Temperature Measurements - PDF Free Download

A transducer is a device that converts some mechanical quantity into some measurable electrical quantity. Through a calibration procedure, the sensitivity of the transducer can be obtained INPUT Physical Phenomenon Pressure, Temperature, Strain, Displacement, Velocity, Acceleration, etc transducer Volts per Engineering Unit V/EU OUTPUT Electrical Signal related to Physical Phenomenon DC voltage, AC voltage, current, resistance, etc Dr. Peter Avitabile University of Massachusetts Lowell Temperature - 122601-2 Copyright 2001

There are two common devices used to measure pressure Thermocouples esistance Temperature Detectors (TD) Both will be considered here Dr. Peter Avitabile University of Massachusetts Lowell Temperature - 122601-3 Copyright 2001

Thermocouples Any two metals connected together develop a voltage which is a function of the temperature of the junction These is due to the Seebeck Effect Fig 9.14 Complications 1) voltage must be made with no current flow 2) connection to measuring device results in added junction 3) voltage depends on composition of metals used Dr. Peter Avitabile University of Massachusetts Lowell Temperature - 122601-4 Copyright 2001

Thermocouples Solutions for Complications 1) high impedance voltmeters used - draw little current 2) extra connection with ref. junction at 0oF (ice bath) 3) only certain well-defined materials used (well-known traceable NBS Standards) Fig 9.15 Dr. Peter Avitabile University of Massachusetts Lowell Temperature - 122601-5 Copyright 2001

Factors for Selecting Thermocouples 1) sensitivity 2) linearity 3) stability 4) corrosion resistance 5) temperature range 6) cost Dr. Peter Avitabile University of Massachusetts Lowell Temperature - 122601-6 Copyright 2001

Alternate Configuration for Connecting Thermocouples For multiple measurements connected to the DAS, each thermocouple is attached to one channel of the DAS in an insulated, temperature controlled box (one channel reserved for the reference junction) Fig 9.19 Dr. Peter Avitabile University of Massachusetts Lowell Temperature - 122601-7 Copyright 2001

esistance Temperature Detector (TD) Electrical resistance of metals is a function of metal temperature. Therefore, a length of wire with a resistance measurement is essentially a temperature measurement -->> TD - more accurate than thermocouples - generally have better linearity characteristics - temperature is measured directly - not relatively - more stable -> less likely to change with time - physically larger -> poor spatial resolution & slower response Dr. Peter Avitabile University of Massachusetts Lowell Temperature - 122601-8 Copyright 2001

Calendar-VanDusen Equation describes the TD T = o ( ( ( )( ) ( )( ) 3 ) 1+ α T δ 0.01T 1 0.01T β 0.01T 1 0.01T The constants in the equation depend on the metal purity Two type of TDs are shown: 1) coiled platinum wire 2) thin film sensor It is critical to minimize the strain due to thermal expansion since strain causes change in resistance Fig 9.20 Dr. Peter Avitabile University of Massachusetts Lowell Temperature - 122601-9 Copyright 2001

Wheatstone Bridge used to measure the TD The bridge is slightly different since the resistance of the lead wire is included - the lead wire may have an effect due to local temperature variations which affect the resistance Neglecting the lead wire resistance gives the following TD = 2 V V supply supply 2V + 2V o o Fig 9.21a Note: The change in resistance is large compared to the strain gage. Simple linear charaterization is not possible for the TD Dr. Peter Avitabile University of Massachusetts Lowell Temperature - 122601-10 Copyright 2001

Wheatstone Bridge used to measure the TD An alternate 3 wire TD bridge uses a third wire which essentially compensates to minimize the effect on the voltage measurement The TD resistance then becomes Fig 9.21b TD = 2 V V s s + 2V 2V o o lead 4Vo V + 2V s o The second term in this equation is very small Dr. Peter Avitabile University of Massachusetts Lowell Temperature - 122601-11 Copyright 2001