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Under ordinary circumstances (at standard pressure), liquid water cannot be heated above 100 °C in an open vessel. Further heating results in boiling, but does not raise the temperature of the liquid water. However, when water is heated in a sealed vessel such as an autoclave, it is possible to heat liquid water to a much higher temperature. As the container is heated the pressure rises due to the constant volume of the container (see the ideal gas law). The boiling point of the water is raised because the amount of energy needed to form steam against the higher pressure is increased. This works well on solid objects; when autoclaving hollow objects, however, (hypodermic needles, tools, etc.), it is important to ensure that all of the trapped air inside the hollow compartments is removed. Autoclaves may achieve air removal by downward displacement, super atmospheric, transatmospheric or sub-atmospheric pulses.
Autoclaves are widely used in microbiology, medicine, veterinary science, dentistry, podiatry and metallurgy. The large carbon-fiber composite parts for the Boeing 787, such as wing and fuselage parts, are cured in large autoclaves.
Autoclaves in medicine
A medical autoclave is a device that uses steam to sterilize equipment and other objects. This means that all bacteria, viruses, fungi, and spores are inactivated. However, in 2003 scientists discovered a single-celled organism, Strain 121, that survives traditional autoclave temperatures. Prions, like those associated with Creutzfeldt-Jakob disease, also may not be destroyed by autoclaving at the typical 121 degrees Celsius for 15 minutes or 134 degrees Celsius for 3 minutes, but can be destroyed with a longer sterilisation cycle of 134 degrees Celsius for 18 minutes.
Autoclaves are found in many medical settings and other places that need to ensure sterility of an object. They were once more common, but many procedures today use single-use items rather than sterilized, reusable items. This first happened with hypodermic needles, but today many surgical instruments (such as forceps, needle holders, and scalpel handles) are commonly single-use items rather than reusable. See waste autoclave.
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 must also be sterilized another way. In all autoclaves, items should always be separated to allow the steam to penetrate the load evenly.
Autoclaving is often used to sterilize medical waste prior to disposal in the standard municipal solid waste stream. This application has grown as an alternative to incineration due to environmental and health concerns raised by combustion byproducts from incinerators, especially from the small units which were commonly operated at individual hospitals. Incineration or a similar thermal oxidation process is still generally mandated for pathological waste and other very toxic and/or infectious medical wastes.
Unlike the humid environment produced by conventional steam, the unsaturated chemical vapor method is a low-humidity process. No time-consuming drying phase is needed, because nothing gets wet. The heat-up time is shorter than for most steam sterilizers, and the heaters stay on between cycles to minimize warm-up time and increase the instrument turnover.
Autoclave quality assurance
There are physical, chemical, and biological indicators that can be used to ensure an autoclave reaches the correct temperature for the correct amount of time.
Chemical indicators can be found on medical packaging and autoclave tape, and these change color once the correct conditions have been met. This color change 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, Geobacillus stearothermophilus. If the autoclave does not reach the right temperature, the spores will germinate, and their metabolism will change the color of a pH-sensitive chemical. Physical indicators often consist of an alloy designed to melt only after being subjected to 121°C or 249°F for 15 minutes. If the alloy melts, the change will be visible.
In addition to these indicators, autoclaves have timers, temperature and pressure gauges that can be viewed from the outside.
There are certain plastics that can withstand repeated temperature cycling greater than the 121°C or 249°F required for the autoclaving process. PFA is an example.
Some 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 or 249°F (e.g: F0 = 15 min.). Since exact temperature control is difficult, the temperature is monitored, and the sterilization time adjusted accordingly.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Autoclave". A list of authors is available in Wikipedia.|