Page 20 - Instrumentation and Measurement

2.3.2 Electrical Thermometry
The expansion or contraction of solids, liquids, or gases, the change in electrical resistance of
conductors and semiconductors, thermoelectric e.m.f.s, and the change in the current across the
junction of semiconductor diodes and transistors are all examples of properties that change when the
temperature changes and can be used as basis of temperature sensors. The following are some of the
more commonly used temperature sensors.
2.3.2.1 Resistance Temperature Detectors (RTDs)
The resistance of most metals increases in a reasonably linear way with temperature (Figure 2.11)
and can be represented by the equation:

Rt = R0(1+αt)
where Rt is the resistance at a temperature t ͦ C, R0 the resistance at 0 ͦ C and α a constant for the metal,
termed the temperature coefficient of resistance. Resistance temperature detectors (RTDs) are simple
resistive elements in the form of coils of metal wire, e.g. platinum, nickel, or copper alloys. Platinum
detectors have high linearity, good repeatability, high long-term stability, can give an accuracy of ±
0.5% or better, a range of about -200 ͦ C to +850 ͦ C, can be used in a wide range of environments
without deterioration, but are more expensive than the other metals. They are, however, very widely
used. Nickel and copper alloys are cheaper but have less stability, are more prone to interaction with
the environment, and cannot be used over such large temperature ranges.

FIGURE 2.11 Resistance variation with temperature for metals.
The most common method for measuring the resistance of an RTD is to use a Wheatstone bridge
circuit. Figure 2.12 shows the arrangement for a two-wire RTD. An electrical excitation current is
passed through the bridge, and the RTD and bridge output voltage is an indication of the RTD
resistance. The circuit uses a very stable excitation power source, three high-precision resistors that
have a very low temperature coefficient, and a high-input impedance amplifier to measure the
resistance change of the RTD with changes in temperature.

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