ME 388 – Applied Instrumentation Laboratory

Temperature Measurement Lab

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.

Instruments for this lab

• Thermometer (reference instrument for lab)

• Thermistor

• Thermocouple

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

Wheatstone Bridge Circuit

R2RT

R3 R4

a

d

Vad5 Vdc

bc

Vcb

Ic Ib

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

Step 2 – Measure Vcb

• Vcb changes with RT

• Determine RT = f(Vcb)

• Determine T from RT

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.