About temperature calculations
Industrial temperature measurement runs on two competing technologies with opposite strengths. RTDs exploit the predictable rise of platinum resistance with temperature and deliver accuracy and long-term stability. Thermocouples generate a small voltage at a junction of dissimilar metals, trading accuracy for range, ruggedness and speed.
Both are deceptively easy to get wrong in ways that produce plausible-looking readings. An RTD measured through long two-wire cable reads high — always high — because the instrument cannot distinguish cable resistance from sensor resistance. A thermocouple wired with ordinary copper silently relocates its reference junction to an unknown temperature. Neither failure announces itself; both show up as a number that looks perfectly reasonable and is quietly wrong.
These calculators implement the actual standards rather than linear approximations: the Callendar-Van Dusen equation of IEC 60751 for RTDs, and the full ITS-90 reference functions for all eight standard thermocouple types, with proper cold junction compensation.
How these tools fit together
- Choose the sensor for the duty. Accuracy and stability below about 500 °C favour an RTD; extreme temperature, vibration or fast response favour a thermocouple.
- Convert the raw signal correctly. Use the PT100/PT1000 calculator for resistance readings and the thermocouple calculator for millivolt readings — the latter needs the cold junction temperature to mean anything.
- Account for the wiring. For RTDs, quantify lead wire error before trusting a long cable run. The difference between 2-wire, 3-wire and 4-wire connection can be several degrees.
- Range the transmitter. Once the sensor is settled, the temperature becomes a 4-20 mA signal scaled with the same loop arithmetic as any other measurement.
Frequently asked questions
Is an RTD more accurate than a thermocouple?
Generally yes. A Class A PT100 is typically accurate to around ±0.15 °C where a Class 1 thermocouple is nearer ±1.5 °C, and RTDs drift far less over time. Thermocouples win on temperature range, ruggedness and response speed rather than accuracy.
What resistance does a PT100 read at 100 °C?
138.51 Ω, following the Callendar-Van Dusen equation of IEC 60751. The relationship is very slightly non-linear, which is why a fixed 0.385 Ω/°C slope is an approximation rather than the standard.
All calculators are provided for reference and education. Verify independently before use in safety-critical work — see our disclaimer.