No Pressure

Low Or No Autoclave Pressure

This chapter covers autoclave pressure and how to determine why your unit is not building or maintaining pressure. Pressure is created & controlled in the first two phases of the Sterilization Cycle:

    YesFill Cycle
    YesSterilize Cycle

In order to have autoclave pressure these processes must occur:

    YesThe Right Amount Of Water has to be in the chamber.
    YesThe water has to be converted to steam
    YesAir has to be removed from chamber
    YesThe Chamber has to be completely sealed

All of these components are required to build and maintain autoclave pressure. If any are faulty, then the pressure process will not occur

Visual Inspection

Remove the Cover On Your Autoclave & Start a Cycle Like You Normally Would

Visually inspect the system for leaks. Any leak, no matter how small will keep the unit from developing and maintaining the autoclave pressure at the required level for sterilization to be completed.

Autoclave Pressure And The Fill & Sterilization Cycles

During the start up phase of the Autoclave Sterilization Cycle, the following process occur:

    Water Water enters the Autoclave chamber and fills to a predetermined level
    Seal The chamber is sealed & heated, turning the water into steam.
    Bleed Air is evacuated from inside the chamber in the early stages of the sterilization process.
    Heat Steam is the gaseous state of water and expands as it is heated - this creates the autoclave pressure
    Steam The heating elements turn off and on as needed to maintain the right temperature & pressure
    Vent At the end of the sterilization cycle, the heat is turned off and the water and steam are vented back into the reservoir.

Open the door to see if water is entering into the chamber and filling to it's recommended level (make sure the autoclave is level)

Autoclave Pressure & The Fill Cycle

If water is not entering the autoclave chamber, or if the water drips in slowly, examine the path the water takes to get into the chamber. If there is no (or not enough) water in the autoclave chamber, no autoclave pressure can be built up or maintained

    Reservoir The water is contained in a reservoir - Make sure the reservoir is full (to it's max level mark)
    Fill Valve In the fill cycle, the water flows through a tube into the main valve body
    Function Switch The Main Valve Body contains the Fill Valve which acts like a gate and is opened & closed by the Function Switch
    Water In Chamber In the Fill Cycle, the Function Switch opens the fill valve and allows water to pass through the main body and on to the inside of the autoclave chamber

Trace this route to determine where the water is stopping along its path between the reservoir and the chamber. Check for filters along the path and make sure they are allowing the water to pass through. Filters should be replaced on a regular basis.

Autoclave Pressure & Heat

If the right amount of water is entering the chamber, the next thing to look at for the loss of autoclave pressure is the heat. To build pressure, the water must be turned into its expanding, gaseous state (steam). The boiling point (212° F) is not enough. 270° F is ideal.Heat

Start by looking at your autoclave pressure gauge or temperature read out. What temperature is being registered?

If the autoclave temperature is below 260°, you will want to test your heating elements and Over Temperature Switch. We have greatly simplified this procedure for you.

Just find the heating element for your model here on this site and follow the step-by-step instructions to determine if the heating element is the source of your lack of autoclave pressure. Do the same for your Autoclave Over Temperature Switch (aka Overheat Thermostat)

If you are getting the right autoclave temperature (between 260°F & 270°F), the next step is to look for leaks where the autoclave pressure (air) can be escaping.

Autoclave Pressure & The Vent Valve

Look inside the autoclaves reservoir prior to starting the fill cycle and make a note at where the water line is


Start the fill cycle and watch the reservoir and make another note where the water line is at the end of the fill cycle

Now start the sterilize cycle and watch the water level

If the water line returns back to its starting point (or close to it) then the vent valve is stuck in the open position. This leaves the chamber empty of water and no autoclave pressure can be made. The vent valve will have to be repaired

Turn the autoclave off if the water level rises back towards it's starting point. Failure to do so will allow the temperature to continue to rise without autoclave pressure and can damage your chamber

Autoclave Pressure & The Door Gasket

Is steam escaping around the door? If it is, this is where your autoclave pressure is going. Replace the Door Gasket.

Door Gasket

As a side note, appearances can be deceiving. The gasket may look perfect, but in fact is not.

The gaskets shrink or become distorted over time. The shrinkage is normally not visual to the naked eye. And this is the reason why replacement is recommended by all manufacturers at a minimum of once every 12 months (every 6 months if heavy use)

Autoclave Pressure & The Bellows

A tube runs from the autoclave chamber to the bellows. The bellows acts like a spigot; opening and closing. At the end of the sterilization cycle, the bellows opens and allows the autoclave pressure to escape and the steam to be returned to the reservoir via the condensing tube


It closes when the chamber heats up, opening occasionally to bleed air from the chamber and then sealing shut. If the bellows is faulty, autoclave pressure can find it's way through, and out of, the chamber. It is in effect leaking. This is a normal wear and tear item that should be replaced on a regular basis (at least once per year for most makes and models).

Testing the autoclave bellows can be difficult. But, you can remove the bellows on most models and visually inspect it. The bellows, if like the one pictured here, when cool (room temperature) is in a contracted state and is approximately 1¾" long from tip to tip. If it is extended beyond that, autoclave pressure cannot be acheived & replacement is required. It is best to just replace the bellows if you hasn't been replaced in a while

Autoclave Pressure & The Condensing Tube

The autoclave condensing tube is located inside the reservoir.

Codensing Tube At the end of the sterilization cycle, the bellows opens and allows the live steam to enter the condensing tube. At this point, most of the autoclave pressure is immediately evacuated.

As the steam makes it through the coil of the condensing tube, it is cooled enough to return to its water state and is returned to the reservoir as water.

Remove the lid from the reservoir and look inside during the sterilization cycle. If there is constant bubbling (like champagne bubbles) throughout the sterilization cycle, look closer.

Upon closer inspection you will be able to see exactly where the bubbles are coming from. If the bubbles are coming from anywhere on the coil, there is a pinhole in the condensing tube and it will have to be replaced. No autoclave pressure can be built or maintained in this state

The Autoclave Pressure Relief Valve

Pressure Relief Valve

The Autoclave Pressure Relief Valve is either located on top of the chamber or connected directly to the chamber. The valve is a safety device for the autoclave pressure.

If the autoclave pressure inside the chamber exceeds the preset limit of the valve, it will open and allow the steam (and pressure) to escape.

Over time, the seals in these valves can become worn and become "leaky" allowing the autoclave pressure to escape

Autoclave Pressure & The Autoclave Chamber

Check for pinholes in the bottom of the autoclave chamber. Remove the trays and, while the chamber is cool), run your fingers along the chamber floor


If the surface flakes, there is a good chance there are pinholes. Refer to the section on replacing your autoclave heating elements and remove them far enough to give you access with your hand to feel the bottom.

If it flakes as well, there is a pinhole. Visual inspection of the bottom of the chamber will also show burnt watermarks where the heater has "fried" the water as it drips

Pinholes are almost always caused as a result of the heat staying on when the autoclave chamber is dry (like leaving it on over night)

If there are pinholes in the chamber, autoclave pressure cannot be maintained and you should replace the autoclave. The cost of Autoclave Replacement Chambers (if you can find one at all) surpasses the replacement costs of the autoclave at this time.

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