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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
320-345°F, 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 375°F (190°C) 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.
Chemical Vapor Sterilization
This process uses a mixture of chemicals,
including alcohol, formaldehyde, ketone, acetone, and
water, that are heated under pressure to form a
sterilizing gas. Sterilization requires 20 minutes at
270°F with 20 psi when instruments are either
unwrapped or bagged following the manufacturer’s
instructions.
10-12
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