Advances in temperature calibration tools for use in sterilisers

Martin Riddett on temperature monitoring and sensor conformance in clinical and laboratory sterilisers

Autoclave sterilisers present temperature sensors with a very demanding operational environment in terms of high levels of pressure and moisture. Temperature sensors specified for autoclave applications, usually type T thermocouple or Pt100 thermometer, must be specially designed and constructed for such environments; reliability, accuracy and stability are essential for ensuring safe sterilisation procedures.

The Labfacility Autoclave Load thermocouple is designed to provide accuracy and stability in the rigorous environment of clinical and laboratory steriliser chambers. The design has been subjected to successful evaluation by CMI Healthcare, including leak testing in a vacuum chamber and processing through normal, porous load sterilisation cycles. The sensor complies with test standard HTM 2010, Clause 6.19/6.21.

The multiple seal construction ensures that moisture ingress is prevented, even at elevated pressure. The combination of a close tolerance type T thermoelement and rugged construction ensures high accuracy, stability and long-term reliability. Alternatively, Pt100 resistance thermometer sensors using similar construction methods can be supplied.

The sensor is protected by a 316-grade stainless steel sheath and bulkhead tube; rugged, flexible stainless steel armour conduit protects the 2m lead with 2.5m of silicone rubber exiting the bulkhead tube. The assembly tolerance complies with IEC 584-2 class 1 and colour code with IEC 584-3.

Validation of the sensors

All temperature sensors used in the sterilisation process must be routinely checked and/or calibrated in accordance with procedures described in HTM 2010 Part 3 or any other national standards. There are various ways of achieving such compliance, which typically involve sending the sensors to an accredited laboratory for certification, which is an expensive and time-consuming procedure.

A practical, cost-effective alternative method is to use an in-house, portable temperature calibration system from Labfacility. This provides a rapid and simple method of sensor checking and traceable calibration ‘on the bench’.

It 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 that allows direct comparison of the steriliser probes with the reference sensor. A simple calculation of difference in 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.

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.

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 steriliser probes are traceably calibrated in-house for full compliance with specified testing and validation procedures.

The accuracy of a type T, Class 1 thermocouple is similar to that of a Pt100 Class B; nominally ±0.5oC and ±0.3oC respectively, yet at 0oC a Class A Pt100 will provide an order greater accuracy.

Measurement uncertainty with such a system will be of the order of ±0.6oC for both types of sensor at a typical working temperature. 

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Martin Riddett is MD of Labfacility.

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