ME 322: Instrumentation Lecture 17

1. ME 322: InstrumentationLecture 17 February 28, 2014 Professor Miles Greiner

2. Announcements/Reminders • HW 6 due now • HW 7 due Friday • Lab 6 next week • See schedule and be on time • http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Labs/Lab%2006%20Fluid%20Flow/Lab%20Index.htm • 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. 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

4. 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

5. Demonstration (three junctions) Iron 2 1 3 VOUT Constantan Ni/Cu • Output is in the 10s 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.

6. Thermocouple Circuit Metal C TT = Terminal Block Temp ≈ uniform + + TS TS • 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) • If terminal block is isothermal, then notdependent on Temperature TT or metal C • If TS = TR, then VOUT = ? (2ndLaw of Thermodynamics) HE VOUT WOUT - TR TR

7. 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) ]

8. How to find VAB(T) 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

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

10. Circuits without a Reference Junction TT TS TT • Problem, we have data for wire pair AB, but not CA or CB • If TS = TT , then by 2nd law • So • Don’t need VCA(T) or VBC(T) data

11. 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?

12. Thermocouple Signal Conditioner ? Out of range Transfer Function 10 • In lab use Omega DRE–TC-J; for Type J (Iron/Constantan) thermcouples • 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 Reading VSC [V] 0 400 Measurand, T [°C] 0

13. Midterm 1 Scores • Sample Average: 74 • Sample Standard Deviation: 18