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Everything You Ever Wanted to Know about Autoclave Gravity Cycles (but were afraid to ask)

Laboratory Autoclave c40 Compact 40 Illustration with 500mL Flasks

Making the jump from a little “semi-disposable” tabletop sterilizer to a true research-grade autoclave means getting a handle on a slew of new terminology. One piece of jargon seems to particularly trip up lab managers and others researching their autoclave options: “Gravity cycle.”

What the heck is a “gravity cycle”?

Short answer: A “gravity cycle” is usually what you’d think of as a “normal” cycle.

Long answer: A steam autoclave relies on direct steam contact with all surfaces in a load in order to kill pathogens present on that load. In order to assure the load is completely permeated–and thus fully sterilized–the entire chamber needs to be filled with steam. This can’t happen if the room-temperature air is still trapped in the chamber.

Since steam is less dense than air, there is a very efficient way to reliably replace all of the room-temperature air with steam: Generate steam in the bottom of the chamber while opening a vent on the back. As the steam rises, it displaces the cold air, which then drains out of the open vent in much the same way that water drains from a water cooler. (Hence the name gravity cycle: the cold air is passively drawn out of the chamber by gravity, rather than being actively pumped out.)

Once the chamber is filled with steam, the vent closes, and sterilization commences.

A gravity cycle has many advantages: It requires no electricity, no extra valves, and no extra pump; there is less energy used to accomplish the task, and fewer parts to maintain (or eventually replace).

One disadvantage is that it is challenging to fully permeate porous loads with steam in this manner, and certain loads (such as narrow-mouthed containers) have a tendency to capture pockets of cold air, thus preventing complete and reliable sterilization. If these are common loads in your lab, we suggest opting for an autoclave capable of “pre-vac cycles.” Such an autoclave is outfitted with a vacuum pump, which gives you the option of drawing a partial vacuum in the chamber during the initial heating period, while the chamber is being flooded with steam. By “pulsing” this vacuum, you can be sure that the cold air is entirely removed from the chamber, and the load is thus completely permeated with steam. (Adding a vacuum pump offers other advantages, including the ability to run cycles with a “post-cycle vacuum” stage, which results in much dryer loads.)

Is this the same as “gravity fill”?

Some manufacturers specify that their autoclaves are “gravity fill”–which is not the same as having a “gravity cycle” option. This occasionally causes confusion.

So, what is a “gravity fill” autoclave?

Short answer: “Gravity fill” refers to how the autoclave gets water to the steam generator between cycles. (There are very few situations where this makes any operational difference in a research setting.)

Long answer: Priorclave’s in-chamber steam generators are similar, in principal, to the immersion heater you might use to prepare a cup of tea at your desk or while traveling. Between cycles the autoclave must replenish the water reservoir that holds these heating elements. In a “gravity fill” system this is done by draining an external water-tank into the in-chamber reservoir. (If you are familiar with the inner workings of a home toilet tank, this is not dissimilar).

In the past, manufactures might specify that a unit had a “gravity fill” option in order to make it clear that it the autoclave could be connected to either standard or high-purity water supplies (the later flummox some types of water-level sensors, and thus can pose challenges). As this is a relatively small niche–and water-level sensing is addressed in a number of different ways now that can handle a wider variety of water types–it is less common to come across the term.