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Also called: Autoclave temperature, Autoclave pressure, Suhu autoclave

Autoclave Temperature and Pressure: 121 °C vs 134 °C

What the numbers on an autoclave display mean, why steam pressure and temperature move together, and which time at which temperature actually sterilises.

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Temperature and pressure are linked

A working autoclave fills its chamber with saturated steam — steam in balance with water at its boiling point. For saturated steam, every temperature has exactly one matching pressure: raise the pressure and water boils hotter, which is why an autoclave is a pressure vessel. If the temperature and pressure on a printout do not match the steam table, the chamber holds air or superheated steam and the load is at risk.

Saturated steam near sea level. Gauge pressure = absolute pressure minus about 1.01 bar of atmosphere.
Sterilising temperatureAbsolute pressureGauge pressureGauge, in psi
121 °C≈ 2.05 bar (205 kPa)≈ 1.04 bar (104 kPa)≈ 15 psi
126 °C≈ 2.39 bar (239 kPa)≈ 1.38 bar (138 kPa)≈ 20 psi
134 °C≈ 3.04 bar (304 kPa)≈ 2.03 bar (203 kPa)≈ 29–30 psi

Gauge vs absolute pressure

Gauge pressure

Measured above the surrounding air, so a reading of zero means normal atmospheric pressure. Most clinic autoclave dials and printouts use gauge pressure, which is why a 134 °C cycle shows roughly 2 bar.

Absolute pressure

Measured from a perfect vacuum. Steam tables are written this way, and many Class B printouts show their vacuum pulses in absolute terms because a vacuum is simply a pressure below atmospheric.

Units on printouts

1 bar = 100 kPa = 0.1 MPa ≈ 14.5 psi. Different brands print bar, kPa or MPa — check whether a figure is gauge or absolute before comparing two machines.

Hold time: time at temperature, not cycle length

The hold (plateau) starts only when the chamber — and in a well-designed cycle, the load — is inside the sterilising temperature band. Air removal, heat-up, exhaust and drying come on top, which is why a 3-minute hold sits inside a 30- to 60-minute cycle. If the temperature drops below the band during the hold, the cycle should fail rather than quietly stretch.

Temperature bandMinimum holdWhere you will see it
121–124 °C15 minutesAutoclavable plastics, rubber and textiles; lab liquids
126–129 °C10 minutesSome hospital and older sterilisers
134–137 °C3 minutesThe standard instrument cycle on clinic tabletop units
134 °C extended18 minutesPrion cycle, where infection-control policy requires it
132 °C4 minutesPrevacuum cycles in US-published instructions

Read next How steam sterilisation worksTests that prove the cycle worked

Liquids and heavy loads lag behind the chamber

  • A bottle of liquid heats far more slowly than the chamber air, so the display can read 121 °C long before the centre of a large bottle gets there.
  • Liquid cycles therefore use longer holds chosen by the largest container, or a load probe in a reference bottle that starts the hold only when the liquid itself is at temperature.
  • Exhaust must be slow so superheated liquid does not boil over, and the door must stay shut until the liquid has cooled.
  • Clinic tabletop units without a validated liquids programme should not be used for liquids.
  • Dense instrument trays and heavy packs lag too — stay within the maker’s maximum load per tray.

Read next Laboratory autoclaves and liquid cycles

Material limits — the instructions for use decide

Under ISO 17664, device makers must publish reprocessing instructions, including the sterilisation temperature and time their product tolerates. The instrument’s instructions for use (IFU) — not the autoclave — set the limit.

Metal instruments

Stainless-steel instruments generally tolerate 134 °C. Carbon-steel and plated instruments can pit or corrode, so follow their IFU.

Plastics and rubber

Polypropylene and silicone tolerate 121 °C and often 134 °C; many other plastics soften or warp. If the IFU says 121 °C, never run 134 °C to save time.

Handpieces and powered devices

Dental handpieces and motors carry a maximum temperature in their IFU. Exceeding it shortens bearing and seal life.

Heat-sensitive devices

Devices that cannot take 121 °C need a validated low-temperature method or single-use replacements.

Plasma and ethylene oxide sterilisers

Related guides

Steam Sterilizer: How Steam Sterilisation WorksTemperatures, pressures, air removal and the common reasons a steam sterilisation load fails — explained for clinic and CSSD buyers.Class B Autoclave vs Class N and Class SThe EN 13060 classes decide which loads your autoclave can legitimately sterilise. Here is what each class covers and how to tell them apart.Autoclave Testing: Bowie-Dick, Indicators and ValidationThe daily, weekly and yearly checks that prove your autoclave is sterilising — and what autoclave tape and pouches can and cannot tell you.

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Frequently asked questions

What temperature does an autoclave reach?
Clinic autoclaves sterilise at 121 °C or 134 °C, with 134 °C the standard for most metal instruments. The hold is at least 15 minutes at 121 °C or 3 minutes at 134 °C.
What is autoclave pressure in psi?
For saturated steam, about 15 psi above atmospheric at 121 °C and about 29–30 psi above atmospheric at 134 °C.
Is 134 °C better than 121 °C?
It is faster, not better. Both sterilise when held for the right time; 134 °C for 3 minutes suits heat-tolerant instruments, while 121 °C for 15 minutes is gentler on plastics, rubber and textiles.
Why can an autoclave show the right temperature and still fail the Bowie-Dick test?
The sensor reads the chamber or drain, not the inside of a pack. Trapped air keeps parts of the load cooler even when the display looks right — exactly what the air-removal test is designed to catch.

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