Page 175 - Mechanical Engineers' Handbook (Volume 2)

164 Temperature and Flow Transducers

Table 6 Variation of Thermistor Resistance with Temperature
Temperature ( C) Resistance ( ) Temperature ( C) Resistance ( )
80 1.66 M 0 7355.0
40 75.79 K 25 2252.0
30 39.86 K 100 152.8
20 21.87 K 120 87.7
10 12.46 K 150 41.9
Source: Reference 43.

The self-heating response is frequently discussed in terms of the dissipation constant of
the probe, in milliwatts per degree Celsium. Dissipation constants can range from 0.005
mW/ C to several watts per degree Celsius and can be estimated with acceptable accuracy,
given the geometry and thermal properties of the probe and its installation. The dissipation
constant should not be used to correct a reading for the self-heating effect; the uncertainty
in the correction is too high. If a calculation shows the self-heating effect to be significant,
pulse interrogation should be used.
Dissipation constants for two representative probes are given in Table 7.
The self-heating effect must be considered in calibration as well as in use. Calibration
baths frequently use stirred oil as the medium. Table 7 shows the self-heating effect in the
oil bath to be very low compared to that in air at 5 m/s. A probe calibrated in an oil bath
and used in air would be subject to different self-heating effects, and the user should be
aware of that.

Figure 24 Resistance ratio of a typical thermistor as a function of 1/T (K).