Measuring Temperature with a Current Excitation Thermistor

A thermistor is a semiconductor made from metal oxides, pressed into a small bead, disk, or other shape at high temperatures, and coated with epoxy or glass.

As with an RTD, by passing a current through a thermistor, you can read the voltage across the thermistor and thus determine its temperature. Unlike RTDs, thermistors have a higher resistance (anywhere from 2,000 to 10,000 Ω) and a much higher sensitivity (~200 Ω/°C). However, thermistors are generally used to measure temperature only up to the 300° C temperature range.

Because thermistors have high resistance, lead-wire resistance does not affect the accuracy of the measurements. Unlike RTDs, 2-wire measurements are adequate.

NI-DAQmx scales the resistance of a thermistor to a temperature using the Steinhart-Hart thermistor equation:

where T is the temperature in Kelvins, R is the measured resistance, and A, B, and C are constants provided by the thermistor manufacturer.