Last Updated on May 1, 2023


Types Of Autoclaves

The way that autoclaves work varies depending on whether they use gravity or vacuum-induced or pre-vacuum (pre vac) sterilization methods. That said, there are some autoclaves that use both methods to sterilize. Both types of autoclaves sterilize through high-temperature steam and the chamber becomes pressurized, so it does not release any air in the chamber which helps heat penetrate all parts of what is being sterilized. The mechanisms differ and this article will outline how these two kinds of systems operate with different media associated with each type respectively.

Gravity Autoclaves

Gravity autoclaving, also known as gravity displacement autoclaving, is the most basic form and is suitable for sterilizing the most common laboratory media. This includes steel utensils, glassware, and bio-hazardous waste. Gravity-induced autoclaving involves pumping steam into the autoclave chamber. Which displaces the ambient air and forces it out of exhaust valves so that the remaining steam can sterilize the contents. This mechanism is helpful in the simplicity of its design and lack of dependency on peripheral mechanisms to displace ambient air with steam. Making these types of autoclaves more affordable and dependable.

Most autoclave media or items to be sterilized are simple in design and do not contain spaces or obstacles for steam to penetrate within, thus the steam displacement function is ample for proper sterilization. It is for these reasons that gravity autoclaves are the most common types of autoclaves in the market and are usually the recommended type of autoclave for most uses.

Vacuum Autoclaves

Vacuum autoclaving, also known as pre-vacuum autoclaving or sterilizing, is more suited where air cannot be easily removed from sterilization media. This may include large or porous items such as animal cages and bedding sterilization as well as wrapped surgical kits. The vacuum function in these autoclaves allows deeper sterilization of the contents, as it completely evacuates the ambient air within, allowing high-temperature steam to penetrate and sterilize areas that would normally be occupied by ambient air, and can be more efficient at sterilizing certain items with hard-to-reach areas within.

Autoclave Media & The Corresponding Autoclave Type

As mentioned above, your choice of an autoclave system largely depends on your autoclave media, ie. the items to be sterilized. Overall, steam sterilization is highly dependable, effective, fast, and non-toxic, and offers an inexpensive way to rapidly heat and penetrate the chamber’s contents, including appropriately contained liquids. However, this method cannot sterilize powders or oils, and can only work with heat and moisture-resistant goods.  Keeping this in mind, below, I summarize the primary media used by gravity and vacuum-autoclave types.

Gravity autoclaves are appropriate for sterilizing non-porous items (i.e. those with a hard surface):

  • Most metals, particularly stainless steel surgical instruments and lab utensils
  • Polypropylene Pyrex® or Type I borosilicate glassware
  • Biohazard waste
  • Unwrapped goods

Vacuum (pre and post) are appropriate for sterilizing large or porous items:

  • Media solutions in containers, such as tissue culture flasks with loose caps for a steam autoclave cycle
  • Pipette tips and other high-density polyethylene products, such as syringes
  • Wrapped dry items that can trap air
  • Animal cages and bedding

Choosing An Autoclave

Choosing the right autoclave for your purposes requires you to not only assess the type of media to autoclave but also throughput capacity, available lab space, and access to utilities, including electricity (with proper power output), water, and house steam.

Because of the simplicity of the gravity-displacement mechanism, which requires an autoclaving chamber, a heating mechanism, intake, and exhaust valves, there is great flexibility in the design for gravity autoclaves, including front and top-loading types.

The top-loading type autoclave is helpful, as it allows for maximum loading space, minimum floor space requirement, and does not require any building steam connection, as the vertical chamber design allows for water to rest at the bottom, which is turned into steam through a heating element at the bottom. Note that many autoclaves on the market, especially compact top-loading autoclaves, often require only access to the proper electrical outlet type and enough space to place your autoclave.

Gravity autoclaves are also particularly helpful when used in geographical areas of high humidity or higher altitudes. As they consistently keep the relationship between pressure and heat within the autoclave chamber and overcome differences in boiling points at higher altitudes by opening and closing the exhaust valve. Known as high altitude autoclaves, their functions allow for proper sterilization to occur in laboratories and hospitals in markets with high altitude terrains.

Regarding the size of your autoclave, to limit energy use and costs, especially for smaller labs, it is important to buy the proper size autoclave that will accommodate the equipment that needs sterilizing, while not going overcapacity. For this, several autoclaves range between 50 L and 70 L, which strike a good balance between relatively high capacity and low energy use.


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