

2 off - 1 near each end.

NOTE: 3 off – arranged at identical intervals along the parallel length.

Measured at the surface of the parallel length. NOTE: T/C junctions should make thermal contact with the surface.

NOTE: suitably screened from direct radiation Not specified

Creep ASTM E21 – 03a2 off - 1 near each end.

3 off - 2 near each end and 1 near the centre. Intimate contact Shielded from radiation Ceramic insulators

Creep EN 102912 off - each end of the parallel length.

3 off - each end of the parallel length third in the middle region.

T/C junctions should make thermal contact with the surface.

screened from direct radiation

Thermally shielded and electrically insulated

Creep ASTM E139 - 002 off - 1 near each end.


Stress Relax EN 103192 off at each end of the parallel length.

3 off - each end of the parallel length third in the middle region.

T/C junctions should make thermal contact with the surface.

screened from direct radiation

Thermally shielded and electrically insulated

Stress Relax ASTM E 328 - 022 off - 1 near each end.


Creep / Rupture ECCC Not specified Not specified Not specified Not specified Not specifiedStress Relax ECCC Not specified Not specified Not specified Not specified Not specifiedStrain-Controlled Fatigue

ASTM E606 – 92 Reapp 2004 Not specified Not specified Not specified Not specified Not specified

Fatigue BS 3518 pt1 Not specified Not specified Not specified Not specified Not specified

Conclusion - with reference to gradient, large samples are penalised.Size matters to the customer.How do you achieve intimate contact? Large samples are fine, small are difficult!

Thermal monitoring sensors at the specimen surface



– Specimen of < 50mm gauge length– 2 sensors positioned near either end

of G.L.– Specimen of > 50mm gauge length– 3 sensors positioned one near either

end of G.L. plus one at near centre.

– Ensure good thermal contact of sensors with specimen.

– Shield sensors from direct radiant heat.

Thermal monitoring sensors at the specimen surface









Measuring equipment accuracy, resolution and stability.Measuring equipment accuracy, resolution and stability.



Test type Procedure Resolution minimum (sensitivity) °CAccuracy

(Precision) °CSurrounding air temperature

max variation °C

ET Tensile EN10002:5 1 +/- 2 Part 1:

CreepASTM E21 –03a

Sufficiently sensitive and reliable to ensure that the temperature of the

specimen is within the limits specified. 6

Creep EN 10291 0.5 +/- 1 +/- 3

CreepASTM E139 -00

Sufficiently sensitive and reliable to ensure that the temperature of the

specimen is within the limits specified. +/- 3

Stress Relax EN 10319 0.5 +/- 1 +/- 3

Creep / Rupture ECCC Not specified Not specified +/- 3

Interrupted Creep / Rupture ECCC Not specified Not specified +/- 2

Uniaxial Stress Relax ECCC Not specified Not specified +/- 3


ASTM E606 –92 (2004) Not specified Not specified Not specified

Fatigue BS 3519 pt1 Not specified Not specified Not specified

Measuring equipment accuracy, resolution and stability



Is test temperature deviation, independent from measuring system accuracy?Examples:

– Temperature deviation limit: +/- 3°C - (BSEN10291)

– Measuring equipment accuracy (precision): +/- 2°C - (BSEN10291)

– Temperature deviation limit: +/- 2°C - (ASTM E139)

– Measuring equipment accuracy (precision): +/- 3°C - (ASTM E139)

Measuring equipment accuracy, resolution and stability








Systematic error correction.


Calibration of the complete monitoring system.Calibration of the complete monitoring system.



Document number

Sensor calibration Frequency Tracability system errors

System Cal Frquency

EN10002-5 : 1992 12 monthstraceable to the international unit (SI)

verification of temperature- measuring system 12 months

ASTM E 21 – 03a

As frequently as is necessary to comply with test method

against secondary standard / E 220

verified against secondary standard Periodically

BS EN 10291 : 2000

12 months or before and after test

traceable to the international unit (SI)

Measurement system (cable, the connection, cold junction, indicator or recorder) 12 months

ASTM E 139 - 00є1 Each lot run Method E 220Calibration of temperature measuring equipment / system 3 Months

BS EN 10319-1 : 2003

12 months or before and after test

traceable to the international unit (SI)

Measurement system (cable, the connection, cold junction, indicator 12 months

ASTM E 328 - 02 Each lot run Method E 220Calibration of temperature measuring equipment / system 3 Months

ECCC Volume 3Temperature dependent National standard

Corrected, where necessary, for all systematic errors not specified

ASTM E 606 – 92 (2004)

no greater than 6 months

National Institue of Standards and Technology

If automated system is used computer output calibration is necessary

no greater than 6 months

BS 3518 pt1 not specified not specified not specified not specified

Conclusion – Maximum system calibration frequency 12 months.Sensor calibration frequency material and temperature / time dependant.

Calibration of the complete monitoring systemCalibration of the complete monitoring system








Systematic error correction.


Taking account of uncertainties within the whole system.Taking account of uncertainties within the whole system.



All areas within the temperature measuring system that may contribute to errors of the actual temperature, should be taken in to account.

– Um Max error in the measurement sensor (e.g. Thermocouple)– Ucs Max error for coupling of the thermocouple to the specimen.– Ums measurement system (including “cold junction" or Reference temp)

– Uu along specimen uniformity (Gradient)

– Ud Sensor drift with time– Uid change in depth of immersion from calibration– Uet environmental temperature influences on measuring system– T Test temperature

Taking account of uncertainties within the whole system



Formula for uncertainty of temperature measurement is as follows:-U = √[Um(T)2 + Ucs(T)2 + Ums(T)2 + Uu(T)2 + Ud(T)2 + Uid(T)2 + Uet(T)2] / √ 3

– Um - from thermocouple calibration– Ucs - will be ‘zero’ for intimate contact– Ums - from system calibration check– Uu - from T/C gradient measurements– Ud - drift published information– Uid - this can be ignored if the calibration immersion is similar– Uet - calculated from voltage readings at 2 ambient temps– T - defined Test temperature




For further details please refer to the following documents:-

– UKAS M3003

– Uncert Codes

– CEN Workshop agreement




Practical ramifications an example– Test temp 550°C– Standard allows +-3°C– T/C cal lab uncertainty = +-1.5°C– Cal of measuring system = +-0.5°C– This leaves +- 1.4°C for gradient, furnace control band

and drift– Is this sufficient?– Can the cal lab uncertainties be reduced?




Thank you

Questions or comments?

