Page 277 - Instrumentation Reference Book 3E

Measurement techniques: electrical 261

Red I

(a)
Blue -
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// 2 White % RT I 100 R @Q"C

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(C)
Figure 14.19 Connectionsfor compensation of resistance thermometer leads.

The total range that can be measured with able in metal encapsulations like those used for
thermistors is from -100°C to +3OO0C. How- platinum resistance thermometers.
ever, the span cannot be covered by one thermis- The big disadvantage of thermistors is that
tor type; four or five types are needed. their characteristics are non-linear. The tempera-
The physical construction of thermistors covers ture coefficient of resistivity cv at any temperature
a wide range. The smallest are encapsulated in within the range of a sensor is given by:
glass or epoxy beads of 1-2.5 mm diameter; bigger
ones come as discs 5-25mm diameter or rods a = -B/T~ (14.15)
1-6mm diameter and up to 50mm length. The
bigger devices are able to pass quite high currents where B is the characteristic temperature constant
and so o?erate control equipment directly with- for that thermistor and Tis temperature ir? Kel-
out need of amplifiers. Thermistors are also avail- vin. The units of cv are ohms K-'.