The principles used in the measurement of pressure are also applied in the measurement of temperature, flow and liquid levels. Hence some of the working principles of the instruments are repeated.
Resistance-temperature detectors, or resistance thermometers, employ a sensitive element of extremely pure platinum, copper, or nickel that provides definite resistance value at each temperature within its range.
Principle of working: The resistance of a conductor changes when its temperature is changed. This property is utilized for measurement of temperature. The variation of resistance R with temperature T (ºK) can be represented by the following relationship for most of the metals as:
RTD - resistance temperature detector:
All metals produce a positive change in resistance with temperature. The requirements of a conductor material to be used in RTDs are :
(i) The change in resistance of material per unit change in temperature should be as large as possible. This implies a metal with a high value of resistivity should be used for RTDs.
ii) The material should have a high value of so that minimum volume of material is used Rt = Rref ( 1 αΔT)
A high value of is desirable in a temperature-sensing element so that a substantial change in resistance occurs for a relatively small change in temperature.
This change in resistance (R) can be measured with a Wheatstone bridge which may be calibrated to indicate the temperature that caused the resistance change rather than the resistance itself.
Fig. given below shows the variation of resistance with temperature for several commonly used materials.
The graph indicates that the resistance of platinum and copper increases almost linearly with increasing temperature, while the characteristic for nickel is decidedly nonlinear.