Checking of thermocouple and platinum resistance thermometer (Pt100) sensors

Paul Boughton

When temperature sensors must be routinely checked and/or calibrated in accordance with in-house quality procedures or other, national standards, there are various ways of achieving such compliance. These typically involve sending the sensors to an accredited laboratory for certification, an expensive and time consuming procedure.
A practical, highly cost effective alternative method is to use an in-house, temperature calibration system from Labfacility; this provides a rapid and simple method of sensor checking and traceable calibration ‘on the bench’.
This comprises a thermal reference (dry block) calibrator, a UKAS certified reference thermometer (Pt100), a calibrated L200 8 channel indicator/logger and PC software: this, when used with any Windows PC represents a complete solution for on-site accreditation which allows direct comparison of the thermocouple or Pt100 probes (up to seven at a time) with the UKAS certified reference sensor.

A simple calculation of difference temperature (if any) between the logged values indicated by the reference and each test probe at a given, stable temperature (chosen by the user) in the dry block calibrator gives the error value for each sensor at particular temperatures. The procedure can be repeated for alternative temperatures set on the dry block as required (eg 70, 100, 125, 150˚C).
The system can even be used independently of a PC if desired; rather than working with logged data, readings for each probe can be taken manually by means of the L200 front panel key and differences calculated by the user. This is an equally valid procedure.
The design of the dry block ensures that all sensors located in its 'well' will be at exactly the same temperature. The necessary calculations can be performed in a spreadsheet by applying formulae to the relevant cells and the logged values imported into the worksheet.
On this basis, the temperature probes are traceably calibrated (UKAS traceable) in-house for full compliance with specified testing and validation procedures.
The accuracy of a type T, Class 1 thermocouple for example, is similar to that of a Pt100 Class B; nominally ±0.5˚C and ±0.3˚C respectively at 0˚C. A Class A Pt100 will provide an order greater accuracy however.
Measurement uncertainty with such a system will be of the order of ±0.6oC for both types of sensor at 250˚C.

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Labfacility Ltd is based in Bognor Regis, West Sussex, UK.

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