The EndThe End

www.alstom.com

HTMTC 15 March 2006 1

Meeting the requirements of temperature measurement

The role of UKAS

Phil EndersbyUKAS


Overview

• Illustration of the effects of thermocouple characteristics

• Specifying calibration requirements

• UKAS assessment of internal temperature calibration


Factors limiting uncertainty in thermocouple calibration

• Thermocouple homogeneity

• Thermocouple output sensitivity

• Thermocouple ageing

• Furnace uniformity

• Traceability


Device Range BMCPlatinum resistance thermometers

0 °C0.01 °C-196 °C-90 °C to 0 °C-0 °C to 250 °C250 °C to 300 °C300 °C to 420 °C420 °C to 660 °C

0.005 °C0.003 °C0.007 °C0.007 °C0.010 °C0.015 °C0.03 °C0.05 °C

Thermocouples -base metal

-196 °C-90 °C to 0 °C0 °C to 40 °C40 °C to 80 °C80 °C to 350 °C350 °C to 420 °C420 °C to 660 °C660 °C to 1100 °C1100 °C to 1300 °C

0.2 °C0.15 °C0.10 °C0.15 °C0.2 °C0.3 °C0.4 °C0.9 °C2.3 °C

Thermocouples -noble metal

0 °C to 280 °C280 °C to 660 °C660 °C to 1100 °C1100 °C to 1300 °C

0.5 °C0.45 °C0.9 °C2.3 °C

Accredited BMC for

specialist temperature calibration laboratory


Factors limiting uncertainty Areas of most significance in italics

• Thermocouple homogeneitybase metal thermocouples

• Thermocouple output sensitivitynoble metal thermocouples, lower temperatures

• Thermocouple ageingbase metal thermocouples, after high temperature

• Furnace uniformityhigher temperatures

• Traceabilityabove 1100°C


Considerations for optimum calibration

• The effects of immersion depth• Annealing• Ageing• Re-calibration• Repeat points• Homogeneity• Batch characterisation


Thermocouple immersionRe-calibration of base metal orcheck prior to annealing for noble metal…

…unlikely to be of any benefit

Immersion during use

Immersion during calibration

…may sometimes be useful

Immersion during use

Immersion during calibration


Repeat pointscan be useful but…

Immersion up to 250°C

Immersion above 250°C

…a repeat point as 250°C for this calibration

Information about repeatability/homogeneity

X Information about ageing/annealing


Ageing, homogeneity and batch characterisation

• Base metal thermocouples can be aged at an appropriate temperature prior to calibration, to give greater stability

• Homogeneity effects can be estimated by changing immersion during calibration

• Batches of suitably annealed/aged base metal thermocouples can be characterised using a representative sample of the batch


UKAS assessment of internal calibration

• Fit for purpose• TPS52 and AC Supp

Internal calibration must be assessed to confirm that it is competent and fit for purpose

• UKAS technical assessorsUKAS will generally use out own specialist staff for assessment of internal calibration in test labs, because of ‘flexibility’ required

Thermocouples : Effects of Long Term Drift, Atmosphere & Strain

Peter Quested, Bryan Roebuck Matt Brooks & Malcolm Loveday

Materials Centre, Division of Engineering & Process Control

NPL

March 2006

Creep Testing

Temperature accuracy is important in test reproducibility.

Source: M.S Loveday and R Morrell ‘Standardisation of Mechanical Testing and Quality Control’ in “Mechanical Testing of Engineering Ceramics at High Temperatures” edited by B. F. Dyson, R.D. Lohr & R. Morrell, Pub:Elsevier Applied Science, London, 1989.

Thomas Johann Seebeck1770 - 1831

Undated engraving,

Deutsches Museum, Munich

T. J. Seebeck’s discovery 1821The emf* is proportional to the temperature difference

http://chem.ch.huji.ac.il/~eugeniik/history/seebeck.html* emf – electromotive force ( voltage)

V » T1 – T2

V

T1 T2

Note: He even found that electrical currents flowed if one portion of a wire was hammered or twisted while the other portion of the same wire was not reshaped, ie influence of Cold Working

J. C. A. Peltier’s discovery 1834Passing a current through the couple generates

a different temperature at the junctions & depends on the polarity of the applied voltage

( Peltier cooling )

I

+ -

Jean Charles Peltier ( French ) 1785 - 1845

T1 T2

William Thomson (Lord Kelvin)British

Sir William Thompson1824 - 1907

( later Lord Kelvin )

Discovered in 1854 that the emf* is generated in Temperature Gradients

rather than at the junctions

* emf – electromotive force ( voltage)

Operating conditions.

• Temperature range, effects choice of wire materials.• Time. The longer the duration of the test, the more likely

thermocouples are to degrade.• Environmental gas. (In oxidising environments Rh can migrate in

noble metal thermocouples and under reducing conditions need stable sheath materials.)

• Pressure. (special interface design. Pressure has little effect on output of thermocouples.)

• Thermal cycling (degrade the thermocouples)• Ionising radiation (transmutations of material change the

thermoelectric voltage characteristic.)• Strong electromagnetic alternating fields. (induction heating)• Strong inhomogeneous magnetic fields along thermoelectric wires

can influence thermoelectric voltages.• Mechanical working This can effect the thermoelectric output.

Design of assembly

• Wire types (use of type N and noble metal thermocouples.)

• Wire diameter (thin wires are more prone to surface reactions but thicker wires can act as heat shunts.)

• Sheath materials must be stable. (danger in reducing atmospheres of reducing some oxide ceramics)

• Avoid mechanical work in thermocouples.

Choice of thermocouples.

There are five base metal and three precious metal thermocouplesThe base metal types are T,J,E,K and N. Types T, J,and E either have restricted temperature ranges or are unstable

from 300°C.

Type + leg - leg Range (°C) long term use

Comments

K Ni-Cr Ni-Al 0 to 1100 High sensitivity; cheap; easy to use but unstable.

N Ni-Cr-Si Ni-Si-Mg 0 to 1100 More expensive and more stable than Type K.

Long term performance of types K and N Thermocouples.

Type N thermocouples have better long term performance than Type K.N A Burley and T P Jones “Practical Performance of Nicrosil-Nisil Thermocouples” Inst.Phys.Conf, Ser, 26,1975, 172-180.

Precious metal thermocouples.

Type +leg -leg Approximate long term °C*

Approximate short term °C

Comments

R Pt-13Rh Pt 0 to 1600 -50 to 1700 Stable in air or oxidising conditions. Care in vacuum especially stability of ceramic sheaths. High cost. Relatively low sensitivity. Considered to be very stable. Easily contaminated

S Pt-10Rh Pt 0 to1550 -50 to 1750 See type R

B Pt-30Rh Pt-6Rh 100 to1600 100 to 1820 Similar to R and S but not as popular. Used in glass industry. Requires no cold junction.

* Also depends on the thickness of the wire.

Selected Ranges for Best Measurement Capability at NPL for Various Types of Thermometer.

Measured Quantity Range Best MeasurementCapability Expressed as an Expanded Uncertainty (k=2)

Resistance thermometers, fixedpoint calibrations

419 °C to 660 °C660 °C to 961 °C

±0.001 °C to ±0.003 °C±0.003 °C to ±0.005 °C

Resistance thermometers,calibration by comparison

200 °C to 550 °C ±0.02 °C

Liquid-in-glass thermometers 0 °C to 50 °C450 °C to 550 °C

±0.003 °C±0.5 °C

Thermocouples, noble metal types 0 °C to 1100 °C1100 °C to 1600 °C

±0.3 °C±1.0°C to ±1.5°C

Thermocouples, base-metal types 200 °C to 550 °C550 °C to 1100 °C1100 °C to 1200 °C

±0.5 °C±1.0 °C±2.0 °C

Infrared Thermometers 500 °C to 1000 °C1000 °C to 3000 °C

±0.4 °C0.05% of Celsius temperature

Source: Schedule of Accreditation for National Physical Laboratory issued by UKAS, October 2004.

