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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
– 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
+
VOUT
1
3
Put into Ice
VOUT
1
Fe/Con
down
2
Cu/Fe
little change
3
Con/Cu
up
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.
Thermocouple Circuit
Metal C
+
TS
TT = Terminal Block
Temp ≈ uniform
VOUT
-
TS
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)
•  = + − − = VCA( ) + VAB( )+ 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 = ?
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)
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
• 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
?
?
•  =   +   +   = ?
– 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 9.22
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
TS
(°C)
0
400
VSC
(V)
0
10
? Out of
range
Transfer
Function
10
Reading
VSC [V]
 =
0
0
•
Measurand, T [°C]
In lab use Omega DRE–TC-J; for Type J (Iron/Constantan) thermocouples


400
– 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:  = 10
•
•
 =


=
10
400°

400℃


℃
= 0.0025  =
= 0.025 ° ;  =


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


= 0.00005 ° ; G =  = 500

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

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