Laboratory Autoclave Steam Sterilization Cycles, Part 9: Air Over-Pressure Cycle

Arthur Trapotsis
Written by: Arthur Trapotsis

MS Biochemical Engineering, MBA, Consultant

In the ninth installment of our Steam Sterilization Cycles Series, we explore Air Over–Pressure, another cycle used to sterilize a specific type of liquid load. An Air Over-Pressure Cycle is used to sterilize small liquid loads that are extremely sensitive to evaporation or boil-over in laboratory autoclaves. Typical applications for this cycle include pre-filled pipet tips, small pre-filled vials, loosely capped flasks, foil sealed glassware, or any partially vented containers where even small amounts of evaporation are not acceptable.

How It Works

The Liquids post in our Steam Sterilization Cycles Series explained how liquid loads (e.g. LB Broth, water, agar, etc.) will boil out of their containers if the autoclave’s chamber pressure is exhausted too quickly. Boil-over is circumvented in a Liquids Cycle by gradually decreasing chamber pressure during the Exhaust Phase. The load may, however, still experience up to a 5% volume loss due to evaporation. For applications that require the sterilization of very precise volumes or liquids under 10ml per container, this evaporative loss may not be acceptable.

The Air Over-Pressure Cycle prevents evaporation using compressed air. As steam is exhausted from the autoclave chamber, cool compressed air is simultaneously introduced so that surrounding pressure on the liquid is maintained while temperature drops. For example, if the sterilization pressure is 15-18 psi, a typical Air Over-Pressure cycle will pressurize the chamber to 30 psi, allowing the load to cool down while under pressure. The temperature of the load is continuously monitored by a Load Probe throughout the cooling phase. Once the load temperature falls to a set-point, typically 200°F-210°F, the risk of evaporative boiling loss has been mitigated and sterilization has been achieved.

Functionally, an Air Over-Pressure Cycle is similar to a Steam-Air-Mix Cycle in that both cycles pressurize the autoclave chamber using a combination of steam and compressed air. A Steam-Air-Mix Cycle, however, uses air during both the sterilization and exhaust phases whereas an Air Over-Pressure cycle uses air only during the exhaust phase. A Steam-Air-Mix cycle is primarily used for applications where sealed containers may develop internal pressure and rupture during the sterilization phase. An Air Over-Pressure cycle is primarily used to minimize loss from evaporation during the exhaust phase.   It is important to note that in the sterilization industry sometimes the Steam-Air-Mix and the Air Over-Pressure are collectively referred to as Air Over-Pressure.


Sterilizing liquid loads can seem intimidating, but Consolidated Sterilizer Systems controllers come preprogrammed with cycles designed for any application. If you are sterilizing small or precise amounts of liquid, the Air-Over-Pressure cycle will prevent evaporation by cooling the load under pressure. To sterilize other types of liquid loads, check out our previous posts on the Liquids Cycle and Steam-Air-Mix cycles.

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17 Questions to Ask Before Buying Your Next Autoclave

17 Questions to Ask Before Buying Your Next Autoclave

With so many models, sizes, options and components to choose from, how can you ever really know exactly what you need to make the most out of your investment?

These questions will help you to make informed decisions by outlining what is most important to consider and know about owning an autoclave.