Why are autoclaves pressurised?

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In this article, we highlight the importance of why autoclaves are pressurised and why devices need to meet the requirements of EN 285 and HTM2031.

So, why are Autoclaves pressurised?

A small amount of moisture is sometimes also important to prevent the steam from ‘superheating’, where steam acts more like hot air rather than a condensing vapour. Superheated steam is steam which has been heated above its saturation temperature and will not condense until it cools to its saturation temperature. Steam is required to heat the sterilizer and its contents to between 1340C and 1370C. As steam at atmospheric pressure at sea level (1.013 bars) is 100º C, it is necessary to pressurise it to achieve the higher temperatures required.

Steam for the sterilization of reusable medical devices should comply with the requirements of EN 285 and HTM2031. The quality of steam is critical. It must be clean so that when it condenses it leaves no undesirable residue on the products which have been processed. It should be ‘saturated’ steam (i.e. ready to condense when it meets a cool surface) but it must not carry additional droplets of water which may result in wet loads which would then require the load to be reprocessed. Dryness value of steam (commonly referred to as a dryness fraction) is measured on a scale of 0 to 1. At a dryness fraction of ‘0’ only liquid water is present i.e. no steam vapour. At a dryness fraction of ‘1’ liquid water has been converted to vapour (steam). Although ideally dry saturated steam should be used, small amounts of water can be tolerated by modern sterilization processes and so a limiting value for the dryness fraction is set at ‘0.9’ for porous loads and ‘0.95’ for loads with heavy metal items which may present particular problems with regard to drying (e.g. heavy surgical instruments such as mallets).

It is necessary to maintain a steam temperature between 1340C and 1370C for a minimum period of three minutes to achieve sterilization. Other temperature and time relationships, as denoted in HTM01-01, may be used for products which are heat-sensitive (thermo-labile). Reference should be made to the medical devices’ manufacturers reprocessing instructions.

For more information on what autoclaves we provide at Sychem, see our full range here.

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