• Drift is a change in thermocouple output while it is within the test environment which in turn remains constant,– Chemical contamination; compositional change; straining of

both limbs.• Depth of Immersion Error is a difference in output of the

thermocouple relative to a known fixed standard when the thermocouple is placed in a different from that of the test furnace.

Calibrate working thermocouples against a Standard T/C in a furnace with similar

depth of immersion to that used on a testing machine

H Robson (1977) ‘Temperature measuring techniques in a large creep laboratory’ J Phys E : Sci Instruments 10 384 - 389

Thermocouples

Temperature versus

position

8.02 mV

Voltageversus

position

glass tube

The reproducibility of the platinum thermocouple at the freezing points of gold, silver and antimony.M de Selincourt. Proc Phys Soc 51 145- 158 ( 1939)

Pure Platinum Wire( data from Selincourt, 1939)

0

2

4

6

8

10

12

0 1 2 3 4 5Number of Strainings

Tem

pera

ture

D

iffer

ence

°C

New Wire

Old wire annealed 1600°CNew Wire

New Wire

Platinum Alloy Wire( Selincourt, 1939 )

-3

-2

-1

0

1

2

0 1 2 3 4 5

Number of Strainings

Tem

pera

ture

D

iffer

ence

°C

New wire

Old wire annealed at1600°COld wire annealed at1600°CNew wire

Old wire annealed at1600°C

8 mm

Influence of straining (1)

[ Early NPL work in 1930’s ]

Annealing Time & Temperature

Strained Alloy Wire: Influence of annealing time ( Selincourt 1939)

0

1

2

0 60 120

Annealing Time, minutes

Tem

pera

ture

Di

ffere

nce

°C

Annealed at 800°CAnnealed at 1000°CAnnealed at 1400°C

Re-plotted by Loveday assuming 10 µV / °CThe reproducibility of the platinum thermocouple at the freezing points of gold, silver and antimony.

M de Selincourt. Proc Phys Soc 51 145- 158 ( 1939)

[ Early NPL work in 1930’s ]

Temperature : 100°C 500°C 1000°CThermocouple Type µV / °C µV / °C µV / °C

Iron- Constantan( Fe-Cu/Ni ) Type J

55 56 -

Chromel- Alumel ( Ni/Cr – Ni/Al) Type K

40 40 40

Noble(10%Rh/Pt – Pt) Type S

8 10 12

Noble(13% Rh/Pt – Pt) Type R

7 11 13

Examples of approximate Seebeck coefficients for some thermocouples

Influence of straining (2)

Thermocouple Error due to Strain (Fenton 1969)

-15

-10

-5

0

5

10

15

0 50 100 150

Strain %

Ap

rox

ima

te

Te

mp

era

ture

Err

or

°C Pt 600°CPt 800°CPt 1000°CPt 1200°CPt-13%Rh 600°CPt-13%Rh 800°CPt-13%Rh 1000°CPt-13%Rh 1200°C

A.W.Fenton ( 1969) ‘ Errors in thermoelectric thermometers’.

Proc. Inst. Engrs. 116 (7) 1277 – 1285.(Data re-plotted by Loveday 2006)

In-situ calibration of thermocouples using a phase transformation

Ti - phase change at 885°C ( α → β )

Eutectoid steel – phase change on heating at 745°C- 750°C Pearlitic ( bcc Fe-C / Fe3C) → austenitic ( fcc Fe-C )

Phase change on cooling ~ 655°C

ETMT Measurements

600 650 700 750 800 850 900600

650

700

750

800

850

900

950

1000

1050

1100

1150

1200R

esis

tivity

n

Wm

Temperature oC

e1 heat e1 cool a1 heat a1 cool

Pt - undeformedPt/Rh - undeformed

Heat to 850°C at 2°C / sCool to RT at 2°C / s

In-situ calibration of thermocouples using a phase transformationRepeat measurements on different thermocouples

( Roebuck et al 2006, NPL )

ETMT Measurements : Strained Thermocouples ( Roebuck et al 2006)

0 200 400 600 800 1000

200

400

600

800

1000

1200

Res

istiv

ity

nΩ

m

Temperature oC

c1 heat c1 cool

Pt - flattenedPt/Rh - undeformed

0 200 400 600 800 1000

200

400

600

800

1000

1200

Res

istiv

ity

nΩ

m

Temperature oC

d2 heat d2 cool

Pt - undeformedPt/Rh - flattened

p

0 200 400 600 800 1000

200

400

600

800

1000

1200

Res

istiv

ity

nΩ

m

Temperature oC

a1 heat a1 cool

Pt - undeformedPt/Rh - undeformed

U – Un-flattened F – Flattened

100 µm diameter 30 µm x 250 µm

Testpiece 1 x 2 x 40 mm

750°C

722°C

702°C

In-situ calibration of thermocouples

using a phase transformation

(~ 60% cold work)

Thermocouples: Influence of straining Measurement on ETMT ( Roebuck et al, NPL March 2006)

Table 2 - Phase transformation temperatures

Pt/Pt-13%Rh U/U*

Pt/Pt-13%Rh U/F*

Pt/Pt-13%Rh F/U*

heating (1) 750 oC 702 oC 722 oC cooling (1) 655 oC 623 oC 623 oC heating(2) 750 oC 702 oC cooling (2) 655 oC 611 oC

U – Un-flattened F – Flattened100 µm diameter 30 µm x 250 µm

Eutectoid Steel . In-situ calibration of thermocouples using a phase transformation

Thermocouples: Influence of straining Measurement on ETMT ( Roebuck et al, NPL March 2006)

In-situ calibration of thermocouples

using a phase transformation

500 550 600 650 700 750 800 850 900600

650

700

750

800

850

900

950

1000

1050

1100

1150

1200

Res

istiv

ity

nΩ

m

Temperature oC

f1 heat and cool d2 heat d2 cool

Pt - undeformedPt/Rh - flattened

Good repeatability inCold worked thermocouple

Inhomogeneity

Inhomogenity of SeebeckCoefficient

Rig with traversing furnace todetermine variation of

response along the length of the wire.

Zvizdic,D et al. “Estimation of Uncertainties in Comparison Calibrationof Thermocouples” TMCSI 7, 2003, 529.

Stability of Thermocouples.Pt-13%RhPt in oxidising atmosphere.

In oxidising conditions at high temperatures above about 1050°C calibration changes which occur in platinum metal thermocouples are associated with selective volatilisation of the metal oxides. Also Rhodium oxide can exist as crystalline form from prolonged exposure at 400-600°C

Darling, A S and Selman, G L “Some Aspects of Environment on thePerformance of Noble Metal Thermocouples” TMCSI,4,Part 3, 1633-1644.

Recommendations by Quinn on construction of thermocouples.

• Use high quality alumina.• Anneal wire for 1 hour at 1250°C in air.• Alumina tube heated at 1200°C in air.• Alumina tube should be sufficiently long

so no gaps between successive sections. • Avoid contamination with bare hands.

