Temperature Probe and Sensor Calibration: RTDs, Thermocouples and Thermistors

Temperature probes and sensors sit at the heart of autoclaves, ovens, freezers, process lines and test rigs. Most of the error in a temperature measurement lives in the probe rather than in the display, so the probe is what has to be calibrated.

This page covers the three common sensor types, how they are calibrated, and how to read the tolerance classes printed on their datasheets.

Three sensor types, three behaviours

Tolerance classes on the datasheet

A new sensor already has an allowed error. For platinum RTDs to IEC 60751, Class A allows ±(0.15 + 0.002|t|) °C and Class B allows ±(0.30 + 0.005|t|) °C, where t is the temperature in °C. At 100 °C that is ±0.35 °C for Class A and ±0.80 °C for Class B. Thermocouple tolerances are set in ASTM E230. A type K thermocouple with standard limits is allowed ±2.2 °C or ±0.75% of reading, whichever is greater. If your process needs better than the class allows, a calibration with individual corrections is the way to get there.

How probes are calibrated

Most probe calibration is done by comparison. The probe under test and a reference thermometer, usually a calibrated platinum resistance thermometer, sit together in a stirred liquid bath or a dry-block calibrator at a series of set points. A few points have special value: an ice-point check at 0 °C quickly shows whether an RTD has shifted. For the trade-offs between equipment types, see dry-block vs liquid bath calibration.

Two things decide the quality of the result. The first is immersion depth: too shallow, and heat conducted along the sheath pulls the reading toward room temperature. The second is stability: readings should only be taken once the bath and both probes have settled.

Sensor-only or system calibration?

A probe can be calibrated alone, or together with the indicator, transmitter or data logger it plugs into. A system calibration gives one correction for the whole measurement chain, but it is only valid while that probe stays with that instrument. A sensor-only calibration lets you swap probes, but the readout needs its own calibration too. For RTDs, the certificate can also include Callendar-Van Dusen coefficients, so the readout can be programmed with the probe’s own curve.

Calibration intervals

Twelve months is a common starting point. Shorten the interval for thermocouples used above about 500 °C, for probes in critical sterilization or cold-chain service, and after any mechanical shock. Use the as-found results from each calibration to decide whether the interval can be extended.

Accreditation and certificates

Temperature measurement is on Techmaster’s ANAB ISO/IEC 17025 scope (certificate AC-1736) within the ranges listed there, so certificates carry the ANAB accreditation symbol along with as-found and as-left data and measurement uncertainty.

Khanh Nguyen

Khanh Nguyen is the Marketing Manager at Techmaster Electronics, a B2B marketing leader covering the test, measurement, and ISO/IEC 17025 accredited calibration industry in the United States.