Also known as a steam sterilizer, an autoclave is an insulated pressure chamber in which high-pressure steam is used to sterilize (decontaminate) laboratory/clinical material or items.For different types of material, sterilization time and temperature may vary depending on recommendations. This ensures that given materials are effectively sterilized for use.

Many autoclaves are used to sterilize equipment and supplies by subjecting them to pressurized saturated steam at 121 °C (250 °F) for around 15–20 minutes depending on the size of the load and the contents. The autoclave was invented by Charles Chamberland in 1879,although a precursor known as the steam digester was created by Denis Papin in 1679.The name comes from Greek auto-, ultimately meaning self, and Latin clavis meaning key, thus a self-locking device.

Types of Autoclaves

Tabletop autoclaves, roughly the size of a microwave oven, are often used in dental offices or small, private medical practices, while larger, horizontal autoclaves are frequently utilized in hospitals in order to handle larger volumes of equipment. Other autoclaves designed for use within the pharmaceutical industry function particularly with current Good Manufacturing Practices (cGMP) regulations in mind.

How does an autoclave work?

An autoclave is essentially just a large steel vessel through which steam or another gas is circulated to sterilize things, perform scientific experiments, or carry out industrial processes. Typically the chambers in autoclaves are cylindrical, because cylinders are better able to withstand extreme pressures than boxes, whose edges become points of weakness that can break. The high-pressure makes them self-sealing (the words “auto” and “clave” mean automatic locking), though for safety reasons most are also sealed manually from outside. Just like on a pressure cooker, a safety valve ensures that the steam pressure cannot build up to a dangerous level.

Once the chamber is sealed, all the air is removed from it either by a simple vacuum pump (in a design called pre-vacuum) or by pumping in steam to force the air out of the way (an alternative design called gravity displacement). Next, steam is pumped through the chamber at a higher pressure than normal atmospheric pressure so it reaches a temperature of about 121–140°C (250–284°F). Once the required temperature is reached, a thermostat kicks in and starts a timer. The steam pumping continues for a minimum of about 3 minutes and a maximum of about 15–20 minutes (higher temperatures mean shorter times)—generally long enough to kill most microorganisms. The exact sterilizing time depends on a variety of factors, including the likely contamination level of the items being autoclaved (dirty items known to be contaminated will take longer to sterilize because they contain more microbes) and how the autoclave is loaded up (if steam can circulate more freely, autoclaving will be quicker and more effective).

Autoclaving is a bit like cooking, but as well as keeping an eye on the temperature and the time, the pressure matters too! Safety is all-important. Since you’re using high-pressure, high-temperature steam, you have to be especially careful when you open an autoclave that there is no sudden release of pressure that could cause a dangerous steam explosion.

Thermal Death Time

Thermal death time (TDT) is used to determine how long it takes to kill specific microorganisms at a specific temperature and specific suspension. Let it suffice to say that death rate is directly proportional to the concentration of microorganisms at any given time. If you increase the temperature of your sterilization process, this has the effect of decreasing thermal death time. Lowering the temperature increases thermal death time. Higher temperatures for shorter periods of time are preferred.

Beyond temperature and time, thermal death time is affected by the materials being sterilized. For example, oily materials slow down steam penetration and therefore increase thermal death time. Materials that are highly acidic or basic tend to decrease thermal death times. Thermal death times are available for a number of microorganisms and specific suspensions they may be in. Keep in mind that thermal death time may not be accurate but a good place to start.

Autoclaving is accepted as being the most effective and most efficient way of sterilization. Moist heat autoclaves work on a time and temperature relationship. Higher temperatures are important for more rapid killing of microorganisms. Longer sterilization times are required for larger loads, large liquid volumes, and dense materials. Temperatures and pressures most often used in a moist heat autoclave are 115°C at 10 psi (pounds per square inch), 121°C at 15 psi and 132°C at 27 psi. Moist heat autoclaving works well for glassware, biological media, surgical dressings, biohazardous waste and much more.

Autoclaves: Pressure and Temperature

Pressure and temperature affect boiling. Water boils when the water molecules contains enough energy to escape the liquid and form water vapor or what is called steam above it. As the water gets hotter, water molecules contain more energy and can more easily escape the liquid. On the other hand, pressure is important as well. The higher the pressure above the water, the more difficult it is for the water molecules to break free of the liquid and vice versa.

Consider going up in altitude. The higher you go, the lower the atmospheric pressure. Boiling water at this higher elevation occurs at a lower temperature because it takes less heat to boil the water. It is much easier for liquid water to go into water vapor because the pressure above the liquid is much less. In fact, if you were on top of Mount Everest, water boils at around 70°C not 100°C.