Heywood, A.E. “Thermocouples and resistance Thermometers”Proc. Comf. On Temperature Measurement and Control,The Chameleon Press Ltd; 22-29 (1979)

100 days at 1,400°C

Stability of Thermocouples.Pt-13%RhPt in reducing atmosphere.

• Under inert or reducing conditions severe reactions can occur between the thermocouple limbs and refractory insulants.

• Showed that alumina; zirconia and thoria can dissociate to give the respective metal which reacts with the platinum to form a low melting point phase.

• Magnesia is very resistant to these effects.Darling, A S and Selman, G L “Some Aspects of Environment on the Performance of Noble Metal Thermocouples”

TMCSI,4,Part 3, 1633-1644

• Impurities in alumina such as Fe and Si (often as silica) can alloy with Pt and effect output.

• Demonstrated higher drifts (about a factor 2) with thinner wires 0.33 mm versus 0.5 mm.

Reported in Glawe, G L and Szaniszlo *Long Term Drift of some noble and Refractory’Metal Thermocouples at 1600 K in Air; Argon and Vacuum” TMCSI,4,Part 3, 1645-1662.

Thermocouples: Long term drift

‘Calibration Drift of PR13 Thermocouples During Long-Time Creep Testing’

S Yokoi, H Iton, T Murata, M Egashira & H Miyazaki ( 1980)

Trans ISIJ 20 (6) 243( also reproduced in Desvaux, 1982)

Note: at 600°C more drift in multi-specimen machines –

due to more handling ( cold-work) ?

~ 5°C –10°C drift may beencountered after ~ 5 years

( Diffusion of Rh across the junction ? )

Thermocouples: Long term drift

~ 5°C –10°C drift may be

encountered after ~ 5 years

( Diffusion of Rh

across the junction ? )

Compensate during test ?

Data from ERA Creep Laboratory,M.P.E. Desvaux ‘The practical realisation of

temperature measurement standardsin high temperature mechanical testing’

Chapt. 7 in Loveday, Day & Dyson, 1982.Based on H Robson (1977) ‘Temperature measuring techniques

in a large creep laboratory’ J Phys E : Sci Instruments 10 384 - 389

Concluding RemarksRemarks

• Avoid introducing cold work into thermocouples. • Compensate for drift in very long term tests ? • Calibrate with similar depth of immersion, or check for

inhomogeneities in the wire. • Consider using in-situ calibration in the testing machine

using a material with well defined phase change. • Remember the Seebeck emf is generated in the

temperature gradient.

R: Thompson

( Lord Kelvin )L: Seebeck

Useful References

M.S.Loveday, M.F.Day & B.F.DysonPublished : HMSO 1982

ISBN 0 11 480049 9

Useful References

• VDI/VDE-RICHLINIEN VDI/VDE 3511 on Temperature Measurement in Industry.

Part 1 Principles and Special Methods of Temperature Measurement

Part 2 Contact thermometersPart 3 Measuring systems and measuring quantity

processing for electrical contact thermometersPart 4 Radiation thermometersPart 4.1 Specification for radiation thermometersPart 4.2 Maintenance of the specification for radiation thermometersPart 4.3 Radiation thermometry. Standard test methods

for radiation thermometers with one wavelength rangePart 4.4 Radiation thermometry Calibration of Radiation

thermometers Part 5 Installation of thermometers.

Useful References

www.evitherm.organd Temperature>Measurement methods>Thermocouples.

http://en.wikipedia.org/wiki/Peltier-Seebeck_effectQuinn, T J “Temperature” Academic Press London. 1990, ISBN 0-12-569681-7 Chapter 6 Thermocouples.Burley,N; Powell, R; Burns,G and Scroger,M “The NICROSIL versus NISIL Thermocouple: Properties and Thermoelectric Reference Data” NBS Monograph 161, 1978.BS1041:Temperature Measurement Part 4 Robin E. Bentley “Thermocouples in Temperature Measurement” CSIRO (Australia) ISBN 0-9750744-4-XJ V Nicholas and D R White “Traceable Temperature: An Introduction to Temperature Measurement and Calibration”, 2001L Michalski, K Eckersdorf and J McGhee, “Temperature Measurement,” 2002

“Temperature Measurement and Calibration course”, National Physical Laboratory

http://www.evitherm.org/

http://en.wikipedia.org/wiki/Peltier-Seebeck_effect

Thank you for listening Thank you for listening

•• Any Questions ? Any Questions ?

HT Double SidedHT Double SidedAveraging TensileAveraging TensileExtensometerExtensometer

( INSTRON)( INSTRON)

Additional Thermocouples

There are also W; W3 and W5 known as “Tungsten-Rhenium.” Poor performance in oxidising atmosphere.

A number of others without designation types:Ir40Re; Ir Very high temperatures 2100°C in air.

Pt:Pd Under investigation for 660-1500°C. Capable of high accuracy.

Mo:Nb Used in nuclear industry because of resistance to nuclear bombardment. Not used in oxidising atmosphere.

See www.evitherm.org and Temperature>Measurement methods>Thermocouples.

Most high temperature mechanical testing uses the noble metals because of their relative stability in oxidising atmospheres; readily available and guaranteed quality of initial homogeneity.

http://www.evitherm.org/

Thermocouples: How they work

• No voltage generated at the junction• Voltage generated along the entire length of both wires by

temperature gradients– Uniform temperature: electrons have no preferred direction of

motion– Temperature gradient: electrons diffuse in a preferred

direction (either along or against the gradient)

• Any changes in structure or composition of the wires where there is a temperature gradient effects the output.

High Temperature Mechanical Testing meeting - Radiation Thermometry

Helen McEvoy

15 March 2006

Restricted Commercial

The talk will cover:

• What is radiation thermometry?

• Applications and advantages

• Calibration issues

• Disadvantages

• Practical considerations


Concept

• All surfaces radiate thermal energy

• Radiation thermometry (radiation pyrometry)

– measures the thermal energy

– converts that measurement into a temperature

• For objects at ordinary temperatures most of the thermal radiation is at infrared wavelengths


Advantages of radiation thermometry

• Non contact, non-intrusive– can measure remote objects, moving objects

• Large temperature range– can measure very hot objects

• Good sensitivity, Lλ∝ exp(-c2/λT)

• Can measure large areas and images

• It is fast

• Fundamental laws can be applied


Examples:Fibre optic extrusion


Steel processing


Plant maintenance


Maintenance – thermal image of an electrical connector


Turbine blade monitoring


Sea surface temperatures


What is a radiation thermometer?

