Temperature monitoring and sensor conformance in sterilisers

Autoclave sterilisers present temperature sensors with a very demanding operational environment in terms of high levels of pressure and moisture. Ken Jefferies reports.

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 specially designed to provide the best possible accuracy and stability in the rigorous environment of clinical and laboratory steriliser chambers. The design has been subjected to successful, thorough 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 special multiple seal construction of this Autoclave Thermocouple ensures that moisture ingress is prevented, even at elevated pressure.

The combination of a close tolerance type T thermoelement and exceptional rugged construction ensure high accuracy, high stability and long-term reliability. Alternatively, Pt100 resistance thermometer sensors using similar construction methods can be supplied.

Validation of temperature 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, an expensive and time consuming procedure.

A practical, highly cost effective alternative method is to use an in-house, c '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 sterilizer 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 (for example, 70, 100, 125, 150°C).

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 (UKAS traceable) in-house for full compliance with specified testing and validation procedures.

System accuracy

The accuracy of a type T, Class 1 thermocouple 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 +/-0.6°C for both types of sensor at a typical working temperature.

For more information at www.scientistlive.com/eurolab

Ken Jefferies is a Technical Consultant to Labfacility Ltd, Bognor Regis West Sussex, UK. www.labfacilty.com

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