The boiling point of water at one atmosphere (sea level, 760mmHg) is 100°C. Raising or lowering the pressure by around 28mmHg changes the boiling point by 1 degree C.

Vapor Pressure

In the case of a closed container such as an autoclave, the process of evaporation will continue until there are as many molecules returning to the liquid as there are escaping. When this occurs, the vapor is said to be saturated. At higher temperatures, more water molecules can escape and the corresponding saturated vapor pressure is greater. In the case of an open container, vapor pressure is actually a partial pressure along with the other components in the air above. In this case, the temperature at which the vapor pressure is equal to the atmospheric pressure is referred to as the boiling point.

What is an Autoclave Used for?

An autoclave chamber sterilizes medical or laboratory instruments by heating them above boiling point. Most clinics have tabletop autoclaves, similar in size to microwave ovens. Hospitals use large autoclaves, also called horizontal autoclaves. They’re usually located in the the Central Sterile Services Department CSSD) and can process numerous surgical instruments in a single sterilization cycle, meeting the ongoing demand for sterile equipment in operating rooms and emergency wards.

How to Clean an Autoclave?

Although an autoclave is very effective when it comes to sterilizing various materials and substances, various organic or non-organic substances can accumulate over time. As a result, they can cause damage to the device or cause it to be less effective.

Some of the issues associated with poor autoclave maintenance include staining, chamber corrosion, and even electric hazards, etc. How an autoclave is cleaned is largely dependent on how often it’s used. While some autoclaves are only used once every few weeks, others are used daily. For autoclaves that are used everyday, the daily cleaning routine varies from the weekly and monthly cleaning routines.

Daily cleaning – An autoclave that is used daily is more likely to collect more organic and non-organic substances. For this reason, it has to be inspected daily for debris and cleaned. Generally, cleaning the autoclave after each cycle is recommended. Here, however, it’s important to ensure that the unit has cooled (make sure that the electric power cord has been disconnected from the power socket) before cleaning to avoid accidental burns.

Generally, cleaning involves wiping with a soft and dry cloth. If stains are present, then a moist cloth with a little detergent may be used. It’s always important to inspect different parts of the autoclave in order to ensure that it’s thoroughly cleaned. In some institutions, particularly larger laboratories, cleaning involves the use of an ultrasonic cleaner (e.g. Durasonic).

* Accumulation of debris has been shown to prevent the unit from effectively sensing temperature.

Weekly – Here, cleaning should start with using a dry cloth to wipe out debris in the unit.

Once the debris has been removed, the following steps should be taken to thoroughly clean the unit:

· Drain the reservoir – One of the most important steps of the weekly cleaning routine involves draining the reservoir. Here, the process involves filling the tube with distilled water as well as a small amount (about an ounce) of speed clean sterilizer.

Draining is particularly important given that it prevents items from being contaminated with residue that may be present in the fluids. The autoclave is then turned on to run a 30-minute cycle at 121 degrees C before draining the reservoir for a second time.

Once the reservoir has been drained, clean distilled water is added before running a 3-minute cycle at 132 degrees C. Lastly, the reservoir is drained again and the unit allowed to cool.

· Remove the tray plates and wipe any remaining debris and again wipe the inside of the autoclave – A soft and dry cloth is recommended for cleaning in order to avoid damaging sensitive parts of the autoclave including the heater elements and the temperature and level sensor components.

· Once the inside part of the autoclave is clean, carefully re-install the tray rack and plates and ensure that they are properly inserted into the unit.

· Add clean distilled water into the reservoir to the appropriate level (usually marked).

Monthly – Generally, monthly cleaning involves the following steps:

· Run the flush system – This involves draining the reservoir, re-filling with clean, distilled water as well as an ounce of speed-clean sterilizer. This part of cleaning also involves some of the steps mentioned in weekly cleaning (e.g. running 30-minute cycle, draining and re-filling the reservoir again, etc)

· Using speed-clean sterilizer cleaner and a small bristle brush, clean the door gasket, gasket ring and the dam gasket – These parts should be removed in order to be cleaned properly.

· Remove the trays and rack and wipe with a soft dry cloth. The filter should be cleaned using speed-cleaner sterilizer and clean water.

· Testing the biological indicators – This step is normally carried out in order to verify sterility. Normally, this involves sending a processed part to a third party lab for culture.

· Pressure relief valve check – One aspect of monthly cleaning involves pressure relief valve check. This is largely dependent on the type of autoclave, instructions are provided in the operation manual for different types of autoclaves.

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