It has:

• an optical system (lenses, mirrors) to collect the radiation

• a detector to measure it

• often a filter to select the wavelength

• the means to relate the detector signal to the temperature


An NPL radiation thermometer


Fundamental principles

Planck's law for spectral radiance distribution

Or Wien's approximation to it

]1 - T)/c(exp[c = L 1-2

5-λ λλ

πελ 1

c1 and c2 are constants; ε is the emissivity; specify λ, measure Lλ, determine T

T)/c(c = L 25-

λ λλπελ −exp1


Plot of the Planck function


Calibration

• To relate radiation thermometer output to temperature requires well-characterised sources of radiation

– (usually) practical blackbodiesradiance closely similar to ideal Planckian radiator

• The radiation field inside a closed, isothermal cavity depends only on the temperature and is independent of the material properties of the cavity


Cavity (blackbody) radiators

• An aperture (hole) is used to observe the radiation in the cavity

• By careful design (e.g. small aperture compared to length) the disturbance this aperture creates can be minimised

• Cavity sources are therefore ideal standard sources of radiation, governed by Planck’s law, and with an emissivity very close to 1


Practical blackbody radiators

• Need:

– blackbody cavity of a good design (large compared to size of aperture, good wall emissivity)

– means of keeping cavity isothermal (stirred liquid bath, heat-pipe)

– means of measuring the temperature (e.g. PRT placed close to, and in good thermal contact with, back radiating surface or calibrate in terms of radiance temperature)


The NPL blackbody radiation sources

Blackbody type Temperature range (°C)

Ammonia heatpipe -40 to +50

Stirred liquid 0 to 80

Water heatpipe 50 to 250

Caesium heatpipe 300 to 600

Sodium heatpipe 500 to 1000

High temperature 1000 to 3000


The NPL blackbody sources (50 °C to 600 °C)

41 mm

Single zone furnace

680 mm

SPRT

Compacted ceramic fibre insulation (caesium heat pipe only)

heat pipe

Compacted ceramic fibre insulation. (caesium heat pipe only) Temperature profiling

tube ( 2 off)


High temperature blackbody source


The NPL fixed-point blackbody cavity


Au point melting and freezing plateaux

5.88E-11

5.89E-11

5.90E-11

5.91E-11

5.92E-11

5.93E-11

5.94E-11

0:00 0:30 1:00 1:30 2:00 2:30time hrs:min

Iph / A


Calibration uncertainties

For a primary standard instrument (includes characterisation, stability, calibration):

< 0.1 °C at 1000 °C increasing to ~1.5 °C at 3000 °C

For a customer instrument (best measurement capability; depends on performance of instrument):

0.2 °C (–40 to 200 °C); 0.3 °C (200 to 500 °C);0.4 °C (500 to 1000 °C);0.05 % of Celsius temperature (1000 °C to 3000 °C)


Disadvantages of radiation thermometry

• A clear line-of-sight is needed

• Radiation may be:– absorbed in the path– scattered into or out of the line of sight– reflected from the object surface or from surrounding

objects

• Radiation emitted depends on the properties and condition of the material’s surface – its ‘emissivity’

• For good results the system must be well understood


• Select a thermometer operating at a suitable wavelength for your application (consider required temperature range, effect of atmospheric absorption, are you viewing through a window?)

• Select a thermometer with a suitable target size for your application

• Be aware of the problems with the emissivity of your surface (more later)

• Consider whether purging of the line-of-sight might be required

Selecting a radiation thermometer


1) Select a thermometer that operates at a suitable wavelength for your application

2) Select a thermometer with a suitable target size for your application



Plot of the Planck function


Detector response


Atmospheric transmission


1) Select a thermometer operating at a suitable wavelength for your application (consider required temperature range, effect of atmospheric absorption, are you viewing through a window?)

2) Select a thermometer with a suitable target size for your application



The size-of-source effect (SSE)

• Caused by scattering and imperfections in the thermometer optics

• Instrument sees radiation from outside its nominal target size (field-of-view)

• Result - as the size of the target increases its apparent temperature (generally) also increases

• Amount depends on quality of optics and how manufacturer defines target size

• Effect can be large for poor quality instruments


Examples of SSE measurements

SSE comparison

40.00

50.00

60.00

70.00

80.00

90.00

100.00

0 2 4 6 8 10 12 14 16 18 20 22 24 26Aperture size (mm)

SSE

(%)

NPL primary pyrometercommercial instrument


Example of some SSE results (source at ~800 °C)

Aperture diameter (mm)

Temperature reading (°C)

4 618.0

25 786.530 792.040 796.6

6 751.89 763.810 765.112 768.2

15 772.820 780.0

50 798.1


So, in summary:

• For high temperature applications use a short wavelength, narrow band filter; for low temperature applications, use a long wavelength, broad band filter

• Avoid wavelengths that correspond to atmospheric absorption bands

• Use as short a wavelength as possible (uncertainties proportional to λ)

• Make sure that the stated target size of the instrument is small enough for your application


Real surfaces and emissivity

• Radiation emitted from a surface depends on the ‘emissivity’ ελ where:

• Blackbody radiators are perfect emitters of thermal radiation and also nearly perfect absorbers of it – their emissivity is 1

)(blackbodyL(surface)/L = λλλε


• Highly polished surfaces are nearly perfect reflectors and radiate very little energy – their emissivity is 0

• For most surfaces 0 < ε < 1

• Emissivity is a material property which depends on:

temperature, wavelength, direction of view and surface conditions

Real surfaces and emissivity


Explaining emissivity


The variation of emissivity with temperature


The variation of emissivity with wavelength


Effect of emissivity

• A radiation thermometer can only respond to radiation it receives

• It will therefore give different answers when viewing surfaces with different emissivities

• For real surfaces the Planck law must be modified:

]1 - T)/c([c= L 1-2

5- λλπ

ελλ exp1

For accurate temperatures the emissivity must be known or measured


Coping with Emissivity

• Estimate and compensate

• Enhance emissivity to ~1

• Paint surface with known emissivity substance

• Ratio two spectral radiances

• Measure emissivity by a secondary technique

• Live with it: if reproducibility is the main concern


Ratio or two colour thermometers

• Principle - take the ratio of the thermometer signals at two wavelengths

• Emissivity term cancels out, can calculate T

BUT:

• The emissivity at the two wavelengths must be the same, or the ratio must be known and remain constant. This assumption is usually not valid.

• Some sensitivity is lost


The variation of emissivity with wavelength


Ratio or two colour thermometers

Generally, uncertainties are difficult to evaluate due to the emissivity uncertainty and such instruments should only be used with caution

However,

•allows some compensation for absorption in the line-of-sight

•Useful if source size is smaller than the field-of-view


Summary

• Introduced fundamental principles

• Described applications and advantages

• Described fundamental disadvantages– and how these might be overcome

• Outlined practical radiation thermometry


Any questions?

Recent developments in applied non-contact thermometry at NPL

A Levick, M Broussely and G Edwards

Thermal MetrologyNational Physical Laboratory

United Kingdom

Non-contact thermometryTechniques

• Photothermal radiometry for temperature and thermal properties measurement including Laser Absorption Radiation Thermometry (LART)

• Radiation thermometry employing background correction algorithms to compensate for reflected radiation, so called “RTHAT”

• Microwave radiometry for non-invasive internal temperature profiling of human tissue or foods

• Multi-wavelength pyrometry – does it work?

