me 322: instrumentation lecture 17
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ME 322: Instrumentation Lecture 17. February 28, 2014 Professor Miles Greiner. Announcements/Reminders. HW 6 due now HW 7 due Friday Lab 6 next week See schedule and be on time - PowerPoint PPT PresentationTRANSCRIPT
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ME 322: InstrumentationLecture 17
February 27, 2015Professor Miles Greiner
Temperature measurements, thermocouple circuits, thermocouple demo
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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
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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
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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
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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
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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.
+
-
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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
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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)
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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
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• Not really linear
• Different sensitivities (slopes)
• Standard wire uncertainty: – Larger of 2.2°C or
0.7% of measurement
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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!
? ?
? ?
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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?
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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