In addition to the high temperature, steam must be saturated so that it will quickly release heat through condensation when it comes into contact with a cool object. Sterilization will not occur unless all air is eliminated from the chamber at the beginning of the process and periodically throughout sterilization. The packaging of supplies and loading of the sterilizer must be done so that steam comes in contact with all areas or surfaces of the items being sterilized. Flash sterilization is defined as the sterilization of unwrapped   items   in   a   gravity   displacement   or prevacuum sterilizer with recommended minimum exposure times and temperatures. Steam sterilization by this unwrapped method is not recommended. It should be used only for emergency sterilization. Types of Steam Sterilizers A steam sterilizer, also know as an “autoclave,” is a pressure-type vessel with a door or cover, valves to control  the  entry  and  exit  of  steam  and  air,  and monitoring  devices  to  allow  the  operator  to  observe conditions inside. It is designed to hold items and allow steam under pressure to penetrate these items. Steam sterilizers are available in many sizes, ranging from  portable  countertop  to  the  fixed  room-size sterilizer. Two of the most common types of steam sterilizers  used  in  the  Navy  are  the  gravity displacement  and  prevacuum  sterilizers. 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 250°F or 121°C 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 270°F   (131°C)   after   4   minutes   exposure   in   a prevacuum  steam  sterilizer.  Consult  the  manu- facturer’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 10-10