Like the RTD, the thermistor is also a temperature-sensitive resistor. The
name thermistors is derived from the term "thermally sensitive resistors,"
since the resistance of the thermistor varies as a function of temperature.
While the thermocouple is the most versatile temperature transducer and
the RTD is the most linear, "most sensitive" are the words that best
describe thermistors. The thermistor exhibits by far the largest value
change with temperature of the three major categories of sensors.

A thermistor's high resistance change per degree change in temperature
provides excellent accuracy and resolution. A standard 2,000-ohm thermistor
with a temperature coefficient of 3.9%/°C at 25°C will have a resistance
change of 78 ohms per °C change in temperature. A 2000 O platinum
RTD would have a change of only 7.2 ohms under the same conditions. So,
a standard thermistor is over ten times more sensitive than a RTD. This
allows the thermistor circuit to detect minute changes in temperature that
could not be observed with an RTD or thermocouple circuit. A thermistor
connected to a bridge circuit can readily indicate a temperature change of
as little as 0.0005°C.

The cost of this increased sensitivity is loss of linearity, as the curves in
Figure 7-19 show. The thermistor is an extremely nonlinear device that is
highly dependent on process parameters. Consequently, manufacturers
have not standardized thermistor curves to the same extent as they have
RTD and thermocouple curves.

You can approximate an individual thermistor curve very closely by using
the Steinhart-Hart equation:

(7-11)

You can find the constants A, B, and C by selecting three data points on the
published data curve and solving the three simultaneous equations. When
you choose data points that span no more than 10°C within the nominal
center of the thermistor's temperature range, this equation approaches a
remarkable +0.01°C curve fit.

Example 7-8 illustrates a typical calculation to obtain the temperature for a
thermistor with a known resistance.

A great deal of effort has gone into developing thermistors that approach
a linear characteristic. These are typically three- or four-lead devices that
require the use of external matching resistors to linearize the characteristic
curve. Modern data acquisition systems with built-in microprocessors
have made this type of hardware linearization unnecessary.

The high resistivity of the thermistor affords it a distinct measurement
advantage. The four-wire resistance measurement is not required as it is
with RTDs. For example, a common thermistor value is 5,000 O at 25°C.
With a typical temperature coefficient of 4%/°C, a measurement lead
resistance of 10 O produces only a 0.05°C error. This is a factor of five hundred
times less than the equivalent RTD error.

Because thermistors are semiconductors, they are more susceptible to permanent
decalibration at high temperatures than are RTDs or thermocouples.
The use of thermistors is generally limited to a few hundred degrees
Celsius, and manufacturers warn that extended exposures, even well
below maximum operating limits, will cause the thermistor to drift out of
its specified tolerance.

Thermistors can be manufactured very small, which means they will
respond quickly to temperature changes. It also means that their small
thermal mass makes them susceptible to self-heating errors. Thermistors
are more fragile than RTDs or thermocouples, and you must mount them
carefully to avoid crushing or bond separation.