Wikipedia:Reference desk/Archives/Science/2020 December 12

= December 12 =

Pressure cooker
I have an electric pressure cooker. Let's say I heat up several liters of water in it, so it is at 1 atm of steam pressure. So there is liquid and pressurized gas in the pot, at (I believe) a temperature of about 125 celsius. Now I release the steam and open the lid. There is liquid water in the pot, bubbling somewhat, which means it is at 100C. There was no cloud of superheated steam or anything like that.

Does that mean the water somehow cooled from 125C to 100C? Just from the gas expansion of the steam being let out? It seems like an awful lot of heat to shed that way. Is some other part of the picture wrong? If not, where did the heat go? Thanks. 173.228.123.178 (talk) 23:27, 12 December 2020 (UTC)

Added: I guess some of the liquid inside turns to vapor when the pressure released, letting out heat of vaporization which for water is pretty high, but I'm still not sure that explains everything. 173.228.123.178 (talk) 00:13, 13 December 2020 (UTC)

You answered yourself when you said "Now I release the steam". Inside the pressure cooker there will be a furious evolution of steam as the superheated water turns to steam and the heat of vaporisation is taken from the thermal energy of the remaining water, which cools down in consequence. From memory my mother let the pressure cooker cool before releasing the steam, the resulting food put me off pressure cookers for life. Greglocock (talk) 00:41, 13 December 2020 (UTC)
 * The water at 125C will boil furiously and probably foam up enormously if the pressure is suddenly released, this will remove heat from the water, but is very dangerous, so usually you cannot safely take the top off a pressure cooker. Also if you are high in elevation it will boil at less than 100°C. Graeme Bartlett (talk) 11:39, 13 December 2020 (UTC)
 * There are many PC recipes that call for releasing the steam early (so-called quick release). You don't take the top off, you press down on a button and that lets the steam shoot through a little valve.  I think I understand now though: the heat is taken away by water vaporization and the steam exiting.  If my math is right, about 5% of the water mass should be vaporized by this process.  That shouldn't be too hard to check experimentally with a kitchen scale (just weigh the water before and after).  I might try it sometime. 2602:24A:DE47:BB20:50DE:F402:42A6:A17D (talk) 09:15, 14 December 2020 (UTC)
 * Yes, many kitchen scales have a plastic or glass pans, which may melt or break if you put a thick, hot pot on them. --CiaPan (talk) 10:56, 14 December 2020 (UTC)
 * Good point. A wood chopping board or any 1" thick piece of wood would be a wise precaution Greglocock (talk) 21:36, 14 December 2020 (UTC)