STERILIZATION BY HOT-AIR OVENS:
These are thermostatic controlled ovens, with an electric heating element, similar to a domestic electric oven.
- Instruments to be sterilized are heated to 160 degree C (320 degree F) for 1 hour.
- Sterilization is achieved, but it is not suitable for rubber or plastic instruments.
- In hospital has been used for sterilizing powders and petroleum products and sharp delicate instruments
The efficiency of dry heat sterilization depends on the initial moisture of the microbial cells, but all microorganisms are killed at 160 degree C for a hold time of not less than 2 hours.
The main advantages of dry heat sterilization are its ability to treat solids, non-aqueous liquids, grease/ointments and to process closed (airtight) containers.
Lack of corrosion is important in the sterilization of non-stainless metals and surgical instruments with fine cutting edges.
WHAT ARE 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 p.s.i.) the temperature reaches 105.5 degree C: at 0.7 kg per cm2 (10 p.s.i.). 115 degree C; and at 1.05 kg per cm2 (15 p.s.i.) 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 fibres 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 2.15
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.
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