©1997,2001 by M. Kostic Ch.8: Temperature Measurement Temperature:Temperature: measure of hotness or coldness indicating the direction in which heat (energy) will spontaneously flow, i.e., from a hotter body (one at a higher temperature) to a colder one (one at a lower temperature). Temperature is not the equivalent of the system energyTemperature is not the equivalent of the system energy Historical backgroundHistorical background

©1997,2001 by M. Kostic Temperature Measurement Devices thermocouples, thermocouples, resistive temperature devices (RTDs and thermistors),resistive temperature devices (RTDs and thermistors), infrared radiators,infrared radiators, bimetallic devices,bimetallic devices, liquid expansion devices, andliquid expansion devices, and change-of-state devices.change-of-state devices.

©1997,2001 by M. Kostic 8.2: Temperature Standards Temperature scaleTemperature scale (ITS-90)(ITS-90) Definition of the degreeDefinition of the degree Kelvin K, defined as the 1/ fraction of the thermodynamic temperature of the triple point of water ( TPW ). Fixed (reference) pointsFixed (reference) points Interpolation between the fixed pointsInterpolation between the fixed points

©1997,2001 by M. Kostic 8.3: Thermometry Based on Thermal Expansion Liquid-in-glass thermometersLiquid-in-glass thermometers Bimetallic thermometersBimetallic thermometers

©1997,2001 by M. Kostic 8.4: Electrical Resistance Thermometry RTD: Resistance Temperature DetectorsRTD: Resistance Temperature Detectors RTD measurement circuits (Bridge circuits)RTD measurement circuits (Bridge circuits) ThermistorsThermistors

©1997,2001 by M. Kostic 8.5: Thermoelectric Temperature Measurements Seebeck effect: in dissimilar metals open circuit emf proportional to  TSeebeck effect: in dissimilar metals open circuit emf proportional to  T Peltier (refrigeration) effect:  T proportional to current I (reversible conversion of energy)Peltier (refrigeration) effect:  T proportional to current I (reversible conversion of energy) Thomson (later Kelvin) effect: effect due to T gradients along a single conductor under current IThomson (later Kelvin) effect: effect due to T gradients along a single conductor under current I For I=0 only Seebeck effect!

©1997,2001 by M. Kostic Fundamental Thermocouple Laws Law of homogeneous material (no emf at  T)Law of homogeneous material (no emf at  T) Law of intermediate material (no emf at uniform T for dissimilar metals)Law of intermediate material (no emf at uniform T for dissimilar metals) Law of successive or intermediate temperature: emf 1-3 =emf 1-2 +emf 2-3Law of successive or intermediate temperature: emf 1-3 =emf 1-2 +emf 2-3

©1997,2001 by M. Kostic Measurement with Thermocouples TC measurement circuits with(out) reference junctionsTC measurement circuits with(out) reference junctions

©1997,2001 by M. Kostic Thermocouple Standards (“+” vs. “-”) E: Chromel vs. Constantan J: Iron vs. Constantan K: Chromel vs. Alumel S: Platinum/10% rhodium vs. Platinum T: Copper vs. Constantan

©1997,2001 by M. Kostic Thermocouple emf (Tables) Tables Emf = E =  c i T i i=0,1,2,…8 (or 14) see Text TABLE 8.7 on p.313 or: see Text TABLE 8.7 on p.313 or: Reference Functions for Thermocouple Types J and T: Table 8.7 (MathCAD) Reference Functions for Thermocouple Types J and T: Table 8.7 (MathCAD)

©1997,2001 by M. Kostic Multiple- Junction TC Circuits Thermopiles (in series)Thermopiles (in series) Parallel arrangementsParallel arrangements

©1997,2001 by M. Kostic Multiple-Junction TC Circuits Thermopiles (in series)

©1997,2001 by M. Kostic For the given data (see the next slide), determine the required number of thermopile junctions N for the T-type thermocouple to yield an uncertainty in heat loss Q of ±5%, assuming the uncertainty in all other variables, but  T, may be neglected. Problem 8.29:

©1997,2001 by M. Kostic Find N Problem 8.29: Table 8.7 (MathCAD) Given Find u  t N=4 N N Q R =Q N