GRAVITY DISPLACEMENT.\Once this sterilizer is loaded and the door is closed as shown in figure 10-10, steam is admitted through an inlet and the sterilization process begins. A typical standard for steam sterilization is achieved at 250F or 121C after 20 to 30 minutes at 15 psi. It is important to refer to the manufacturer's instructions for operation, since exposure times can vary according to the design of the particular sterilizer.

You should observe the following precautions when loading the sterilizer chamber:

Do not overload. The passage of steam from the top of the chamber to the bottom should not be blocked.

Place all packages on edges, with large packs at the bottom of the chamber, and small packages in an upper layer crosswise to the lower layer. This allows free passage of steam.

If mixed loads of metal items and linen are sterilized together, the linen is placed on the upper shelf and the metal items on the lower.

Articles that require the same amount of time and the same final steps should be sterilized together.

Enclosed fluids are sterilized separately because the pressure must be slowly released.

Load all packages at the same time when you are ready to sterilize.

A standard operation chart for the correct exposure period of all supplies should be prepared and posted for easy daily reference. It is important to note that sterilizing conditions are based on temperature rather than on pressure. Effective steam sterilization and exposure time are measured from the moment the thermometer in the discharge line indicates the desired preset temperature. The pressure inside the sterilizer is not an indication of positive sterilization because other factors determine the pressure inside the sterilizer. Pressure merely maintains temperature.

PREVACUUM STEAM STERILIZER.\The prevacuum steam sterilizer (fig. 10-11) was designed to help overcome the trapping of air in the chamber. Trapping of air is one of the greatest dangers encountered when using saturated steam under gravity cycles. When errors are made by improperly packaging items or overloading the sterilizer chamber, cool air pockets may form resulting in items not being sterilized. The speed and efficiency of the steam sterilizer may be improved by removing air from the chamber with a powerful pump, creating a nearly perfect vacuum before steam is introduced into the chamber. This procedure allows fast and more positive heat to penetrate the entire sterilizer load. The improved sterilizer is referred to as the prevacuum steam sterilizer.

Full heating of the loads is faster in the prevacuum sterilizer than in the gravity displacement sterilizer. For example, wrapped instruments can be sterilized at 270F (131C) after 4 minutes exposure in a prevacuum steam sterilizer. Consult the manufacturer's instructions for specific details on operation and user maintenance information.

The Bowie-Dick type test was developed for prevacuum sterilizers to determine if the air has been removed from the chamber during the prevacuum stage. Air must be removed so that steam can penetrate the load instantaneously. It must be understood that this is not a test for adequate exposure to heat in terms of time-at-temperature. A commercially prepared Bowie-Dick type test can be used by carefully reading and following the manufacturer's instructions. All

Figure 10-10.\Gravity displacement steam sterilizer.

Navy prevacuum sterilizers will be tested daily using the Bowie-Dick type test.

Level One Maintenance

The interior of the steam sterilizer should be cleaned each day before being heated. This simple procedure can easily be accomplished by using a mild detergent to wash the surfaces. Follow the wash with a thorough rinse of plain water. Unless this is done, the chamber walls will collect mineral deposits and may become greasy. Do not use wire brushes, steel wool, or any type of abrasive cleaning compounds on the sterilizer. The manufacturer's directions must be followed to maintain a properly functioning sterilizer. If the sterilizer does not appear to function properly, dental equipment repair personnel should check it at once. Sterilizers should be spot checked frequently for leaks in lines and improperly functioning gauges, dials, thermometers, doors, drain strainers, and valves.

Dry Heat Sterilization

The least expensive form of heat sterilization of instruments is dry heat. Destruction of microorganisms by dry heat in a DTF is accomplished by using a unit that has been tested and approved by the FDA as a commercial sterilizer.

Figure 10-11.\Prevacuum steam sterilizer.

Dry heat is suitable for sterilizing metal instruments that rust or dull in the presence of water vapor. A disadvantage is that the high temperatures destroy many rubber and plastic based materials, melt the solder of most metal impression trays, and weaken some fabrics, as well as discolor other fabrics and paper materials.

A complete cycle involves heating the dry heat oven (fig. 10-12) to the appropriate temperature and maintaining that temperature for the proper time interval. Depending on the location, dry heat ovens can use one of the following heating elements to achieve sterilization:

Conduction (direct contact with a heat source)

Radiation (long electromagnetic waves)

Convection (heated air)

Because dry air is not as efficient a heat conductor as moist heat at the same temperature, a much higher temperature is required for sterilization. One of the most common problems with the use of dry heat sterilization is the failure to properly time the exposure. A typical dry heat cycle is 90 minutes at 320-345F, plus the time required to preheat the chamber before beginning the sterilization cycle. A common misuse of the dry heat method occurs when the oven door is opened, and an instrument is quickly removed during the timed cycle. This interrupts the cycle and timing must begin all over again.

Advances in the design of the dry heat oven resulted in the development of the dry heat convection unit, which uses forced air at higher temperatures. This method of rapid heat transfer achieves sterilization in 12 minutes at 375F (190C) for wrapped items and in 6 minutes for unwrapped items. Biological monitoring will be performed weekly. Consult the manufacturer's instructions of each type of dry heat sterilizer for specific details on its operation and user maintenance.