The Complete Handbook of Autoclaves

Chapter 1: Understanding Autoclaves

The machine and the process

An autoclave is a pressure vessel that uses saturated steam under pressure to destroy microorganisms, including bacterial spores, which are the most difficult biological material to kill. You can think of it as a very powerful, very controlled pressure cooker. The difference between an autoclave and a sanitizing dishwasher is not the temperature but the quality of the steam and the attainment of a specific physical state. The machine uses high-pressure steam to push the temperature of the chamber well above the boiling point of water at atmospheric pressure, and it maintains that state for a specified period known as the hold time.

To understand why this works, look at the physics. At sea level, water boils and turns to steam at 100°C. This is the point where the vapor pressure of the water equals the ambient pressure on the surface of the liquid. When you put water in a sealed vessel and add heat, that vessel pressure rises. As the pressure rises, the boiling point of the water rises too. When the chamber pressure is held at 15 pounds per square inch above atmospheric pressure, the boiling point rises to about 121°C. That is the standard sterilization temperature. Under these conditions, it takes about 15 minutes of steady temperature to achieve a lethal effect on the most common heat-resistant spores. Increase the pressure to 30 psi above atmospheric, and you reach 134°C, which allows a much shorter hold