Introduction
This is a sterilisation method which is used for aqueous preparations and surgical dressings by saturating items in steam under pressure. It is the introduction of pressure to steam which raises the temperature and so therefore the more pressure introduced the higher the temperature will rise. There have been a number of time/temperature combinations trailed but both the USP 23 (1995) and BP (1993) recommend a temperature of 121˚C maintained for 15 minutes is the preferred combination.
To ensure effective sterilisation takes place the quality of steam needs to be correct. Steam production can be achieved in two ways depending on the scale in which it is required;
- Small scale: produced from water within the steriliser and because the water is present this is known as Wet Saturated Steam.
- Large Scale: Steam may be piped from a separate boiler and is known as Dry Saturated Steam.
Saturated Steam
This is where steam is at a temperature which corresponds to the liquid boiling point appropriate to its pressure.
Super heated Steam
This is where the temperature is no longer at the boiling point which corresponds to its liquid boiling point at its appropriate pressure. This type of steam is usually avoided because it becomes more like hot air and so therefore less effective.
The Advantages of saturated Steam
It flows quickly to every item in each load including porous items.
It heats up rapidly
Condensation produced on cooling contributes to lethality by coagulating microbial protein.
Items sterilised are free from toxic contamination
Presence of Air
Air is a poor conductor and so can provide a barrier to penetration. In small sterilisers there is more likely to be turbulence which disturbs air and therefore disperses the air film, however in large sterilisers is it vital that air is removed as it can form barriers on surfaces and along with the lack of turbulence to disperse air it can stop penetration of the steam.
An Autoclave is a sterilizer which uses pressurised steam in order to eradicate micro-organisms from equipment and other objects. The optimum temperature to ensure that all micro-organisms are eradicated is 121°C which should be carried on for 15 minutes at a pressure of 202kPa (2.0 bar). There are different types of autoclaves; Portable autoclaves which are used in small scale production and laboratory work, they are generally found in two types; pressure controlled autoclaves, where the only way to measure who goes in is by pressure gauges, the air must then be removed before sterilization begins, therefore making the general process longer. There are also temperature controlled autoclaves where the thermometers will show the correct exposure temperature has been reached. There are also large scale autoclaves, which are used in larger areas or manufacture.
Most autoclaves work in the same general way:
- Loading the items which need to be sterilized, this should be so the items are evenly through the autoclave, allowing for even heat distribution throughout.
- The Air should them be removed from the autoclave by creating a vacuum before the steam is introduced.
- Steam which has been passed through a separating system to reduce moisture content will then be introduced into the autoclave, until the correct temperature is reached. Some plastic material may burst is too much steam pressure introduced so special care may need to be taken.
- Once the required temperature has been reached the correct exposure time should be applied
- Once the time for correct sterilization has been counted down the steam supply should be cut off and allowed to leave the autoclave
- The cooling process can begin. This will vary depending on the items inside the autoclave; some items may be affected if the autoclave is opened whilst the items are still too hot. A reduction in cooling times may be reduced using a fine mist of cool sterile water. This is thought to dramatically reduce the cooling temperature from 3hours to 10 minutes of a load of 24 X 500ml bottles.
We have already established that increasing the pressure increases the heat of the steam. The table below shows the temperature ranges available at different levels of pressurisation.
PSI/ lbf/in2 Temperature Range
10 115 to 116
15 121 to 123
20 126 to 129
32 134 to 138
The practical use of the different levels of pressure and the effect on heat ranges is the effect this have on the minimum time required for sterilisation to occur. The table below shows the minimum time required for sterilisation to occur at different levels of pressurisation.
PSI/ lbf/in2 Minimum Time Required (min)
10 30
15 15
20 10
32 3
Higher pressures not in result in higher temperatures but also a reduction in the minimum time required for sterilisation to be complete. This can be advantageous in larger industries or hospital where large amounts of sterilisation are required as each individual load can be completed at a faster rate however the machinery required to do this can be expensive. Higher pressures can also be useful if large quantities are required to be sterilised as this can effect the minimum time required. Smaller pressures may be required if the material or product being sterilised can not withstand the higher levels of temperature or pressure and as 121°C is the optimum temperature for eradicating micro-organisms there is no need to go above the pressure of 15lbf/in2 for effectiveness.
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