Thermocouples measure temperatures over a broad range and at high temperatures. But when a task calls for precision measurements over a smaller, lower temperature range, somewhere from about -80 to 300C, thermistors do the trick. And because thermistors come in small packages, they exhibit fast response times. Manufacturers offer devices with either a positive temperature coefficient (PCT) or a negative temperature coefficient (NTC), which describes how their resistance changes with temperature.
You can indirectly measure resistance by passing a current through a resistor and measuring the voltage across it (R = E/I), but a current can heat a thermistor, which alters its resistance and throws off your calculated value. A Wheatstone bridge or a commercial instrument that applies only a few microamps to a thermistor will provide accurate resistance or temperature information.
In some cases, self heating works to your advantage. A known preset current will heat a thermistor to a specific temperature. But, if air blows by the device or if a liquid level reaches it, for example, its temperature will change. So, you now have a wind or level sensor instead of a “thermometer.”
The graph above shows the nonlinear resistance vs. temperature relationship for an NTC thermistor. After you measure a resistance, you can pick temperature values from a thermistor’s resistance chart or you can compute a temperature value in units of Kelvins (°C = K -273.15). In 1968, John Steinhart and Stanley Hart published the relationship:
1/T = A+B(ln(R)) +C(ln(R³))
Engineers now call this the Steinhart-Hart equation. (The equation does not include an R² term.)
Thermistor manufacturers supply the three coefficients, A, B and C, for each thermistor type, so users can calculate a temperature in units of Kelvins based on a resistance measurement. If you must make high-accuracy measurements, you can fine tune the three Steinhart-Hart coefficients for your specific thermistor. Take resistance measurements at three known well-temperatures and simultaneously solve three Steinhart-Hart equation for the three unknown coefficients. The graph shown here points out three calibration temperatures, but you can choose others across the temperature range you plan to work with.
If you’d rather not solve equations, ILX Lightwave offers two worksheets you can use to calculate the Steinhart-Hart coefficients. The company uses thermistors to regulate the temperatures of its laser sources.
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 This graph shows the nonlinear relationship between resistances and temperatures for a commercial thermistor. Calibration at three points lets manufacturers or users calculate temperature based on a measured resistance. Courtesy of Measurement Specialties - Temperature Products Group )BetaTHERM). |
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