How does an autoclave work?

Autoclaves are often mentioned in discussions concerning piercing or tattoo shops, sterilization, and even in regards to medical procedures, but what are they and what do they do?

An autoclave is an instrument used to sterilize equipment and supplies by subjecting them to high pressure saturated steam at 121 °C for around fifteen to twenty minutes depending on the size of the load and the contents. It was invented by Charles Chamberland in 1879, and the machines themselves vary in size and function depending on the media to be sterilized. Autoclaves are widely used in microbiology, medicine, tattooing, body piercing, veterinary science, mycology, dentistry, chiropody and prosthetics fabrication. Typically they are used to sterilize laboratory glassware, surgical instruments, medical

waste, patient care utensils, animal cage bedding, piercing needles, and other tools. Because damp heat is used in the process, products (such as some plastics) cannot be sterilized this way or they will melt. Paper and other products that may be damaged by steam must also be sterilized another way. In all autoclaves, items should always be separated to allow the steam to penetrate the load evenly.

When using an autoclave, it is very important to ensure that all of the trapped air is removed from the inner chamber before activation; hot air is actually a very poor medium for achieving sterility (steam at 134 °C can achieve in three minutes the same sterility that hot air at 160 °C takes two hours to achieve). There are different methods of air removal including:

Downward displacement: As steam enters the chamber, it fills the upper areas first as it is less dense than air. This compresses the air to the bottom, forcing it out through a drain which often contains a temperature-sensing device. Only when air evacuation is complete does the discharge stop. Flow is usually controlled by a steam trap or a solenoid valve, but bleed holes are sometimes used, often in conjunction with a solenoid valve. As the steam and air mix it is also possible to force out the mixture from locations in the chamber other than the bottom.

Steam pulsing: air dilution by using a series of steam pulses, in which the chamber is alternately pressurized and then depressurized to near atmospheric pressure.

Vacuum pumps: a vacuum pump sucks air or air/steam mixtures from the chamber.

Superatmospheric cycles: achieved with a vacuum pump. It starts with a vacuum

followed by a steam pulse followed by a vacuum followed by a steam pulse. The number of pulses depends on the particular autoclave and cycle chosen.

Subatmospheric cycles: similar to the superatmospheric cycles, but chamber pressure never exceeds atmospheric pressure until they pressurize up to the sterilizing temperature.

After the autoclave has run through it’s cycle, how can you be sure that the autoclave worked? There are physical, chemical, and biological indicators that can be used to ensure that an autoclave reaches the correct temperature for the correct amount of time. Most shops that use an autoclave for sterilization have medical packaging and autoclave tape with chemical indicators that change color once the correct conditions have been met. This tells you that the object inside the package has been appropriately processed and is safe for use. This however is not enough; a spore test is also required to ensure an autoclave is reaching both a high enough temperature and the correct pressure to truly sterilize. If the temperature is correct but the pressure is not, sterilization will not have occurred and vice versa. A shop can buy a service through an outside company and then run a spore test once a month. The lab sends the shop a  test kit that includes a biological pathogen, this kit is run through the autoclave and sent back to the lab which determines if the spore is dead. This is an inexpensive service and a huge assurance that the needles, tapers, or body jewelry about to be put in your piercing are safe and clean!