handbook of modern sensors

466 16 Temperature Sensors
Fig. 16.5. Transfer function of a KTY silicon temperature sensor.
is determined by it physical dimensions and the material resistivity. The relationship
between the resistance and temperature is highly nonlinear (Fig. 3.18 of Chapter 3).
Whenever high accuracy is required or the operating range is wide, the thermistor
characteristics should not be taken directly from a manufacturer’s data sheet. Typical
tolerances of the nominal resistance (at 25 ◦ C) for mass-produced thermistors are
rather wide: ±20% is quite common. Unless it was adjusted at the factory to a better
tolerance, to reach a high accuracy each such thermistor needs to be individually
calibrated over the operating temperature range. Manufacturers can trim a thermistor
by grinding its body to a required dimension that directly controls the nominal value of
resistance at a set temperature. This, however, increases cost. An alternative approach
for an end user is to individually calibrate the thermistors. Calibration means that a
thermistor has to be subjected to a precisely known temperature (a stirred water
bath is often employed 3 ) and its resistance is measured. This is repeated at several
temperatures if a multipoint calibration is needed. Naturally, a thermistor calibration
is as good as the accuracy of the reference thermometer used during the calibration.
To measure the resistance of a thermistor, it is attached to a measurement circuit
that passes through it electric current. Depending on the required accuracy and the
3 Actually, water is not used. Mineral oil or Fluorinert ® electronic fluid are more practical
liquids.