Page 276 - Instrumentation Reference Book 3E

260 Temperature measurement

Figure 14.1 8 Typical industrial resistance thermometers. Courtesy ABB Instrument Group

0 "C. At any other temperature Vt = ZtRt and the consist of a piece of the semiconductor to which
meter will indicate the difference between V, and two connecting wires are attached at opposite
VR which will be proportional to the tempera- sides or ends. Thermistors have a negative tem-
ture. The indicator must have a very high resist- perature coefficient; that is, as the temperature
ance so that the current in conductors 1 and 2 is rises the electrical resistance of the device falls.
essentially zero. See Part 3. This variation of resistance with temperature is
much higher than in the case of metals. Typical
14.4.2 Thermistors resistance values are 10kb2 at 0°C and 2000
at 100°C. This very high sensitivity allows
14.4.2.1 Negative temperature coefficient measurement or control to a very high resolution
tlzerrnistors
of temperature differences. The accuracy is not as
An alternative to platinum or nickel for resistance good as for a metallic resistance thermometer
thermometer sensing elements is a semiconductor owing to the difficulty in controlling the
composed of mixed metal oxides. The compos- composition of the thermistor material during
ition of these materials depends on the particular manufacture. The resolution differs across the
properties required. Combinations of two or usable span of the devices due to their non-line-
more of the following oxides are used: cobalt, arity. With the right choice of device char-
copper, iron, magnesium, manganese, nickel, acteristics it is nevertheless possible to control
tin, titanium, vanadium, and zinc. Devices made a temperature to within very close limits: 0.001
of these materials are called thermistors. They degree Celsius temperature change is detectable.