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RTD & Thermocouple Converter

Convert resistance or millivolts to exact temperatures.

Sensor Configuration

Ω

Input resistance in Ohms (Ω) for PT100.

Estimated Temperature

0.00 °C
32.00 °F
info
Disclaimer of Liability: This calculator is provided for educational and estimation purposes only. By using this tool, you agree to discharge AutomationView of any liability for direct, indirect, or consequential damages resulting from its use. It is your strict responsibility to independently verify all calculations, validate the results against official manufacturer documentation, and ensure compliance with all applicable safety and engineering standards before implementing any parameters in a production environment.

Converting Temperature Sensor Signals

In industrial control systems, temperature measurement relies heavily on Resistance Temperature Detectors (RTDs) like the Pt100, and Thermocouples (Type K, Type J, etc.). Because these sensors operate on different physical principles, their signal conversion methods are entirely distinct.

1. RTD (Pt100) Conversion: The Callendar-Van Dusen Equation

A Pt100 RTD has a resistance of precisely 100 Ω at 0°C. While the resistance-to-temperature relationship is somewhat linear, highly accurate industrial applications rely on the Callendar-Van Dusen equation.

For a basic linear approximation over small ranges:

$R_t = R_0 times (1 + alpha times T)$

  • $R_t$: Measured resistance at temperature T.
  • $R_0$: Base resistance (100 Ω for Pt100).
  • $alpha$: Temperature coefficient (typically 0.00385 Ω/Ω/°C).

For high precision, the full IEC 751 standard polynomial is used, applying different coefficients ($A$, $B$, and $C$) depending on whether the temperature is above or below 0°C.

2. Thermocouple (Type K & J) Conversion: NIST Polynomials

Thermocouples generate a small millivolt (mV) electromotive force (EMF) based on the temperature difference between the measuring junction and the reference junction. This relationship is highly non-linear.

Temperature is calculated using high-order polynomials provided by the National Institute of Standards and Technology (NIST):

$T = a_0 + a_1E + a_2E^2 + a_3E^3 + … + a_nE^n$

Where $E$ is the measured voltage in mV, and $a_0…a_n$ are specific coefficients for the thermocouple type and temperature range.

Cold Junction Compensation (CJC)

Because thermocouples only measure a differential temperature, the absolute temperature of the connection terminal (the “Cold Junction”) must be measured separately—usually by a thermistor or RTD. This CJC temperature is added to the calculated thermocouple value to yield the true process temperature.