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After cooking using a pressure cooker, it usually takes a while for the pressure to be released and for the cooker to be opened. However when I think the food may have been slightly overcooked, I take the pressure cooker from the stove and put it under running cold water from a tap. Within a few seconds, I am able to easily remove the lid off the cooker.

Why does reducing the temperature on the surface of cooker help me accelerate the reduction of pressure inside the cooker? Is this method safe?

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  • $\begingroup$ Sounds like you are talking about a conventional, stove-top pressure cooker. In that case, Why can't you simply release the pressure by removing the weighted cap off the top of the "regulator?" Just be careful! Wear an oven mitt so that you don't get burned by the escaping steam. $\endgroup$ Jun 28 '20 at 20:14
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The pressure in a pressure cooker (that is the excess pressure over atmospheric) is due to water vapour (steam). The hotter the water the greater the evaporation from it and the greater the vapour pressure.

"Why does reducing the temperature on the surface of cooker help me accelerate the reduction of pressure inside the cooker?" Cooling the surface will cause heat to flow through the metal from the inside wall. Water vapour will condense back to steam on the cooled parts of the inside wall, reducing the pressure.

As far as I can tell, there is no danger in cooling the outside of the vessel, but don't take my word for it: check with the makers.

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  • $\begingroup$ I would definitely say check with the makers. If it's mostly metal I wouldn't be as worried, but if there are any ceramic parts or anything, thermal shock could be something to worry about, at least for the health of the pressure cooker. $\endgroup$
    – JMac
    Jun 28 '20 at 17:24
  • $\begingroup$ Agree with you 100%. $\endgroup$ Jun 28 '20 at 17:29
  • $\begingroup$ @PhilipWood, hopefully you will not consider this "nitpicking". When cold water is poured on the outside of a hot pressure cooker, heat flows through the metal from the inside wall. Steam will condense back to water on the inside of the pressure cooker. You may want to check the wording in your reply, which states that water vapour will condense back to steam (no doubt confusing some readers). $\endgroup$ Jun 28 '20 at 22:13
  • $\begingroup$ It's not nitpicking, but a welcome pointing out of a silly typo. Thank you. $\endgroup$ Jun 28 '20 at 22:48
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I think you may be referring to the partial vacuum that develops when cooling a sealed pressure cooker, as evidenced by the Ideal Gas Law:

$$pV=nRT$$

As $V$, $n$ and $R$ are all constants, this reduces to:

$$\frac{p}{T}=\text{constant}$$

So reducing $T$ demands $p$ also decreases.

However, pressure cookers are built to withstand (modest) over-pressures and for that reason will also withstand the partial vacuum that arises when rapidly cooling a sealed, hot pressure cooker.

If the cooker starts at room temperature and atmospheric pressure, pressure then builds up as you heat it on the stove.

Cooling the hot cooker back to room temperature then its pressure also reduces back to atmospheric pressure. No danger at all, IMO.

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    $\begingroup$ Gas law pressure changes are not the problem unless the pressure cooker is empty;; vapor pressure changes are much larger... $\endgroup$
    – DJohnM
    Jun 28 '20 at 18:41

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