Thermistor Calibration for High Accuracy Measurements

CLEVELAND - March 7, 2024 - PRLog -- Calculating Steinhart-Hart Coefficients for High-Precision Readings

Thermistors are commonly used by technicians and engineers to measure temperature with high accuracy. They change resistance in a predictable but non-linear way, providing better accuracy than thermocouples or RTDs within a narrower temperature range. Thermistor calibration is therefore crucial.

One challenge with thermistors is the non-linear behavior when calculating temperature from resistance. The Steinhart-Hart equation is used for more accurate readings, especially in data-logging applications like measuring water or skin temperatures. It helps provide precise results across different temperature ranges.

There are high-quality data loggers like dataTaker DT8x, Grant SQ16, and VersaLog VL-TH that let you input coefficients to convert resistance to temperature. We provide tech support to help with this.

Thermistor manufacturers may not provide Steinhart-Hart coefficients, but users can derive them from resistance vs temperature tables or self-validate by measuring resistance at known temperatures. Online calculators can speed up this process.

Deriving Steinhart-Hart Coefficients for Thermistor Calibration:

If your thermistor manufacturer doesn't give you the coefficients, you can find them yourself. Get three resistance values at known temps, then use the formula to find the A, B, and C coefficients.

The Steinhart-Hart equation is commonly defined as:
where:

• T is the temperature (given in kelvins)
• R  is the resistance at T (given in ohms)
• A, B, and C are the Steinhart–Hart Coefficients which differ according to your thermistor model/type and its particular temperature range
• Ln is the natural logarithm

The equation is sometimes presented as containing a term, but this does not significantly improve the conversion accuracy.

To find the Steinhart–Hart coefficients, you need to know at least three operating points. For this, we use three values of resistance data for three known temperatures.

After inputting the values for〖 R〗_1, R_2 and R_3 giving resistance at the temperatures at, and  (in degrees Kelvin), you can determine the Steinhart-Hart coefficients A, B, and C:

If instead, you want to find the resistance of a thermistor given its temperature, you must use the inverse of the Steinhart–Hart equation.

Our engineers developed a Steinhart-hart thermistor coefficient calculator in the form of an Excel spreadsheet that does this automatically. The figure below shows an example of how to enter your actual temperature in Celsius and Kelvin (Col. B and C) and your sensor's resistance (Col. D) at 3 reference points (T1-T3) to derive the 3 Steinhart-Hart coefficients (A, B, C).

To get more information on thermistor calibration or thermistor data loggers, contact a CAS Data Logger Applications Specialist at (800) 956-4437 or visit us at https://www.dataloggerinc.com.