Laser Absorption Radiation Thermometry (LART)

Photothermal Radiometry for Temperature Measurement

LART is independent of:

•Emissivity•Reflected radiation•Gaseous absorptions•Laser intensity profile

At least in principle

Photothermal Radiometry

Wavelength λ1

Diode laser

Wavelength λ2

InGaAsphotodiode

Temperature

Hot Sample

Laser

Detector

2

1S λωλ

1

2S λωλ

Temperature

( )( )

2

1

1

2

1

2

,,LART

S L TC

S L T

λωλλ

ωλ

λλ

′= ⋅

′

2

1

1

2

SS

λωλλ

ωλLARTC ( )

( )1

2

,,

L TL T

λλ

′′

2

0 2 1

1 1626

1

CTe λ λλ

λ

⎛ ⎞−⎜ ⎟

⎝ ⎠= ⋅

Temperature

( )( )

2

1

1

2

1

2

,,LART

S L TC

S L T

λωλλ

ωλ

λλ

′= ⋅

′LARTC

T

LART1

The modulated thermal radiance at λ1 due to photothermal heating induced by a laser at λ2 is: -

1

2

11 1

( , ). . .L TS C TT

λω λ

λε ∂= ∆

∂

Measure two modulated thermal radiances at λ1 and λ2 due to lasers at λ2 and λ1, and take the ratio:-

1

2

2

1

1 2

2 1

'( , ) ( ). .'( , ) ( )LART

S L T PCS L T P

λω λλ

ω λ

λ λλ λ

= No emissivity terms!LEFT1

2 2 2 2. ( ). ( )T C Pα λ λ∆ =where

Kirchhoff’s law ( ):-

1

2

11 2 1 1 2 2 2

( , ). . ( ). ( ). . ( )L TS C C PT

λω λ

λε λ ε λ λ∂=

∂

ε α=

LART2

Measure two modulated thermal radiances at λ3 due to lasers at λ2 and λ1

No emissivity terms! – but still sensitive to reflected background radiation

Also, measure two dc thermal radiances at λ2 and λ1

3 2

1

3 1

2

2 1

1 2

. ( , ) ( ). .. ( , ) ( )LART

S S L T PCS S L T P

λ λω λλ λ

ω λ

λ λλ λ

=

Take the quantity:

LART Instrument Overview

LART Capabilities

•Measurement of temperature and thermal properties

•Single instrument

•Transportable

•Non-invasive

•Flexible

•Independent of emissivity, reflected radiation, absorption

Photograph of Instrument

LART Instrument Specifications

• Temperature range: 500°C and above

• Measurement time: 1 second

• Operates using thermal radiation laser wavelengths of 840 nm, 970 nm,

1320 nm, 1550 nm and 1970 nm

• Laser powers: 1 Watt

• Target materials include opaque solids and liquids; some semi-transparent materials and moving targets

• Target distance: 200mm – 2000mm

• Transportable in an a small car

Temperature – Results

-20

-15

-10

-5

0

5

10

15

20

0 5 10 15

InconelPlatinum

Dev

iatio

n fro

m T

herm

ocou

ple

Rea

ding

(Cel

sius

)

Point

847 oC 940 oC 1033 oCInconel vsplatinum

• RF Levitated metal drops

• Laser-heated aerodynamic levitated particles

• Silicon wafers in a Chemical Vapour Deposition (CVD) apparatus

LART thermometry -Fieldwork

Having demonstrated the LART technique in the laboratory, the next stage is to investigate “real” field applications. So far investigated:-

How to we verify the temperature readings?There are no other suitable thermometry methods

for cross-comparison

LART fieldwork - Electromagnetically Levitated drop

•Metals – liquid or solid

•RF heats and levitates

•Contactless/containerless

•Inert or reducing purge gas

•Measure various properties

•Radiation thermometry•Emissivity problem

•Work by Brooks et al, NPL UK and Egry et al at DLR Germany

LART Fieldwork - RF Levitated drop

LART – levitated iron drop Results

Levitated drop – Solid Temperature gradients

Drop

LART

Two colourpyrometer

TemperatureD

ista

nce

Solid

Levitated drop – Liquid No temperature gradients

Drop

LART

Two colourpyrometer

TemperatureD

ista

nce

Liquid

Convection

LART – Levitated iron drop Results

Temperature profile on the drop surface

Solid state (1260 C)

Liquid state (1538 C)

LART – Considerations

Weakness of photothermal signals is an issue for field work – Ideally need higher power lasers

Difficult finding alternative thermometry methods for cross-comparison

Temperature range limited by laser wavelengths available

Expensive due to high cost of high power lasers

Eye hazards associated with high power lasers

Conclusions

Technique Immune to problems associated with radiation thermometry

In-situ multi-property measurements

Instrument – Use of fibre optics

NowLART proven in the laboratory

LART demonstrated on some field applications

Promising results for multi-properties

Next… Single fibre-optic transportable instrument

Further field LART measurements

Tests on well-characterised reference targets

Commercialisation – cheap high power lasers?

Radiation Thermometry

Radiation Thermometry in HighAmbient Temperatures (RTHAT)

RTHAT is independent of:

•Emissivity•Reflected radiation

At least in principle

RTHAT apparatus

M

SPyrometer 1

Pyrometer 2

Background at TB

Target surface at TS

fibre-coupled pyrometer 1

fibre-coupled pyrometer 2

Incandescent lamp

concave mirror

Furnace & targetCorrelation: S1=a•S2+Starget

Ripple technique - background radiation subtraction from measured signal

0

0.5

1

1.5

2

2.5

3

0 20 40 60 80 100 120Time

Rad

iatio

n / a

rb.

Background (S2) Sample signalSample+reflection (S1) Recovered

Pyrometer 1

Pyrometer 2

a=0.6094 (0.6)Ssample=0.9797 R2=0.9865

Hot target with reflected radiation

Hot target with no reflected radiation

Experimental procedure:

Apply least

squares fit

Surface radiance (em itted + re flected)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0 0.05 0.1 0.15 0.2

t / s

Si/ V

Background Radiance

0.0

0.5

1.0

1.5

2.0

2.5

0 0.05 0.1 0.15 0.2

t / s

Mi /

V

Correlation between background radiance and surface radiance.

0.00

0.02

0.04

0.06

0.08

0.10

0 0.5 1 1.5 2 2.5

Mi / V

Si /

V

. ( )i iS L T r Mε= + ⋅

Results for temporal RTHAT

1r ε= −

Therefore get T and ε

Results for temporal RTHAT

Least squares background correction method• Inconel target

• Diffuse reflector• High emissivity• ∆T < 1%

Platinum target• Specular reflector• Low emissivity• ∆T <10%• Better for high T

0

10

20

30

40

50

60

70

80

90

100

-20 -10 0 10 20 30 40 50

The ta / °

T(FI

T)-T

(DC

) / K T=789K

T=882K

T=976K

T=1103K

-3

-2

-1

0

1

2

3

4

5

6

7

-20 -10 0 10 20 30 40 50

The ta / °

T(FI

T)-T

(DC

) / K T=789K

T=882K

T=976K

T=1103K

Radiation Thermometry

Multi-wavelength Pyrometry

Claimed to be independent of emissivity!

Or is it?

Multi-wavelength Pyrometry

Measure thermal radiation signals at m different wavelengths:

15

2

1( , )exp( ) 1

cL T cT

λλ

λ

=−

nε

where( , )n n nS L Tε λ=

= emissivity

We have m equations and m+1 unknownsTherefore introduce an “emissivity relationship”, e.g.

