How does the autoclave itself work? The diagram of an autoclave depicts the simplicity of its operation. Basically, steam enters the chamber jacket, passes through an operating valve and enters the rear of the chamber behind a baffle plate. It flows forward and down through the chamber and the load, exiting at the front bottom. A pressure regulator maintains jacket and chamber pressure at a minimum of 15 psi, the pressure required for steam to reach 121ºC (250ºF). Overpressure protection is provided by a safety valve. The conditions inside are thermostatically controlled so that heat (more steam) is applied until 121C is achieved, at which time the timer starts, and the temperature is maintained for the selected time.Howard Judelson 6/28/04 The temperature profile for a saturated steam–vented cycle. t0 to tn is the time interval in the computation of the F value. F1 to Fn areas represent the lethal dose provided to the material being treated. The shaded area under the curve obtained through summation of F1 to Fn values represents the total F value. In the heating phase, saturated steam is admitted into the chamber, displacing the cold air until the exposure temperature and corresponding saturated steam pressure are attained. In the exposure phase, the sterilizing temperature is maintained by saturated steam for the prescribed exposure time. The cooling phase can be achieved by slow exhaust (for containers filled with liquids) or fast exhaust (for goods required to be dry after sterilization). This phase is completed when the pressure reaches atmospheric pressure. Are all autoclaves the same? No! There are two general types of steam sterilizers: gravity displacement, in which the displaced air flows out the drain through a steam-activated exhaust valve; and pre-vacuum, in which a vacuum is pulled to remove the air before steam is introduced into the chamber. With both types, as the air is replaced with pressurized steam, the temperature in the chamber increases. However, the latter is more efficient. Both approaches should result in temperature increases within the load that, under most conditions, are sufficient to treat the materials to be sterilized. How to achieve perfect sterilization every time.
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This is obviously an important issue! Improperly autoclaved materials results in contamination, lost time, and wastes money. Hopefully, by understanding the operation of the autoclave and other issues associated with your sample, you will be successful every time! Time/Volume/Mass/Insulation/Microbe concentration/Etc. You must think about all of these variables! There is no simple formula for how long a certain item needs to be autoclaved to acheive sterility. Consideration must be given to the type of primary container (the beaker or flask or packet containing the item to be sterilized), the volume of liquid, amount of solid material, and the secondary container (such as a tray containing the primary container). Some examples of the considerations needed are described in the following sections. Time is critical. As the cycle time will vary with the composition of the load, it is important to determine the appropriate time requirement. Some (ignorant) people assuming that a time of 30 minutes is sufficient, however this often proves to be a very costly mistake. Secondary Containers. Plastic or steel containers (trays) are commonly used to contain material during autoclaving, since it is important to contain spills. But don’t forget that such containers alter the nature of the autoclave run! Polypropylene plastic pans with 6 inch sides are favored over polyethylene and polystyrene because it can withstand autoclaving without melting. (Don’t ever autoclave a plastic item — like a tray– if you are not sure it can handle the heat. It is no fun to pry out a melted tray!). However, the use of a plastic container increases the time needed for autoclaving, since plastic is a good insulator. Always add 5 minutes when using a plastic tray. On the other hand, stainless steel containers are not only durable, they are a better conductor of heat so the run time will be a little faster.Howard Judelson 6/28/04 Do not use overly deep containers (greater than 6 inches), which may prevent displacement of air from the bottom. Also, ensure that steam can flow around the secondary container. Volume.