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Why bother with calibration and performance qualification testing of Autoclaves.

05 August 2009

Calibration is the process of determining the actual temperature inside an autoclave when a given temperature is set. So why bother?

Well, without a calibration – even with its uncertainties, the actual temperature inside the autoclave is unknown.  The electronics may be wrong, the temperature probe may be damaged, the pressure gauge – a very useful backup - may also be wrong.

But is it worth the risk if the process is not sterilising properly? Is it worth the risk, in this litigious world, if the autoclave is not covered by a calibration?

Calibrating, particularly if a UKAS calibration laboratory is employed provides a powerful demonstration, in a court of law, that due diligence has been properly exercised. It shows that ‘all reasonable care’ has been taken. They are the sole organization recognised by the UK government to assess calibration laboratories against international criteria.

What does it involve?
The calibration engineer will insert a calibrated probe into an autoclave and run it at a set temperature, taking sufficient readings to produce a calibration certificate.  The actual procedures used will vary from company to company but all UKAS accredited calibration laboratories will work to approved procedures.  The data is then used to produce a calibration certificate. The engineer may also attach seals to prevent access to areas that allow the calibration to be altered.

Adjustment.
Calibration does not always include adjustment of the autoclave controls. Often the manufacturers’ control software is not accessible to untrained, third party engineers, for obvious reasons. If the autoclave cannot be adjusted and is significantly out, then the set temperature can be adjusted to compensate the difference by using the data on the calibration certificate. If for any reason adjustment of an autoclave is not required the service engineer should be informed - preferably beforehand.

What does it cost?
This depends. The largest cost in the calibration is often getting the engineer to and from the site. Once on site the work is generally straightforward, if rigorously controlled. It therefore makes financial sense to choose a calibration company that has local engineers and is also UKAS accredited for the calibration. Calibration can also be performed as part of a routine service visit at agreed intervals. This is often an economical solution.

Performance qualification.
This is one of several generic terms for testing an autoclave - and its load - to ensure the load is properly processed.  Usually a calibrated, multi channel, data logger is employed for the measurement so that considerable numbers of points in the load, together with salient points on the autoclave, can be logged. Analysis of these results can point to many problems in the setup of the autoclave or in the way the load is contained.  Significant differences can be seen, for instance, between glass and plastic bottles or plastic and metal discard containers, or between single and double bagged discard loads.

Performance qualification is the only way to know what is actually happening to the load.

It is vital that the test load is typical of loads actually processed as the results cannot necessarily be read across.  A good calibration laboratory will consider and discuss these factors with the customer prior to their engineer arriving.  However, once the performance qualification is done, calibration alone thereafter is often sufficient – at least if the load does not change.

Uncertainties explained.
All measurements involve error. There are two sorts of error – Known errors, which can be partially compensated for and errors due to uncertainty, which cannot. The calibration certificate will indicate the result and show uncertainty and confidence levels. In other words, in the case of a 95% confidence level, the chance of the true temperature at any particular time during the test period being within the stated temperature, plus or minus the uncertainty,was  95%.  On a UKAS certificate the uncertainty will have been calculated to recognised criteria.
To give an example, if the result of a calibration was 121.5°C and the uncertainty  ±0.5°C with a 95% confidence level then, when the autoclave was calibrated, the true temperature inside had a 95% probability of being between 121°C and 122°C. Note that with a 95% confidence level there is still a 5% probability that it is not. This confidence level can easily be increased to 99% or further but at the expense of the uncertainty limits. In this instance with 99% confidence the uncertainty increases to ±0.75°C.

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