11

1

mi

n i ni

cε λ−

−

=

= ∑ We now have m equations and m unknowns

However……

Multi-wavelength PyrometryHowever…..

• The “emissivity relationship” is not very valid and may lead to large errors in the estimated temperature

• The set of equations is often “ill-conditioned”. Thus, small errors in the measured signals lead to large errors in measured temperature value.

• Increasing the number of wavelengths does not reduce the errors

2

1 1 ln nT T cλ

λ ε= − extrapolating to λ=0

Multiwavelength PyrometryTungsten target – errors due to emissivity relationship

Two wavelengths:Delta T = 260 CLambda1 = 1.000e-006Lambda2 = 2.000e-006

Three wavelengths:Delta T = 89.23 CLambda1 = 1.000e-006Lambda2 = 1.500e-006Lambda3 = 2.000e-006

Five wavelengths:Delta T = -123.67Lambda1 = 1.000e-006Lambda2 = 1.270e-006Lambda3 = 1.500e-006Lambda4 = 1.800e-006Lambda5 = 2.000e-006 Contour plot of versus T and c1 – note the valley!2χ

Conclusions

• LART is independent of emissivity, reflected radiation and gaseous absorptions

• RTHAT is independent of reflected radiation and possibly emissivity

• Multi-wavelength pyrometry can measure tagets that underfill the field of view and possibly immume to gaseous absorptions. It is NOT immune to emissivity

However all the techniques in their current form only work with a limited number of applications

Thermal Measurement Awareness Network, NPL, 15 March 2006

Issues of Dynamic Temperature Measurement and Control

in Thermo-Mechanical Fatigue Testing

Peter HähnerEuropean Commission, Joint Research Centre, Institute for Energy

NL-1755 ZG Petten, The Netherlands

Klaus Rau, Tilmann BeckUniversität Karlsruhe, Institut für Werkstoffkunde I,

D-76128 Karlsruhe, Germany


Thermo-Mechanical Fatigue (TMF):

Design & residual life analysis of safety critical components

exposed simultaneously to thermal & mechanical loads

(gas turbines, aero engines, automotive & process industries):

Internal thermal constraints of representative volume element

simulated by uniform temperature and mechanical strain fields

cyclically imposed on a specimen with uniform gauge section.


temperature: T = T0 + δT F(ωt) , mech. strain: εm = εm,0 + δε F(ωt − ϕ)

phase angle ϕ , F(x) = F(x + 2π) , ∀ x


0 50 100 150 200-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

Mec

hani

cal S

trai

n [%

]

Time [sec]

350

450

550

650

750

850

950

Tem

pera

ture

[o C

]

0 50 100 150 2000.0

0.2

0.4

0.6

0.8

1.0

1.2

Mec

hani

cal S

trai

n [%

]

Time [sec]

350

450

550

650

750

850

950

Tem

pera

ture

[o C

]

Triangular TWF waveforms (without dwell):

in phase ϕ = 0, Rε = 0

out-of-phase ϕ = 180o, Rε = − ∞

mech. strain ratio:

Rε = εm,min/εm,max


-800

-600

-400

-200

0

200

400

600

800

-0,4 -0,3 -0,2 -0,1 0 0,1 0,2 0,3 0,4

Mechanical strain %

Stre

ss M

Pa

OP cycleIP cycle

400°C

900°C

650°C

650°C

400°C

900°C

Mechanical Response ?

Hysteresis loops:

Number of TMF Cycles to Failure ?

Out-of-phase

In-phase

εm = εel + εin


• Load train alignment• Precision extensometry

LCF Thermalfatigue

Load-contr.TMF

Strain-contr.TMF

Dynamic temperature measurement & control

Therm. – mech. load phasing

• Pre-cycling, start-up procedures• Thermal strain compensation

εtot = εm + εth


Outline

Introduction

TMF-Standard Project

Dynamic T Measurement & Control• heating method• specimen geometry• type of thermocouple (TC), methods of TC attachment• pyrometry Recommendations and Concluding Remarks


TMF-Standard GRD2-2000-30014Thermo-mechanical fatigue – the route to standardisation(funded within the GROWTH Programme of the Europ. Comm.)

Objectives: to establish a TMF testing platform in Europeto issue a validated code-of-practiceto disseminate and exploit the results

Starting date: 1. October 2001Duration: 48 monthsBudget: ~ 1.800.000 Euro (~ 1.080.000 EC contribution)Consortium: 10 principal contractors

9 assistant contractors, 1 external participantNo. of tests: ~ 400 incl. 120 validation tests


Three distinct phases of the TMF-Standard project:

1. Preparatory phase:• Inventory of TMF testing procedures in Europe (WP1)• Material procurement, characterization & distribution (WP2)

2. R&D phase:• Pre-normative testing (WP3, Task 1 – 5)• Validation testing (WP4)• Statistical sensitivity analysis (WP5)

3. Exploitation and dissemination phase:• Drafting of Code-of-Practice (WP6)• Dissemination of CoP, workshop (WP7)


WP 3: Pre-normative research on TMF procedures & tolerances

Task 1: Pre-cycling, start, interrupt, restart procedures

Task 2: Dynamic T measurement and control

Task 3: Thermal strain comp., deviations from nominal T

Task 4: T gradient effects in 3 different sample geometries(solid cylindrical, hollow cyl., solid rectangular)

Task 5: Deviations from nominal phase angle

Reference testing as preparation for WP4


Challenges in temperature measurement and control for TMF:

Temperature rates (heating & cooling) up to 50°C/s to be controlled with sufficient accuracy (± 5°C or 1% of ∆T)

-> specimen design, heating system, fast temperature control system

T measurement must not affect crack initiation and TMF life

-> Avoid any microstructural damage, e.g. by thermocouple attachment

Heating method (induction, radiation, ...) must not influence temperature readout in the absence of thermal equilibrium

-> no cold spot at thermocouple, nor direct over-heating of thermocouple

Long-term stability of dynamic temperature fields must be ensured

-> no drift of T profile

Repeatable & reproducible results with desired accuracy of ± 5°C


Issues affecting dynamic T measurement and control:

• heating method

• specimen geometry

• type of thermocouple (TC)

• method of TC attachment

All these issues are interrelated and cannot be optimized independently !


Heating methods:

Resistance furnace: too slow

Direct ohmic heating: practicable for miniaturized, e.g. thin-walled tubular, specimens only

radiation (bulb) furnace: appropriate, if

- excessive radial temperature gradients are avoided

- issue of reflectivity differences: specimen – TC is addressed

Induction heating: most commonly used in TMF

- skin effect to be kept sufficiently low

- cold spot to be avoided by proper TC type and attachment method


Radiation heating:• surface heating: use of thin-walled tubular specimens• ensure good thermal contact of TC

(tends to remain colder due to its higher reflectivity)

Set-up of Centre des Materiaux / Ecole des Mines de Paris:


Induction heating:

• heating of subsurface layer: Skin effect d = (4πσµ0ν)−1/2

• ensure very good thermal contact of TC (not heated directly)

TC spot-welded outside GL Ribbon TC wrapped around centre

Water coolingfor T profile stability


Measurement by spot welded thermocouples in the gauge length:+ Good thermal contact+ Direct measurement in the gauge length− Damage (crack initiation) at specimen surface possible? Formation of „cold spots“ due to thermal conduction in

thermocouple wires, in particular, for Pt-based TCs:

? Which configuration of welding gives best accuracy:separately welded TC wires!

