Page 174 - Mechanical Engineers' Handbook (Volume 2)

4 Thermistors 163

High-temperature thermistors (1000 C) have been made from doped ceramic materials
but with only moderate success.

4.3 Electrical Characteristics of Typical Probes
Thermistor probes vary in resistance from a few hundred ohms to megohms. Resistance is
frequently quoted at 25 C with no power dissipation in the thermistor. The commercial range
is from about 2000 to 30,000 .
The resistance–temperature characteristic of a thermistor depends on the material of
which it is made. Representative values of the sensitivity coefficient (percent change in
resistance per degree Celsius) are given in Table 5.
To illustrate the range of resistances encountered in practice, Table 6 shows the resis-
tance as a function of temperature for a typical probe whose resistance is 2252 at 25 C.
Thermistor resistance data are frequently shown as logarithm of resistance ratio versus
1/T. Such a presentation emphasizes the ‘‘almost linearity’’ of a single-term exponential
description of the resistance–temperature characteristic. The data listed in Table 6 are shown
in Fig. 24.
Proprietary probes are available that linearize thermistors by placing them in combi-
nation with other resistors to form a circuit whose overall resistance varies linearly with
temperature over some range. These compound probes can be summed, differenced, and
averaged like any linear sensor.
Modem manufacturing practices allow matched sets of thermistors to be made inter-
changeable within 0.1 C.

4.4 Thermal Characteristics of Typical Probes
Thermistor probes are generally interrogated using a current of 1–10 A, either ac or dc.
With a probe resistance of 10 , this results in power dissipation of 0.01 W that must be
transferred from the probe into its surrounding material. At the first instant of application,
this current has no significant effect on the measured resistance of the probe, but in steady
state, it results in the probe running slightly above the temperature of the medium into which
it is installed. This is referred to as the self-heating effect. Since thermistors are often used
where very small changes in temperature are important, even small amounts of self-heating
may be important.

Table 5 Variations of Thermistor Temperature Coefficient
with Temperature

Temperature ( C) Condition %/ C
183 Liquid oxygen 61.8
80 Dry ice 13.4
40 Frozen mercury 9.2
0 Ice point 6.7
25 Room temperature 5.2
100 Boiling water 3.6
327 Melting lead 1.4
Source: Reference 39.