Thermistors
A thermistor is a piece of semiconductor made from metal oxides, pressed into a small bead, disk, wafer, or other shape, heated at high temperatures, and coated with epoxy or glass.
Like RTDs, Thermistors have a resistance that varies nonlinear with respect to 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 only up to the 300 °C temperature range.
NI-DMM 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.
Because thermistors have high resistance, lead-wire resistance does not affect the accuracy of the measurements. Unlike RTDs, 2-wire measurements are adequate.
NI 4065/4070/4071/4072 devices support the following sensors from Omega's 44000-Series. NI-DMM also supports user-specified A, B, and C coefficients through the custom thermistor type if none of the presets below match the transducer.
| Sensor | Nominal Resistance (25 °C) | Steinhart-Hart Coefficients |
|---|---|---|
| Omega 44004 | 2.25k | A =1.468e-3 B =2.383e-4 C = 1.007e-7 |
| Omega 44006 | 10k | A = 1.032e-3 B = 2.387e-4 C = 1.580e-7 |
| Omega 44007 | 5k | A = 1.285e-3 B = 2.362e-4 C = 9.285e-8 |
Cabling
Thermistor measurements are wired using the HI and LO connectors on the DMM:
Underlying DMM Measurement
To measure temperature with a thermistor, the DMM performs a 2-wire resistance measurement with the range, resolution, timing, and other measurement options that have been specified by the user. The resulting resistance is then converted to degrees Celsius by software using the Steinhart-Hart equation and A, B, and C coefficients.
When configuring a temperature transducer measurement, the user must specify the range and resolution of the underlying voltage or resistance measurement while configuring the DMM. The recommended range for a 1 to 10kΩ thermistor is 100 kΩ