κ heat conductivityρ densityc specific heat

TCTCTCTCcycle rc

tTT

21 ρκ

κπδ

&≈


Measurement by ribbon thermocouples in the gauge length:

+ Direct measurement in the gauge length

+ No significant damage in the gauge length

+ No formation of „cold spots“ by thermal conduction

? Thermal contact to specimen surface may be insufficient and prone

to degradation by oxidation, surface rippling, micro-cracking…T measurement point

Cylindrical specimen Flat specimen


Measurement by spot welded thermocouples outside the gauge length:+ Good thermal contact+ No damage of test piece within the gauge section− Additional temperature calibration necessary

? Formation of „cold spots“ due to thermal conduction in thermocouple wires, in particular, for Pt-based TCs

? Configuration of welding: wires to be welded separately !


Non-Contacting Measurement by pyrometry:

+ Direct measurement in the gauge length

+ Non-contacting, hence no damage in the gauge length

+ No formation of „cold spots“ by thermal conduction

? Temperature drift due to oxidation (One Colour Pyrometry)

? Interference effects due to oxide scale growth (Two Colour Pyrometry)


Overview of tests performed at IWK

TC-types: R(Pt-PtRh), S(Pt-PtRh), K(Ni-CrNi), N(NiCrSi-NiSi)

TC-designs: - (Coaxial (type R, d = 1 mm))

- Ribbon type (type S, as developed by MTU)

- Spot welded (all types with d = 0.25 mm, type K additionally with d = 0.5mm)

Spot welded TCs with different contacting lengths to the specimen surfaceby wrapping the TC wires circumferentially around the gauge length

(Coaxial TCs placed in holes with different depths)

Thermocouple position inside and outside the gauge length

Pyrometry: One-Colour (IWK) and Two-Colour (NPL)


Test Setup & Calibration at IWK

Optimization of induction coil

and air-cooling

- dT/dt up to 50°C/s

- Axial and circumferential

gradients below +/− 5°C

Tmax = 850°C, Tmin = 400°C

Inconel 617

One type R-TC every 5 mm

T1

T5T4

T3T2

X: Thermocouple locations

X X

X


Ribbon and Spot-Welded Thermocouples

lw<

1mm

∅

d

Spot-WeldedRibbon


0 10 20 30 40 50 60 70 80 90 100200

300

400

500

600

700

800

900dT/dt = 10°C/s

Spotweld, 90°-wrapping Ribbon-Type Spotweld, non-wrapping

T [°

C]

t [s]30 35 40 45 50 55 60

750

800

850

900 Spotweld, 90°-wrapping Ribbon-Type Spotweld, non-wrapping

dT/dt = 10°C/s

T [°

C]

t [s]

-> Using spot welded TCs the development of „cold spots“ at the welding point is avoided by wrapping the TC wires along at least 90° of the circumference of the specimen.

Ribbon and Spot-Welded Thermocouples(Pt based)

lw

∅

d


Comparison SW vs ribbon TC (Pt based)dT/dt [K/s] SW (non-wrapping) SW (90° wrapping)

Tmax - 14 K (1.6% of Tmax) - 3 K (0.4% of Tmax) 2

Tmin - 9 K (2.2% of Τmin) - 3 K (0.8% of Tmin)

Tmax - 21 K (2.5% of Tmax) -3 K (0.4% of Tmax) 5

Tmin - 7 K (1.8% of Tmin) -2 K (0.5% of Tmin)


Tmin - 8 K (2.0% of Tmin) - 1 K (0.3% of Tmin)


Tmin - 11 K (2.8% of Tmin) - 1 K (0.3% of Tmin)

-> Spot welded TCs must have a sufficiently high contacting length to the specimen to give accurate results

-> Using ribbon TCs, sufficient thermal contact to the test piece must be ensured in presence of oxidation and/or surface rippling by plastic deformation

-> For Ni-based TCs the cold-spot effect tends to be less pronounced

Reference:Ribbon TC


GROWTH Project n° GRD2-2000-30014

TMF-Standard

T-control using thermocouples outside the gauge length

? Possible accuracy? Achievable heating and cooling rates? Accuracy of thermocouple location and specimen position in test rig? Best practice procedure


GROWTH Project n° GRD2-2000-30014

TMF-Standard

0 50 100 150 200 250 300 350 400200

300

400

500

600

700

800

900

ribbon type TC in the gage section controll: spot welded TC on the

specimen shoulder

T [

°C]

t [s]

dT/dt = 5°C/s desired T-path in the gauge section

0 20 40 60 80 100 120 140 160 180 200200

300

400

500

600

700

800

900dT/dt = 10°C/s

controll: spot welded TC on the specimen shoulder

ribbon type TC in the gage section

T [

°C]

t [s]

desired T-path in the gauge section

T-control using thermocouples outside the gauge length (specimen shoulder)

-> Using appropriate methods for the optimisation of the set point signal, temperature control using a TC located at the specimen shoulder is possible for dT/dt up to 5°C/s (or even 10°C/s for control TC sufficiently close to GL)

-> The accuracies of the TC position within the specimen and of the specimen position with respect to the induction coil have to be respected within +/- 0.5 mm

-> Long term stability of T field has to be assured by appropriate cooling


Temperature Measurement by Pyrometry

0 10 20 30 40 50 60 70 80 90 100200

300

400

500

600

700

800

900dt/dT = 10 K/s

IN 617,preoxidised for 2h @ 850°C

increasing N = 1 ... 50

T [°

C]

t [s]

One-Colour (IWK)

0 5000 10000 15000 20000500

600

700

800

900

1000

Tem

pera

ture

oC

Time s

Thermocouple Pyrometer

IN718

Two-Colour (NPL)

-> One colour pyrometry: stable emissivity of surface must be ensured

-> Two-colour pyrometry: oscillations due to interferences must be avoided

These requirements will exclude the pyrometric T control in most cases !


Further Recommendations• To reduce the effects of cold spots on the specimen, the diameter of the spot-

welded and ribbon TC wires should not exceed 0.5mm.• K (Ni-CrNi) and N (NiCr-NiAl) type TCs are recommended for Tmax up to

850°C because of their higher thermo-voltage and lower thermal diffusivity.Above 900°C type R (Pt-PtRh) or S (Pt-PtRh) TCs are recommended.

• Thermocouple wires should be spot welded separately with parallel non contacting wires.

• Observe heater output and machine stroke to make sure that there is no long term drift of the T profile.

• More comprehensive set of recommendations can be found in the TMF-Standard Code-of-Practice to be published in Int. J. Fatigue,or upon request to [email protected]


Final Remark

At present there is no generally accepted, optimized method of dynamic

temperature measurement & control:

- awareness of critical issues is most important;

- use of complementary means is necessary to ensure accuracy of method;

However, what does „accuracy“ mean in the absence of appropriate methods of

Dynamic Temperature Calibration

? ? ?

temperature measurement in high temperature mechanical...

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