Welcome to our second post in the series "Medical Autoclaves." Here we will discuss the ever-popular Class B autoclave.

Class B autoclaves are great space-savers built for high performance. They are microwave-sized autoclaves that fit easily on the counters of busy clinics and dental offices. Available in the compact sizes of 19 and 29 liters, the Class B is the top-of-the-line of tabletop autoclaves.

Essentially they are a miniature version of the larger EN 285 autoclave found in hospitals. The industry consensus about what the “B” in Class B stands for is “big,” not as a reference to size, but rather as a reference to performance. Class B tabletop autoclaves are expected to offer the same advanced performance as the much larger hospital sterilizers EN 285.

The main advantage of Class B autoclaves is the use of a vacuum pump for both removing air and drying the load. Class B autoclaves are able to remove more than 99% of the air inside the chamber, ensuring no trapped air that would prevent steam from reaching the load surface, thus killing microbial life on the load, without external hookups to water or steam (some tabletop autoclaves are connected to water pipes for auto-filling, so operators don’t need to fill them). In other words, the Class B tabletop autoclave is the most advanced “plug-n-play” model on the market today.

Tuttnauer Elara 9D, Class B Tabletop Autoclave
Tuttnauer Elara 9D, Class B Tabletop Autoclave

How Does the Class B Autoclave Work?

1. First we remove the air. How? By use of a vacuum pump, the Class B autoclave uses vacuum pulses to extract the air in the autoclave chamber. This process is able to remove more than 99% of the air inside the chamber.

Let’s say we are going to use three pulses to extract the air. The first pulse will extract about 80% of the air, leaving 20%. Then in the second pulse, 80% of the 20% remaining will be extracted (leaving 4% of the initial air). And finally the last pulse will remove 80% of the last 4% of air, which brings us to the low overall number of 0.8% of air left in the chamber. In other words, each pulse extracts 80% of the air, so that with each pulse, that number gets much smaller, leaving us with a final number of 99% total air removal. This whole process is called fractionated pre-vacuum air removal. It eliminates air pockets and thus improves the penetration of the steam into the deepest point of the load.

In this graph you can see the pre-vacuum air removal pulses (in pink) achieved by the built-in vacuum pump
In this graph you can see the pre-vacuum air removal pulses (in pink) achieved by the built-in vacuum pump

2. After the vacuum pulses have removed almost all of the air inside the chamber, the pressure and temperature increase.

3. We have now reached the sterilization stage where the temperature and pressure are held constant for the duration of the sterilization time, also known as holding time. This is the phase when the actual sterilization takes place, i.e., goodbye to germs.

4. Next the exhaust valve releases the pressure to atmospheric pressure.

5. The last phase is the drying phase in which the load is dried completely. Again we use the vacuum pump, but this time the vacuum pump sucks out the moisture on the load. How? By decreasing the pressure to lower than atmospheric pressure, we create a vacuum inside the chamber and reduce the boiling temperature to lower than the actual boiling temperature of 100℃. This lower boiling temperature makes the water evaporate into a gas, which is then sucked out by the vacuum pump, and voilà, the load is dried!

At this point, sterilization is complete, the load is dry, and the autoclave is safe to open.

Autoclave Cycles

There are generally two cycles for autoclaving in a Class B tabletop autoclave:

  1. The standard cycle lasts about three minutes and reaches 134℃. This cycle is intended for standard materials like glassware or metal solids, hollows, porous items and textiles.
  2. The delicate cycle lasts fifteen minutes and operates at the lower temperature of 121℃. This cycle is designed for delicate materials like plastics and other delicate items that would not be suitable for the standard cycle.

Be Ready to Use Your Class B Tabletop Autoclave

Let’s review what we just learned about Class B autoclaves. We explained that the main advantage of Class B autoclaves is the use of a vacuum pump for extracting air before sterilization and drying the load after sterilization. We showed how the Class B autoclaves use a set of pulses to extract about 99% of the air inside the chamber, called fractionated pre-vacuum air removal. We looked at each step in the autoclaving process to understand exactly what happens inside the chamber when the cycle takes place. And finally, we described two common cycles, the standard and delicate cycles, and for which materials we would use each of them.

Check out our next post in this series in which we will explore large autoclave sterilizers (EN 285) found in hospitals, medical centers, laboratories and other industries that require sterile processing.

Comments and questions are welcome, as always, in the comment section below. 

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