ME 322: Instrumentation Lecture 17 February 27, 2015 Professor 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 𝐴 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

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 = 0.00001 V VOUT depends on all three junction temperatures The sensitivity of VOUT to temperature is not the same for all the junctions.

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 = ?

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) 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

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

Circuits without a Reference Junction TT 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! ? ? ?

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?

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 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: 𝑉 𝑆𝐶 =10𝑉 𝑇 𝑆 400℃ =0.0025 𝑉 ℃ 𝑇 𝑆 = 𝑆 𝑆𝐶 = 𝜕 𝑉 𝑆𝐶 𝜕𝑇 = 10𝑉 400°𝐶 =0.025 𝑉 °𝐶 ; 𝑆 𝑇𝐶 = 𝜕 𝑉 𝑇𝐶 𝜕𝑇 = 0.00005 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)