Autoclaves

Last updated: August 31, 2009.

Be glad, be very glad that you're eyes aren't as powerful as electron microscopes. If they were, you'd see the world around you crawling with all kinds of horrible bugs. How filthy and nasty life would seem! Just as well, then, that we have autoclaves: machines for sterilizing such things as surgical instruments and hospital equipment. They're a bit like giant pressure cookers that use the power of steam to kill off germs that would survive a simple washing with boiling water and detergents. Autoclaves have many other scientific uses too. Let's take a closer look at what they are and how they work!

Photo: Looking inside the open door of a large autoclave. This one is used by the United States Air Force at Wright Aeronautical Laboratories for assembling composite materials. Photo by US Air Force courtesy of Defense Imagery.

What does an autoclave do?

Pressure cookers were all the rage until microwave ovens became popular in the 1980s. They're like oversized saucepans with lids that seal on tightly and, when you fill them with water, they produce lots of high-pressure steam that cooks your food more quickly. Autoclaves work in a similar way, but they're typically used in a more extreme form of cooking: to blast the bugs and germs on things with steam long enough to sterilize them. The extra pressure in an autoclave means that water boils at a temperature higher than its normal boiling point—roughly 20°C hotter—so it holds and carries more heat and kills microbes more effectively. A lengthy blast of high-pressure steam is much more effective at penetrating and sterilizing things than a quick wipe in ordinary hot water!

Photo: Autoclaves are widely used in hospitals. Here, sterilized bedding is being unloaded from an autoclave at a US military hospital in Japan in 1976. Photo by courtesy of US Army and Defense Imagery.

What is an autoclave?

An autoclave is essentially just a large steel vessel through which steam is circulated to sterilize things, perform scientific experiments, or carry out industrial processes. Typically the chambers in autoclaves are cylindrical, because cylinders are better able to withstand extreme pressures than boxes, whose edges become points of weakness that can break. The high-pressure makes them self-sealing (the words "auto" and "clave" mean automatic locking), though for safety reasons most are also sealed manually from outside. Just like on a pressure cooker, a safety valve ensures that the steam pressure cannot build up to a dangerous level.

Photo: Closing the door on a typical laboratory autoclave. Note the large handle on the right being used to seal the door completely. Also note the dials on the right-hand side that indicate temperature and pressure. Photo by PHAA Sarna courtesy of US Navy and Defense Imagery.

How do you use an autoclave?

Once the chamber is sealed, all the air is removed from it either by a simple vacuum pump (in a design called pre-vacuum) or by pumping in steam to force the air out of the way (an alternative design called gravity displacement). Next, steam is pumped through the chamber at a pressure of at least 15psi so it reaches a temperature of about 121–140°C (250–284°F). Once the required temperature is reached, a thermostat kicks in and starts a timer. The steam pumping continues for a minimum of about 3 minutes and a maximum of about 15-20 minutes (higher temperatures mean shorter times)—generally long enough to kill most microorganisms. The exact sterilizing time depends on a variety of factors, including the likely contamination level of the items being autoclaved (dirty items known to be contaminated will take longer to sterilize because they contain more microbes) and how the autoclave is loaded up (if steam can circulate more freely, autoclaving will be quicker and more effective).

Autoclaving is a bit like cooking, but as well as keeping an eye on the temperature and the time, the pressure matters too! Safety is all-important. Since you're using high-pressure, high-temperature steam, you have to be especially careful when you open an autoclave that there is no sudden release of pressure that could cause a dangerous steam explosion.

What else are autoclaves used for?

Although best known as sterilizers, autoclaves can also be used to carry out scientific experiments that work best at high-temperatures or pressures. Thus, nylon is made by "cooking" a concentrated salt solution in an autoclave to encourage the formation of long-chain polymer molecules.

Photo: Inspecting a crystal grown in microgravity inside a cylindrical autoclave. This scientific experiment was carried out onboard the Space Shuttle in October 1995. Photo by courtesy of NASA Marshall Space Flight Center (NASA-MSFC).

Who invented autoclaves?

• Ancient Greeks use boiling water to sterilize medical tools.
• 1679: French engineer Denis Papin (1647–1712) invents the steam pressure cooker—an important step in the development of steam engines.
• 1860s: French biologist Louis Pasteur (1822–1895) helps to confirm the germ theory of disease. He realizes that heating things to kill germs can prevent diseases and extend the life of foodstuffs (which leads him to the invention of pasteurization).
• 1880s: Pasteur's collaborator Charles Chamberland (1851–1908) invents the modern autoclave.