PRESSURE AND AUTOCLAVING

One of the essential components in the use of autoclave is pressure. Autoclaves are usually operated at a higher pressure than the environment. High pressure is needed in order to achieve the sterilizing temperature of 121 degrees centigrade. Without pressure water can only boil at 100 degrees centigrade under normal atmospheric pressure. ( Of course at higher elevation such as on mountain tops, where the atmospheric pressure is less water boils at lower temperature!)
Generally pressure of about 15 psi is required for autoclaving. This pressure is achieved by heating the water in the sealed compartment of the autoclave. High pressure is due to the built up of the steam as the water is heated. There is a pressure safety valve in autoclaves that the high pressure is not overshot. ( This explains the regular hissing sound produced during autoclaving as excess pressure is regulated)
While it is very important to build the pressure to the correct value before autoclaving is initiated, it is also important that at the end of a sterilization that the pressure is brought back to normal values before the autoclave is opened to remove the sterilized items.
Heating and cooling processes take time. Most users are impatient for the time it takes to cool and lower the pressure. Impatience often results in users trying to speed up the release of pressure by opening the release valve too quickly. This action would result in the boiling over of the contents of the flasks and test tubes which could result in the wetting of the cotton plugs and increase the possibility of contamination.
Opening the doors to quickly could result in the sudden release of internal pressure by the autoclave. This could result in scalding of the body as the hot steam rushes out. Release of pressure must be done slowly and only when it reaches zero is the door of the autoclave opened

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