The Importance of Venting a Pressure Cooker or Autoclave

Two test have been conducted so far one with a presto 23q and one with an AA 25x sterilizer.

A nearly new presto 23q pressure cooker was used for this test
There was one quarter taped to the weight.
The first trial I put the lid on the cooker with 1q of the hottest tap water I have inside. Put the weight on the lid immediately. no venting done. Turned the stove to 10.
As soon as the weight started to jiggle I killed the heat.


244F ±2F

The second trial I put the lid on the cooker with 1q of the hottest tap water I have inside. I turned the stove to 10. vented for 10m after steam appeared from the vent. put the weight on and as soon as the weight started to jiggle I killed the heat


258-259F ±2F

For both test runs I put the trivet in and two empty mason jars upright with no lid to stand the thermometer on.
In both tests the needle on the gauge got up in between 17-18psi
at 18psi I shouln't have seen such high temperatures on the thermometer but considering the thermometer is calibrated to NIST and the gauge on the presto is from china I'm going to go with the gauge being a hair off on the low side. The gauge is accurate to one scale division so ±2F

This may seem like a small difference but the difference is huge in terms of sterilization effectiveness. Not only is the temperature lower but the lower heat capacity of air compared to steam means you are not as energetically pumping the heat into the media being sterilized. Steam has a high latent heat.

Steam has more "heat torque" than air

Quote:

Latent heat can be understood as heat energy in hidden form which is supplied or extracted to change the state of a substance without changing its temperature.

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Because of a much higher heat capacity, steam and liquids transfer heat more rapidly than air. For example, the hot air inside an oven at 200 °C (392 °F) will not immediately burn your skin, but the steam from a boiling kettle at 100 °C (212 °F) will scald skin almost instantly and feel hotter, despite the steam (and water) in the kettle being at a lower temperature than the air inside a hot oven. Thus the internal temperature of material in a pressure cooker will rise to the desired value much more quickly than if it were in an oven.

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It is very important to ensure that all of the trapped air is removed from the autoclave before activation, as trapped air is a very poor medium for achieving sterility. Steam at 134 °C can achieve in three minutes the same sterility that hot air at 160 °C can take two hours to achieve

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Quote:

Effective moist-heat sterilization depends on having saturated steam conditions. When air is present in the system to be sterilized, saturated steam temperature and pressure conditions no longer apply.

The air dilutes the steam and a lower temperature than expected is imposed at a given pressure. Trapped pockets of pure air completely prevent steam contact and provide dry-heat sterilization conditions.

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Test 2 with an All American 25x sterilizer with the vent tube removed
Using no basket just the trivet rack. Starting with hot ~140F tap water for both runs.

for both runs I'm going to bring it up to 15 psi and toggle the on off switch to keep it at 15psi for 5m. Then kill it.

For the venting run ill vent 10m. Get to 15m and hold it there 5m then kill it. For the non vent run I'm putting the lid on with the toggle closed and skipping the venting entirely

Interesting enough is that with the toggle closed from the beginning (0 venting) I could stand to hold my hand on the lid with pressure registered on the gauge. Because air was getting compressed before the water was even boiling. And once it started to boil it quickly compressed air giving an abnormally high pressure for a low temperature.




Got up to 15.5-16psi kept it there 5m before shutting down. This is what I got off the thermometer. Very interesting.

212±2F

Here's the reading from the vented run. Also hit close to 16 psi during the 5m hold.

252±2F

that's a 40F difference and there was a 5 minute hold just to make extra sure the thermometer registered the temp at 15psi.


The toggle holds in air better than the jiggle weight on the presto. so the presto sort of vents itself during the run, the toggle on the AA really trapped the air in it seems.




Vent your pressure cookers.

bodhisatta said:

I've wanted to see this for myself for so fucking long. For $60 dollars I don't know why I waited so long

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I might know why

   

bodhisatta said:

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