Autoclaves
This is the most efficient method of sterilizing instruments, packs and dressings, and is suitable for most materials. An autoclave is basically a pressure cooker and in fact, there is no reason why a domestic pressure cooker should not be used to sterilize instruments in a small clinic. The small autoclaves produced for the doctor’s surgery offer a choice of temperatures, pressures and sterilizing times.
The highest temperature that can be reached by boiling water at sea level in an open vessel is 100 degree C. With increased pressure, the water can be raised to much higher temperatures before it boils, e.g. at a pressure of 0.35 kg per cm2 (5 psi) the temperature reaches 105.5 degree C: at 0.7 kg per cm2 (10 psi). 115 degree C; and at 1.05 kg per cm2 (15 psi) the temperature will reach 121 degree C, etc.
In a sterilizer chamber (autoclave) which has been well exhausted of air the steam entering promptly fills the free spaces surrounding the load. As steam contacts the cool outer layers of the fabrics a film of steam condenses, leaving a minute quantity of moisture in the fibers of the fabrics. Air contained in the fabric interstices, being heavier than steam, is displaced by gravity in a downward direction, and the latent heat given off during the process of condensation is absorbed by that layer of the fabrics (Fig. 1.15).
The next film of steam immediately fills the space created when the first film condensed into water, and it does not condense on the outer layer of the fabrics but penetrates into the second layer, condenses and heats it. This process continues until the whole load is heated through and no further condensation occurs, the temperature within the pack remaining at that of the surrounding steam.
• ‘Quick’cycle would heat the water to 134 degree C (273 degree F) for 3.5 min under a pressure of 30 lb/in2.
• Slower cycle, more suitable for plastics, would heat the water to 121 degree C under a pressure of 15 lb/in2 15 minutes.
• In practice, instruments are placed in the trays or in packs, the autoclave turned on, and left for the desired time.
• At the end of the cycle, the instruments are ready for use.
• The main disadvantage of the smaller autoclave is that instrument packs cannot be sterilized as there is not a vacuum cycle to extract air and dry the packs. However, most materials including rubber, plastics and metal can be readily sterilized, the only exception being sealed containers.
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