ME 388 – Applied Instrumentation Laboratory Temperature Measurement Lab

1. ME 388 – Applied Instrumentation Laboratory Temperature Measurement Lab

2. References • Omega Temperature handbook • Experimental Methods for Engineers, J.P. Holman (Ch. 4 & 8)

3. What is temperature? • Latin word Temperare • To observe proper measure • Temperature – a measure of hotness or coldness

4. Temperature • An index of an objects thermal condition • Related to molecular motion • Provides indication of average molecular kinetic energy

5. So What? • Critical engineering parameter • Affects… • Material properties • Chemical and metallurgical reactions • Heat transfer rates • etc.

6. Instruments for this lab • Thermometer (reference instrument for lab) • Thermistor • Thermocouple

7. 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

8. 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)

9. Thermistor • Omega OL-703-PP • 44018 linear thermistor element rated to 100C • Semiconductor device • Negative coefficient of resistivity

10. 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

11. Thermistor Resistance • RT = thermistor resistance • R0 = reference resistance •  = characteristic parameter (3500 – 4600K) • T = Temperature • T0 = reference temperature

12. Determining  • Plot 1/T versus lnRT • Slope of line = 

13. Wheatstone Bridge Circuit

14. 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

15. Step 2 – Measure Vcb • Vcb changes with RT • Determine RT = f(Vcb) • Determine T from RT

17. 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 range • Chromel = Ni Cr alloy (+) • Alumel = Ni Al alloy (-)

18. 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”

19. 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

20. 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

21. 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.