1. ME 322: Instrumentation Lecture 17
February 27, 2015
Professor Miles Greiner
Temperature measurements, thermocouple circuits, thermocouple demo

2. 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

3. 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

4. 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

5. Thermocouples Employ the Seebeck Effect𝐴 𝐵
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

6. Demonstration (three junctions)
Iron 2
Put into Ice
VOUT 1
Fe/Con
down 2
Cu/Fe
little change 3
Con/Cu up +
VOUT 1 3 -
Constantan
Ni/Cu
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 = V
VOUT depends on all three junction temperatures
The sensitivity of VOUT to temperature is not the same for all the junctions.

7. TT = Terminal Block Temp ≈ uniform
Thermocouple Circuit
Metal C
TT = Terminal Block Temp ≈ uniform + TS TS
VOUT HE
WOUT - TR TR
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)
𝑉 𝑜𝑢𝑡 = 𝑉 + − 𝑉 − =VCA( 𝑇 𝑇 )+VAB( 𝑇 𝑆 )+ VBA( 𝑇 𝑅 )+ VAC( 𝑇 𝑇 )
How are these voltage related?
𝑉 𝐶𝐴 𝑇 𝑇 = ? 𝑉 𝐴𝐶 𝑇 𝑇
𝑉 𝐵𝐴 𝑇 𝑅 = ? 𝑉 𝐴𝐵 𝑇 𝑅
𝑉 𝑜𝑢𝑡 = 𝑉 𝐴𝐵 𝑇 𝑆 − 𝑉 𝐴𝐵 𝑇 𝑅 (transfer function, 𝑇 𝑆 desired, 𝑇 𝑅 undesired)
If terminal block is isothermal, then 𝑉 𝑜𝑢𝑡 not dependent on Temperature TT or metal C
How to find 𝑉 𝐴𝐵 𝑇 ?
2nd Law of Thermodynamics (heat engine)
If TS = TR, then VOUT = ?

8. 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)

9. How to find VAB(T) T Material Science Calculations, or Calibration:
VOUT T
TR= 0°C
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

10. Different sensitivities (slopes) Standard wire uncertainty:
Not really linear
Different sensitivities (slopes)
Standard wire uncertainty:
Larger of 2.2°C or 0.7% of measurement

11. Circuits without a Reference JunctionTT TS TT ?
𝑉 𝑜𝑢𝑡 = 𝑉 𝐶𝐴 𝑇 𝑇 + 𝑉 𝐴𝐵 𝑇 𝑆 + 𝑉 𝐵𝐶 𝑇 𝑇 = ?
Problem, we have data for wire pair AB, but not CA or CB
“Thought” experiment: If TS = TT , then by 2nd law
𝑉 𝑜𝑢𝑡 = 𝑉 𝐶𝐴 𝑇 𝑇 + 𝑉 𝐴𝐵 𝑇 𝑇 + 𝑉 𝐵𝐶 𝑇 𝑇 = ?
So 𝑉 𝐶𝐴 𝑇 𝑇 + 𝑉 𝐵𝐶 𝑇 𝑇 = −𝑉 𝐴𝐵 𝑇 𝑇 (effect of C cancels out)
𝑉 𝑜𝑢𝑡 = 𝑉 𝐴𝐵 𝑇 𝑆 − 𝑉 𝐴𝐵 𝑇 𝑇
Don’t need VCA(T) or VBC(T) data to find this transfer function! ? ? ?

12. Problem 9.22
A 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?

13. Thermocouple Signal Conditioner
Reading
VSC [V]
Measurand, T [°C]
400 10
? Out of range
Transfer
Function
𝑆 𝑆𝐶 = 𝜕 𝑉 𝑆𝐶 𝜕𝑇 TS
(°C)
VSC
(V)
400 10
In lab use Omega DRE–TC-J; for Type J (Iron/Constantan) thermocouples
Wiring: Iron (white insulation) goes to +Tc; Constantan (red stripe) goes to (-Tc)
Transfer Function: 𝑉 𝑆𝐶 =10𝑉 𝑇 𝑆 400℃ = 𝑉 ℃ 𝑇 𝑆 =
𝑆 𝑆𝐶 = 𝜕 𝑉 𝑆𝐶 𝜕𝑇 = 10𝑉 400°𝐶 =0.025 𝑉 °𝐶 ; 𝑆 𝑇𝐶 = 𝜕 𝑉 𝑇𝐶 𝜕𝑇 = 𝑉 °𝐶 ; G𝑎𝑖𝑛= 𝑆 𝑆𝐶 𝑆 𝑇𝐶 = 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)