Page 21 - Instrumentation and Measurement

Figure 2.12. Two-wire RTD bridge circuit
2.3.2.2 Thermistors (Thermally sensitive resistor)
Thermistors are semiconductor temperature sensors made from mixtures of metal oxides, such as
those of chromium, cobalt, iron, manganese, and nickel. The resistance of thermistors decreases in a
very non-linear manner with an increase in temperature, Figure 2.13 illustrating this. The change in
resistance per degree change in temperature is considerably larger than that which occurs with metals.
For example, a thermistor might have a resistance of 29 kΩ at -20 ͦ C, 9.8 kΩ at 0 ͦ C, 3.75 kΩ at 20 ͦ
C, 1.6 kΩ at 40 ͦ C, 0.75 kΩ at 60 ͦ C. The material is formed into various forms of element, such as
beads, discs, and rods (Figure 2.14).
Thermistors are rugged and can be very small, so enabling temperatures to be monitored at virtually
a point. Because of their small size they have small thermal capacity and so respond very rapidly to
changes in temperature. The temperature range over which they can be used will depend on the
thermistor concerned, ranges within about -100 ͦ C to +300 ͦ C being possible. They give very large
changes in resistance per degree change in temperature and so are capable, over a small range, of
being calibrated to give an accuracy of the order of 0.1 ͦ C or better. However, their characteristics
tend to drift with time. Their main disadvantage is their non-linearity. Thermistors are commonly
used to monitor the coolant temperatures and the outside and internal air temperatures in automobiles.

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