How Does an Autoclave Work?

An autoclave is a pressure chamber that uses steam to bring every surface of a load to a set temperature and hold it there. Why steam works so much faster than hot air, why the air has to go first, what the pressure is actually for, and what happens in each phase of a cycle.

WhatsApp for a quote

Also called How autoclaves work, Steam sterilisation principle, Moist heat

Published: 17 October 2026

Last updated: 16 October 2026

Moist heat, not dry heat

An autoclave sterilises with moist heat. Steam and water break down the proteins microorganisms depend on far more readily than hot, dry air does, so a steam cycle kills bacteria, viruses, fungi and bacterial spores at lower temperatures and in much less time than a hot-air oven.

Method	How it kills	Typical sterilising conditions
Steam (moist heat) — autoclave	Coagulates and denatures enzymes and structural proteins	Held at 134 °C for at least 3 minutes, or 121 °C for at least 15 minutes
Dry heat — hot-air oven	Oxidises cell components, much more slowly	160 °C for 2 hours to 180 °C for 30 minutes

Why steam carries so much heat

Condensation releases latent heat

When steam touches a cooler instrument it condenses back into water and gives up its latent heat — roughly 2,200 kJ for every kilogram condensed, several times the energy needed to bring the same water from room temperature to the boil. That heat goes straight into the surface the steam touched.

Cold spots pull in more steam

Condensing steam shrinks to a tiny volume of water, which draws fresh steam towards the cooler surface. The coldest parts of the load attract the most steam until everything reaches the same temperature.

Saturated, not wet or superheated

Saturated steam sits at exactly the temperature for its pressure, so it condenses readily. Wet steam carries water droplets that soak packs; superheated steam behaves more like hot air and condenses poorly. Both sterilise less reliably.

Steam quality and why loads fail

The surface has to be clean

Steam has to touch the surface to work. Dried blood, tissue, oil or cement shields what lies beneath, which is why cleaning always comes before sterilising.

Cleaning instruments before sterilisation

What the pressure is for

Pressure is the means, not the goal. At normal atmospheric pressure water boils at 100 °C, which is too cool to kill spores quickly. Sealing the chamber and raising the pressure lets steam reach higher temperatures: about 1 bar above atmospheric gives saturated steam at 121 °C, and about 2 bar gives 134 °C. It is that temperature, held at every surface for long enough, that sterilises.

For saturated steam, pressure and temperature move together, so the controller can check one against the other.
If the pressure is right but the temperature is low, air is usually still in the chamber.
Pressure on its own does not kill microorganisms; hot air at the same pressure would sterilise far more slowly.

Air has to come out first

Air is the enemy of steam sterilisation. A pocket of air insulates the surface beneath it, and air mixed with steam lowers the temperature at a given pressure. How a machine removes air decides which loads it can sterilise.

Passive (gravity) air removal

Steam enters and pushes the heavier air out through a drain. This works for unwrapped solid instruments, but air can stay trapped in pouches, porous packs and narrow lumens. EN 13060 Class N machines work this way.

Class N autoclaves

Fractionated pre-vacuum

A vacuum pump pulls air out, steam is let in, and the cycle repeats several times, so air is drawn out of the inside of hollow and wrapped items before the hold begins. This is the principle behind a Class B autoclave.

Class B autoclaves

Proving air removal

A daily Bowie-Dick or helix test shows whether steam reached the centre of a test pack or the far end of a long tube. A temperature display cannot show that on its own.

Bowie-Dick and helix tests

Inside the machine

Part	What it does
Chamber and door	A pressure vessel with a locking door and silicone seal; the door stays locked while the chamber is under pressure
Steam generator	Turns distilled or deionised water into steam, either in the chamber or in a separate generator
Vacuum pump (Class B)	Removes air before the hold and pulls moisture out of the load during drying
Valves and filters	Let steam in, vent the chamber, and filter the air that re-enters at the end of the cycle
Sensors and controller	Temperature and pressure sensors feed a controller that runs the cycle and aborts it if values drift out of range
Safety valve	Releases pressure if every other control fails
Printer or data logger	Records each cycle’s time, temperature and pressure for the steriliser log

A cycle, phase by phase

Phase	What happens	If it goes wrong
Air removal and heat-up	Vacuum pulses or steam displacement remove air while the chamber and load heat up	Leaks, a weak pump or overloading leave cold spots — caught by a failed Bowie-Dick, helix or vacuum test
Exposure (hold)	Saturated steam keeps every surface at the set temperature for the hold time	The controller aborts the cycle if temperature or pressure leaves its band
Exhaust	Steam is vented and the pressure falls back towards atmospheric; liquid programmes do this slowly	Fast exhaust boils liquids over; a blocked drain leaves water in the chamber
Drying	Vacuum or chamber heat drives off condensate so packs come out dry	Overloading, stacked pouches or poor water leave wet packs, which are not treated as sterile
Air admission and end	Filtered air returns the chamber to atmospheric pressure and the door unlocks	A cycle that ends with an error has not sterilised, whatever the indicators show

How you know it worked

Physical: the printout or log shows the hold temperature, pressure and time for every cycle.
Chemical: indicators inside each pack change colour only when time, temperature and steam have all been present.
Biological: a spore test, commonly at least weekly, shows that highly resistant Geobacillus stearothermophilus spores were actually killed.
“Sterile” under European standard EN 556-1 means a sterility assurance level of 10⁻⁶ — no more than a one-in-a-million chance that a single viable microorganism remains on an item.

Autoclave supply near you

Written quotes for clinics across Malaysia, handled from Batu Caves, Kota Kinabalu and Kuching.

See all 28 locations

Frequently asked questions

Is an autoclave moist heat or dry heat?

Moist heat. An autoclave sterilises with saturated steam under pressure. Dry-heat sterilisers use hot air instead and need higher temperatures and much longer times, such as 160 °C for 2 hours.

How do sterilizers kill bacteria and spores?

Heat denatures and coagulates the proteins microorganisms need to live. Steam does this quickly because condensing steam delivers a large amount of heat straight to every surface it touches.

What is the principle of a Class B autoclave?

Fractionated pre-vacuum: several alternating vacuum and steam pulses remove air from inside hollow, porous and wrapped items before the sterilising hold, followed by vacuum drying. That is why EN 13060 Class B is proven on those loads.

Why does an autoclave need pressure?

To raise the boiling point of water. Pressure lets steam reach 121 °C or 134 °C instead of 100 °C, and it is that temperature, held long enough at every surface, that sterilises.

Get a written quotation

Tell us your clinic or lab type, daily workload and location. We match suitable models and send a written quotation that states who handles delivery, installation and training, and what each costs.

WhatsApp us