me 388 – applied instrumentation laboratory temperature measurement lab
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References
• Omega Temperature handbook
• Experimental Methods for Engineers, J.P. Holman (Ch. 4 & 8)
What is temperature?
• Latin word Temperare– To observe proper measure
• Temperature – a measure of hotness or coldness
Temperature• An index of an objects thermal condition
• Related to molecular motion
• Provides indication of average molecular kinetic energy
So What?
• Critical engineering parameter
• Affects…– Material properties– Chemical and metallurgical reactions– Heat transfer rates– etc.
Thermometer Operation
• Principle of different expansion coefficients of different materials
• Liquid (i.e., Hg, Alcohol) expands at a greater rate than glass
• Liquid predictably moves in capillary tube relative to temperature
Thermometer - pros and cons
• Limited measurement range (-20 to 150 C)
• Fragile
• Inexpensive
• Precision ~±0.5 C
• Not conducive to electronic monitoring (i.e., computer data acquisition)
Thermistor
• Omega OL-703-PP– 44018 linear thermistor element rated to 100C
• Semiconductor device
• Negative coefficient of resistivity
Thermistor – pros and cons
• Very precise ~±0.01 C
• Expensive
• Fragile
• Limited measurement range (100 C max)
• Requires Wheatstone bridge circuit
• Adaptable to electronic or computer data acquisition
Thermistor Resistance
• RT = thermistor resistance
• R0 = reference resistance
= characteristic parameter (3500 – 4600K)• T = Temperature
• T0 = reference temperature
00
11exp
TTRRT
Determining
• Plot 1/T versus lnRT
• Slope of line =
y = 3808.4x - 4.0937
R2 = 0.9987
5.00
5.50
6.00
6.50
7.00
7.50
8.00
8.50
9.00
1.E-03 2.E-03 3.E-03 4.E-03
Inverse absolute temperature (1/K)
Nat
ural
log
of T
herm
isto
r R
esis
tanc
e
Step 1 – balance the bridge
• Vcb = 0 when RTR4 = R2R3
• Place Thermistor in ice bath and measure resistance RT
• Measure R2
• Pick R3 and R4 such that RT/R2 R3/R4
• Take measured values for RT ,R4 ,R3 and calculate R2 for Vcb = 0
• Adjust R2 to your calculated value• Measure supply voltage and record all
measurement uncertainties
42
3
2
RR
VI
RR
VI
IRVV
IRVV
adb
T
adc
badb
cTadc
R2RT
R3 R4
a
d
Vad5 Vdc
bc
Vcb
Ic Ib
342
2
2
RR
VR
RR
VRV
IRVIRVVVV
T
adTadcb
badcTadbccb
Thermocouples
• Principle of operation - Seebeck Effect
• V T at junction of two dissimilar metals
• This lab will use K-Type TC-200 to 1250 C rangeChromel = Ni Cr alloy (+)Alumel = Ni Al alloy (-)
Thermocouples – pros and cons
• Simple
• Durable
• Inexpensive
• Wide temperature ranges
• Precise ~±2 C
• Lends itself to electronic data acquisition
• Provides millivolt signal
• Signal requires “compensation”
Compensation
• Connection of the dissimilar TC leads to a measuring device causes unwanted EMF
• The unwanted EMF is controlled (compensated) by an additional junction held at a reference temperature (0 C)
• Use Omega table which is based on (0 C) reference temperature
• In practice, compensation is done electronically through conditioners
Lab Summary• Organize into groups
• Set-up thermistor bridge with ice bath
• Set-up thermocouple (TC) circuit
• Record all component values and uncertainties
• Make hot water
• Place TC and thermistor in water at about 75 C
• Take 12-15 readings from 75 to 40 C
Analysis Summary• Thermocouple data
– Plot TC emf vs. Temp. for your data and the Omega data on one graph
– Do regression analyses on both
• Thermistor data– Determine RT from Vcb
– Plot 1/T versus lnRT to determine – Plot thermistor resistance vs. temperature
(measured by the thermometer) and fit eqn.
• Uncertainty: - value for thermistor.
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