How to calibrate a Pt100

A Pt100 is a platinum resistance thermometer with a nominal resistance of 100 Ω at 0 °C. Industrial Pt100 sensors are made to tolerance classes defined in IEC 60751, but a tolerance class is not a calibration: calibration establishes the actual relationship between the sensor's resistance (or the indicated temperature of the instrument it is connected to) and temperature, with an uncertainty.

Calibration by comparison

• Choose calibration points across the range of use, including the extremes.
• Generate each temperature in a suitable medium — a stirred liquid bath, a dry-block calibrator or a chamber — and allow full stabilisation.
• Immerse the Pt100 and the reference thermometer to sufficient depth, close together.
• Record repeated readings of both; the reference must be traceably calibrated.
• Compute the correction (reference minus indicated) at each point and its uncertainty.

Where the uncertainty comes from

Typical contributions are the reference thermometer's calibration uncertainty and drift, the medium's stability and uniformity, immersion effects, self-heating, the readout resolution and, for the unit under test, repeatability and hysteresis. Each is expressed as a standard uncertainty, combined in quadrature, and expanded with a coverage factor (usually k = 2) following the GUM.

Reporting

The certificate should give the measured values, the corrections or deviations, the uncertainty and its coverage factor, the reference standards used with their traceability, and the environmental conditions. If a conformity statement to a tolerance class is made, the decision rule must be stated.

ValiTrac's AI assistant can retrieve evidence on this procedure from its approved sources and, where a numeric combination is needed, calls the deterministic uncertainty engine rather than estimating the arithmetic.