Industrial Temperature Measurement Standards

Industrial temperature measurement standards address sensor characteristics, tolerances, cable properties and instrument requirements in separate documents. A Pt100, thermocouple and temperature controller are parts of the same measurement chain. One standard number does not describe the accuracy of the complete system. Choosing between a Pt100 and a thermocouple also requires consideration of the application's temperature range and conditions.

Which standard covers which subject?

How should Pt100 and Pt1000 tolerances be assessed?

The names Pt100 and Pt1000 indicate nominal resistance at 0 °C: 100 Ω and 1000 Ω, respectively. These values are not accuracy classes. Read IEC 60751 tolerance classes together with sensor construction and the stated temperature range. The Callendar–Van Dusen conversion explains the mathematical relationship used to convert resistance into temperature; one linear coefficient is unsuitable for the entire range.

Two-, three- and four-wire RTD connections deal with lead resistance differently. In a two-wire connection, lead resistance adds to the measurement. Three-wire compensation depends on equal lead resistances; four-wire measurement largely removes the lead contribution. Also assess RTD self-heating and lead resistance errors: the measuring current can heat the sensing element.

Why select the thermocouple, reference junction and cable together?

The thermocouple type determines the applicable EMF–temperature relationship. Use reference tables, polynomials and ITS-90 for the correct type and range. IEC 60584-1 reference functions assume a 0 °C reference junction. In field instruments, cold-junction compensation accounts for the temperature at the connection point; measuring millivolts alone is insufficient to determine temperature accurately.

The difference between extension and compensating cables matters when selecting wiring. Check the type, polarity, cable class and operating temperature together; ordinary copper cable cannot replace every thermocouple cable run. For grounded, ungrounded and exposed junctions, assess response time and electrical isolation requirements. IEC 61515 requirements for the delivered product do not automatically establish the performance of a used sensor.

Does a calibration certificate cover the complete system?

Calibration involves comparison with a reference under specified conditions; uncertainty describes the evaluated dispersion associated with the result. Thermocouple system uncertainty extends beyond sensor tolerance. Review the certificate's measurement points, results, uncertainty and traceability. Where accredited service is required, the laboratory's scope should also match the selected method and range.

Calibrating the sensor separately and checking the chain, including sensor, cable, transmitter and display, are different tasks. Field installation and thermal contact require separate consideration. Set the calibration interval using service conditions, previous results and process requirements. Do not automatically assume the same annual interval for every application.

How is non-contact temperature measurement assessed?

RTD or thermocouple tolerance classes do not apply to non-contact measurement. ASTM E1256-17(2022) addresses performance tests for single-waveband radiation thermometers. For infrared instruments, also check emissivity and whether the measurement spot fits the target; a laser dot does not show the entire measured area.

Does a measurement standard also guarantee successful control?

EMC, electrical safety and temperature tolerance require separate assessments. Select requirements applicable to the instrument's scope. In PID temperature control, well-tuned parameters cannot correct the wrong sensor type or faulty wiring. A procurement specification stating sensor type, range, tolerance, connection, response time and calibration needs is more informative than requesting a standard number alone.