Page 92 - The Veterinary Laboratory and Field Manual 3rd Edition

The selection, use, maintenance and quality control of laboratory equipment and supplies 61

To ensure that an autoclave is working • Heating up of the water and air removal.
efficiently it is necessary to check that the tem- The water is heated up to its boiling point
perature/pressure readings are correct. The of 100°C. During this heating up the air is
usual method to verify this is to apply special being removed from the chamber. In order
autoclave tape to objects for sterilization. This to improve the removal of air, the water is
strip of self-adhesive ‘autoclave tape’ is impreg- allowed to boil some additional time after
nated with a chemical that changes colour if the the water has reached 100°C. The air in the
correct temperature/time has been employed. chamber will be flushed out with the steam.
A large range of chemical sterilization products Care should be taken to expel all air from
is available, such as Bowie and Dick test sheets, the autoclave before it is put under pressure
helix test. otherwise pockets of air will hinder penetra-
tion of steam into the articles to be sterilized.
• Building up pressure. Temperature increases
The basic steam sterilization process
to sterilization temperature. By closing a
Air is about twice as heavy as pure steam of the valve, the vessel is sealed and pressure and
same temperature. Therefore, empty containers temperature increase to the required level.
should be put upside down, the heavier air will • Sterilization time (holding time). During this
sink down and will be replaced by the lighter time, the temperature/pressure is held at the
steam (downward displacement). In order to let required level for sterilization.
the steam reach all parts, leave sufficient space • Reducing the pressure to atmospheric by
between the goods. releasing the steam. By opening a valve the
A full sterilization cycle based on the removal steam can escape and the pressure goes down.
of air by downward displacement will go through • Cooling down of the load. After sterilization
the following phases (Figure 2.22). the pressure and the temperature should

Figure 2.22 Sterilization cycle in graphic format.

Vet Lab.indb 61 26/03/2019 10:25