ME 322: InstrumentationLecture 17

February 27, 2015Professor Miles Greiner

Temperature measurements, thermocouple circuits, thermocouple demo

Announcements/Reminders• HW 6 due now• HW 7 due Friday• Lab 6 next week– Only 4 wind tunnels (we are constructing a 5th)–Watch your WebCampus to find when your group

is scheduled to attend lab. • Bring Excel from HW 6 and use it to process

the data you acquire. • This will help check the data as you take it and allow

you to complete the data acquisition phase of the lab in one hour

Midterm I Scores

• Average 75, St. Dev 18– In 2014 it was 74 and 18 (very similar)

• Solutions posted outside PE 213– I will only consider revising scores before

Wednesday, March 4, 2015

Phenomena used to Measure Temperatures• Liquid density change (in

glass thermometer)• Metal Deformation (Coil,

bimetallic strips)• Gas Pressure• Wire resistance• Problem

– All devices act line fins and affect the temperature of the locations that they are measuring

Thermocouples

• Employ the Seebeck Effect–When two dissimilar metals (A & B) are in contact, a

small electrical potential (voltage) is produced that depends on the junction temperature.

• Probes can consist of two wires and be inexpensive• Rugged shielded probes can be expensive

𝐴 𝐵

Demonstration (three junctions)

Put into Ice VOUT

1 Fe/Con down

2 Cu/Fe little change

3 Con/Cu up1

2

3

Iron

ConstantanNi/Cu

VOUT

• For demo use type-J thermocouple pair (Iron/Constant) connected to a copper (Cu) digital voltmeter

• Output is in the 10’s of microvolts – 10mV = 0.01 mV = 0.00001 V

• VOUT depends on all three junction temperatures– The sensitivity of VOUT to temperature is not the same for all the junctions.

+

-

Thermocouple Circuit

• Four junctions, including reference• Let VCA(T) be voltage decrease going from C to A at junction temperature T

– VCA(T) = VC(T) - VA(T)

• + VBA()+ VAC()– How are these voltage related?

•

• (transfer function, desired, undesired)– If terminal block is isothermal, then not dependent on Temperature TT or metal C– How to find ?

• 2nd Law of Thermodynamics (heat engine)– If TS = TR, then VOUT = ?

TT = Terminal Block Temp ≈ uniform

TR

TS VOUT HE

TS

TR

WOUT

+

-

Metal C

Standardization

• Industry uses standard wire material pairs (page 276)• The composition of the two wires must be well-controlled

and sufficiently-different to give predictable (small uncertainty) and useful (sensitive) voltages

• Different wire pairs have different operating ranges and sensitivities, S = dVTC/dT = d(Reading)/d(Measurand)

How to find VAB(T)

TR= 0°C

TVOUT

• Material Science Calculations, or • Calibration:• Put reference junction is pure water/Ice Slurry, TR = 0°C • Measure VOUT for a range of T

• See Page 277 for results

• Not really linear

• Different sensitivities (slopes)

• Standard wire uncertainty: – Larger of 2.2°C or

0.7% of measurement

Circuits without a Reference Junction

TT

TT

TS

– Problem, we have data for wire pair AB, but not CA or CB

• “Thought” experiment: If TS = TT , then by 2nd law

– So (effect of C cancels out)

– Don’t need VCA(T) or VBC(T) data to find this transfer function!

? ?

? ?

Problem 9.22A type E thermocouple is placed in an oven and connected to a computer data-acquisition system. The junction box temperature is independently measured to be 30°C. The thermocouple voltage is found to be 37.0 mV. What is the temperature of the oven?

Thermocouple Signal Conditioner

• In lab use Omega DRE–TC-J; for Type J (Iron/Constantan) thermocouples – http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Labs/Lab%2007%20Boiling%20Water%20Temperature/Lab7%20Index.htm – Wiring: Iron (white insulation) goes to +Tc; Constantan (red stripe) goes to (-Tc) – Transfer Function:

• ; = 500

– Inverted transfer function: TS = (40°C/V)*VSC

• Conditioner Provides– Reference Junction Compensation– Amplification – Low Pass Filter (RF noise rejection) – Linearization– Galvanic Isolation (avoid ground loops even in water)

TS

(°C)VSC

(V)0 0

400 10

ReadingVSC [V]

Measurand, T [°C]00

400

10? Out of

rangeTransferFunction

𝑆𝑆𝐶=𝜕𝑉 𝑆𝐶

𝜕𝑇