Thermistors
About thermistors devices
Thermistors are manufactured from beads
of semiconductor material prepared from oxides of the iron group of
metals such as chromium, cobalt, iron, manganese and nickel. The
resistance of such materials varies with temperature according to the
following expression:
R = Ro exp[β(1/T — 1/To)]
This relationship exhibits a large negative temperature coefficient
(i.e. the resistance decreases as the temperature increases), and so
is fundamentally different from the relationship for the resistance
thermometer, which shows a positive temperature coefficient. The form
of equation above is such that it is not possible to make a linear
approximation to the curve over even a small temperature range, and
hence the thermistor is very definitely a non-linear instrument.
The major advantages of thermistors are their relatively low cost and
their small size. This size advantage means that the time constant of
thermistors operated in sheaths is small. However, the size reduction
also decreases its heat dissipation capability, and so makes the
self-heating effect greater. In consequence, thermistors have to be
operated at generally lower current levels than resistance
thermometers and so the measurement sensitivity afforded is less. Like
resistance thermometers, the resistance of a thermistor is usually
measured with d.c. bridge.
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Temperature Measurements
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