What is an Autoclave?
An autoclave is a pressurized device that enables the heating of
aqueous solutions up to temperatures above the boiling point of water.
Under ordinary circumstances, heating water above the boiling point in an open vessel is impossible. Whenever the temperature rises up to 100 °C, water ceases to warm any more. This is due to extensive evaporation that occurs during boiling. The process of evaporation takes all the heat, thus preventing water from further heating. If water is boiled long enough, it all turns to vapor.
An autoclave is essentially a pressure cooker primarily used in the medical field for sterilizing medical instruments. An autoclave must maintain a temperature of at least 246 degrees for 30 minutes in order to fully sterilize the equipment.
There are several different kinds of autoclaves, all acceptable to use in the sterilization of the equipment. It is a good idea to ask to see the autoclave. Is it clean? More importantly, was the shop personnel more than happy to show it to you, or did they act like they had something to hide?
Also, keep in mind that the presence of an autoclave does not mean effective sterilization. Autoclaves need to be regularly tested to ensure that they are working properly.
If water is heated in a sealed vessel, however (such as an autoclave), it is possible to increase the boiling point. When the temperature reaches approximately 90 °C, extensive evaporation takes place. Water vapor, being a gas by nature, creates excessive pressure within the vessel. As vapor pressure in the vessel reaches the value that corresponds to the temperature, evaporation ceases. Thereby, not all water turns into vapor. The higher the temperature, the higher is the pressure of the vapor. The heat generated under pressure is called latent heat and has more penetrative power to squeeze through bacteria and even their dormant, heat-resistant form - the spores.
Autoclaves in Medicine and Autoclave Quality Assurance
A medical autoclave is a device that uses steam to sterilise equipment and other objects. This means that all bacteria, viruses, fungi, and spores are inactivated. However, in 2003 scientists discovered a single-cell organism, Strain 121, that survives autoclave temperatures. Prions also may not be destroyed by autoclaving.
Autoclaves work by allowing steam to enter, and maintaining pressure at 103 kPa (15 psi). This causes the steam to reach 121 °C, and this is maintained for at least 15 minutes.
The high pressure means autoclaves are constructed of strong metal, and are shut very tightly and securely.
Autoclaves are found in hospitals, microbiology labs, and other places that need to ensure sterility of an object.
Because damp heat is used, heat labile products (such as some plastics) cannot be sterilised this way or they will melt. Some paper, or other products that may be damaged by the steam, should also be sterilised another way. In stovetop autoclaves, items should always be separated to allow the steam to penetrate the load evenly.
There are physical, chemical and biological indicators that can be used to ensure an autoclave reached the correct temperature for the correct amount of time.
Chemical indicators can be found on medical packaging and autoclave tape, and these change colour once the correct conditions have been met. This indicates that the object inside the package, or under the tape, has been autoclaved sufficiently.
Biological indicators include Attest devices. These contain spores of a heat resistant bacterium Bacillus stearothermophillus. If the autoclave did not reach the right temperature, the spores will germinate, and their metabolism will change the colour of a pH-sensitive chemical.
Physical indicators often consist of an alloy designed to melt only after being subjected to 121 °C for 15 minutes. If the metal has melted, the change will be visible.
As well as these separate indicators, autoclaves have temperature and pressure gauges visible from the outside.
There are certain plastics that can withstand repeated temperature cycling greater than the 121 degrees Celsius required for this process. PFA is an example.
Computer-controlled autoclaves use an F0 (F-naught) value to control the sterilization cycle. F0 values are set as the number of minutes of equivalent sterilization at 121°C (e.g: F0 = 15 min.). Since exact temperature control is difficult, the temperature is monitored, and the sterilization time adjusted accordingly.
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