Steam Autoclave Testing Methods and Maintenance: Leak Testing and the Bowie-Dick Test
Category: Lab Practices
In most cases, autoclave installation includes both vacuum leakage testing and a Bowie-Dick test (also known as a “dynamic air removal test”). These confirm that your steriliser can create a vacuum, maintain that vacuum, and force steam to fully permeate even tricky porous loads. This is especially important in any lab that regularly handles potentially dangerous biological waste, as it’s notoriously tricky to get steam to fully penetrate “red bag” and similar waste loads.
But autoclave testing should never be treated as a “one and done” affair. At the very least, seals and gaskets degrade over time. They will leak if not properly maintained or replaced in a timely manner. In most labs, vacuum leak testing and Bowie-Dick tests should be a regular part of the maintenance schedule.
How Often should I Run a Bowie-Dick Test or Perform Vacuum Leakage Testing?
That varies by lab. If you regularly run critical porous loads daily, it might make sense to perform a daily Bowie-Dick test. In many labs, vacuum leak and Bowie-Dick tests are part of the scheduled weekly or monthly maintenance (usually at the end, after checking and greasing the door gaskets).
Whatever you schedule, regular testing will give you an early warning if something is starting to go awry and greater confidence in the reliability of your steam steriliser and sterilisation protocols.
Autoclave Vacuum Leak Testing
As the name implies, vacuum leak testing checks the air-tight integrity of the autoclave chamber. On most autoclave models, this will be a specific factory-programmed cycle that will put your autoclave through its paces and log how much “vacuum depth” is lost under specific conditions over a given time period. A loss of “vacuum depth” is indicative of leaks permitting cold air to enter the steriliser while it’s running; such leaks disrupt steam penetration and circulation and can prevent complete sterilisation of the load. Built-in leak testing programs usually allow labs to set their own pressure and leak-rate parameters (for reference, in many industries the standard acceptable leak rate is less than ~1mmHG/min, which is ~0.013 bar over 10 minutes).
Priorclave autoclaves include a fully automated vacuum Leak Test Cycle program. Running this program automatically checks the integrity of the chamber and interconnecting pipe-work. During a Leak Test Cycle, the autoclave evacuates the chamber to your established vacuum set-point, then holds for five minutes before checking the pressure again. After another ten minutes, the pressure is checked a final time. The pressure readings are then compared. Any increase is the result of air leaking into the chamber. If that “relative pressure increase” exceeds your established allowance, the system logs a failed Leak Test.
At each stage of the test, the control system automatically logs the pressure and elapsed time, creating a convenient record not simply of passed tests, but also of the system’s overall performance throughout the testing cycle. (This may prove handy later, if you’re trying to diagnose an issue that could be related to plumbing seals, door gaskets, the vacuum pump, etc.)
The Vacuum Leak Test confirms that your steam steriliser is creating and maintaining a good vacuum, which is necessary for good steam saturation throughout the load. Without good steam saturation, you cannot reliably reach sterilisation temperatures. The Bowie-Dick Test confirms that this steam saturation is actually occurring and that your entire load is reaching the expected temperatures, with no “cold pockets” left unsterilised.
You can either use a commercially available Bowie-Dick test pack, or make your own using a few dozen folded surgical towels (also called “huck towels”) and some standard autoclave tape. In either case, the test is essentially the same:
A thermochromic test paper (i.e., a test strip that changes color once it’s reached a specific temperature) is sandwiched in the middle of a porous load. If the steam has uniformly saturated the load during your sterilisation cycle, then the paper will uniformly change color. If the steam isn’t saturating the load, the color change will be inconsistent.
Performing a Bowie-Dick Test
Before performing a Bowie-Dick test, start by running a warm-up cycle with an empty chamber. This ensures that you’ve cleared the chamber of residual air. Once that warm-up cycle is complete, place your test pack (store-bought or DIY) on the bottom shelf of your autoclave and over the drain (this is generally the coldest point in the chamber). If you’re using a purchased test pack, you’ll want to place it label up. Now run your test cycle.
Once the cycle is complete, remove your test pack, let it cool, and check the results. The test paper (or autoclave tape in a DIY pack) should be uniformly dark. If there are light patches, the autoclave has failed the test. Failed Bowie-Dick tests may indicate issues with the autoclave hardware or software or with your utilities. Start troubleshooting by confirming that the chamber is not leaking (i.e., run a Vacuum Leakage Test), then confirm that the pre-vac cycle is properly drawing a vacuum.
Don’t trust your steriliser until it’s passed a Bowie-Dick Test.
A Note on Temperature Settings for Bowie-Dick Testing Cycles
In the United States, there are several schools of thought regarding the appropriate temperature for a Bowie-Dick test cycle. Because of the dominance of healthcare-style autoclaves in North America, it was once very common to run a Bowie-Dick test at ~134°C for 3 to 4 minutes (which was the widely recognised healthcare standard for many years).
As research-grade autoclaves have become more common in the life sciences and other industries, many manufacturers have developed lab-specific Bowie-Dick test packs. These are designed to run at the lower temperatures and longer times common in research labs (generally ~122°C for ~8 minutes).
When running a Bowie-Dick test, you should use temperature and time settings that the pack’s manufacturer advises. If the pack hasn’t specifically been validated by the manufacturer to run at 122°C, then it should be run using the older “higher temperature/shorter time” settings.
If you’ve made your own DIY test pack, then we assume you have your own preferred temperatures and cycle times that you are validating.