Steam Sterilization Cycles, Part 1: Gravity vs. Vacuum

Steam sterilization relies on three (3) parameters to eliminate microbes and organisms: Time, temperature, and pressure. And these parameters can be manipulated into different cycle recipes to sterilize various types of loads. However, the cycle requirement for every load type—such as red bags, media, glassware, cages, animal bedding, and pipette tips—can vary significantly.

While the vast majority of common laboratory loads can be sterilized with one of the following three (3) basic cycles: Gravity, Vacuum (or Prevac), and Liquids; it is important to ensure that your sterilizer offers the cycles appropriate for ALL of all of your load requirements.

In this article, we explore the first two basic steam sterilization cycles: Gravity and Vacuum.

Basic Cycles Description Typical Application or Load Type
Gravity The most basic sterilization cycle. Steam displaces air in the chamber by gravity (i.e. without mechanical assistance) through a drain port. Glassware, unwrapped goods, waste, utensils, redbags.
Pre-Vacuum and/or Post-Vacuum Air is mechanically removed from the chamber and load through a series of vacuum and pressure pulses. This allows the steam to penetrate porous areas of the load that couldn’t otherwise be reached with simple gravity displacement. Wrapped goods, packs, animal cage bedding, cages, porous materials, redbags.

Then, in future articles, we will discuss additional cycle types, such as Liquids, Air-Overpressure, and a myriad of other, more advanced cycle types for special applications.

Discover the Top 12 Steam Sterilization Cycles Here >

The Gravity Cycle

The traditional “Gravity Cycle” is the most common and simplest steam sterilization cycle. During a Gravity Cycle, steam is pumped into a chamber containing ambient air. Because steam has a lower density than air, it rises to the top of the chamber and eventually displaces all the air. As steam fills the chamber, the air is forced out through a drain vent. By pushing the air out, the steam is able to directly contact the load and begin to sterilize it.

At the end of the cycle, the steam is discharged through the drain vent. However, the load can still be hot and possibly wet. To address this issue, gravity autoclaves can be equipped with a post-cycle vacuum feature to assist in drying the load. If equipped with this feature, the sterilizer runs a normal Gravity Cycle and after the load is sterilized, a vacuum pulls steam and condensation through the drain vent. The longer the vacuum system runs during the dry phase, the cooler and dryer the goods will be when removed from the chamber.

Gravity Cycles are commonly used on loads like glassware, bio-hazardous waste (red bag waste), vented containers, and certain types of unwrapped instruments.

The Vacuum Cycle

There are certain types of applications where air is not easily displaced from the chamber. Gravity air displacement (as described above) is not as effective on porous loads or partially vented containers. For example, when sterilizing loads such as cages with animal bedding, wrapped goods, surgical packs, etc., it is best to use a Vacuum Cycle (also called Prevac Cycle).

A sterilizer configured to run a Vacuum Cycle will be equipped with a vacuum system. A typical Vacuum Cycle will begin with a series of alternating steam pressure injections and vacuum draws (also called pulses) to dynamically remove the air from the chamber. Drawing a vacuum to remove ambient air from the chamber allows steam to be sucked into areas where it would otherwise have difficulty penetrating. The absence of air within the chamber allows “steam to penetrate the load almost instantaneously” resulting in more reliable sterilization and shorter sterilization cycle times (Perkins 124).

Once sterilization is complete, a post-cycle vacuum can be programmed to enhance and quicken the drying process. Consolidated sterilizers has two options for vacuum systems: a water ejector with optional booster pump, as well as a liquid ring vacuum pump.

Common Vacuum Systems

Common Vacuum Systems

Gravity and Vacuum cycles are the most commonly utilized steam sterilization cycles, as they can accommodate a wide variety of autoclave load types. Some loads, however, (e.g. syringes, contact lenses, certain types of media, etc.) require special cycle configurations that employ pressure or temperature ramping, for instance. Fortunately, today’s advanced controllers can manipulate sterilization parameters to accommodate these loads.

At CSS, we recognize that many of our customers need assistance in identifying and creating the proper sterilization cycle for their load type. As such, we hope this article helps, and we encourage you to check back for our next series of blog articles too—where we will first introduce the Liquid Cycle and then focus on exploring advanced sterilization cycles and their applications. Stay tuned!

In the meantime, if you have any questions, we are always happy to help. Contact us today.

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Read this comprehensive 31-page guide about when and how to use the top 12 steam sterilization cycles. This guide outlines how today’s steam autoclaves can be configured with specific parameters suitable for nearly any load and application type.

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