A pressure cooker heats water up higher than boiling point in a heavy-duty sealed container. When the cooker is getting up to temp, steam comes out. Then when it’s at temp it seals and there is no more steam. When it’s in this state with super-heated water, what is the humidity of the air inside the pressure cooker? Is the air like a super thick fog or does the water condense out?
First of all, the water in a pressure cooker is not superheated. Superheated water remains liquid at above its boiling point. The water in a pressure cooker is at its boiling point. It’s just that its boiling point is higher at that pressure than it would be at standard pressure.
Second, whenever a system is at equilibrium, and there’s a reservoir of liquid water in the system, it’s at 100% humidity, by definition. There’s always some water evaporating, and some water condensing, and at equilibrium, they balance out. If they don’t balance out, then you’ll either end up with net water condensing until they do, or with net water evaporating until the air is either saturated, or the liquid water is used up. And at the kinds of conditions found in a pressure cooker, this process of reaching equilibrium will happen very quickly.
It probably will not, however, form a fog, or at least not a significant one. While there is constantly liquid water condensing out of the vapor, and fog is one way of that happening, it’s not the only way, nor the easiest. It’s much easier for water to condense out at a surface, either the inside of the pot or the top of the liquid water. You only get significant fog where there isn’t enough surface available for that to happen, which is why clouds are usually far above the ground.
Relative humidity only makes sense in terms of how much water vapor air contains. A pressure cooker is nearly completely purged of air by the time the safety relief valve closes. When the service relief valve (weight on old ones, spring on many newer ones) starts cycling, it will quickly purge any remaining air.
So what you have above the food is wet steam. It is wet because the liquid under it prevents superheating. More liquid will convert to vapor to absorb any excess heat.
If the humidity is 100% inside, is that the same as relative humidity? When the weather report says it’s 100% humidity, the air is not 100% water. There’s still air, but it’s just that the air is holding 100% of the water it can normally hold.
I guess I’m wondering how much more humid is the air inside the pressure cooker relative to the air outside. So say I’m in my 72 degree kitchen with 50% humidity sanding next to my hot pressure cooker. How much water is in a cubic foot (or whatever) of pressure cooker air and how much water is in a cubic foot of kitchen air?
When the pressure cooker begins to boil air will begin to vent out, that air will begin to approach 100% relative humidity. As the cooker keeps continuing to boil all the air will be vented. The pressure cooker will no longer contain a mixture of air and water vapors, but only water vapor (steam). There is no relative humidity.
I am not going to dig out my thermo dynamics or steam tables books so I can tell you how much water vapor per cubic foot will be in each condition. but I can tell you a cubic foot of water vapor will have more water in it than a cubic foot of air at 100% relative humidity.
And at that point you talk about the concept of vapor quality.
At 72 degrees and 50% relative humidity, the density of atmospheric water vapor is 0.0006145 pounds per cubic foot.
According to Google, a pressure cooker can hit 250F. At this temp, the density of the saturated water vapor inside the cooker is 0.07231 pounds per cubic foot. This is about 118 times more than your described ambient conditions, but it’s actually pretty close to the total atmospheric density (O2+N2+misc) of 0.07491 pounds per cubic foot. So it’s not some super-dense soupy fog over your chili; it’s just that the air in there has been replaced with hot water vapor.
“Relative humidity” is generally reserved for description of indoor/outdoor ambient environment, not for pressure vessels, although I don’t think that’s part of the strict definition of the term. If you wanted to apply it to your pressure cooker, you could say it’s at 100% relative humidity in there, since you can’t increase the density of the water vapor in there without raising the temperature.
Thanks. That’s what I was wondering about but was using the wrong terms.
Relative humidity is relative to the saturation point, where this cubic foot of air is holding the maximum amount of water vapor. In this case this saturation level would be 0.0012290 pounds of water vapor (at sea level). We only have 0.0006145 pounds of water which is half what the air can hold, thus we say the air is at 50% relative humidity. If we had 0.0009275 pounds of water vapor, relative humidity would be 75%.
This, exactly this, and nothing more or less. Question answered completely.
No. If saturated air is holding the maximum amount possible, then what is supersaturation? Is that when the air is doing the impossible?
Relative humidity is relative to equilibrated air with a reservoir of water available, like Chronos said.
By the way, the reservoir has to have a flat air water interface, or else you get a different amount of vapor per the Kelvin effect. But that’s for another day…
I think you are misunderstanding how a pressure cooker works. When the pressure cooker ‘seals’ that means that the pressure cooker starts to build pressure over atmospheric. But that pressure buildup is short lived and soon a venting device does not allow a further buildup. So as you cook steam is always venting.
In that the gas in the pressure cooker starts as air. As it is heated and pressurized, more and more humidity is added and eventually that mix is continually vented. So and the end of a long enough cooking cycle, the air inside the pressure cooker is pretty much all steam. So not the max amount of water the air can hold (which is what we get the humidity scale from), but that’s all there is, just H2O no air to hold anything, thus no humidity - or infinite humidity, if you prefer, as there is no air, so a division by zero situation.
The supersaturated condition occurs during transitions. For example, if we lower the temperature faster than the air can precipitate the water out, then at some instant during the process we would see relative humidity > 100%. The supersaturated condition cannot remain, once we stop lowering the temperature, the air will continue to precipitate water until RH = 100%. As far as I know, there is no equilibrium condition where RH > 100%.
We wouldn’t have a supersaturation condition inside the pressure cooker.
Air doesn’t really “hold water vapor”. It just coexists with it. At any given temperature, there is some amount of pressure of water vapor that will be present anywhere that there’s water. You will get that same amount of water vapor at that temperature regardless of what other gases are present, or in what quantity. Take away the nitrogen and oxygen entirely, and you’ll still have that same amount of water vapor. It doesn’t need for the other gases to be there to “hold” it.
Unless you’re my dad, in college, and you’ve overfilled the pressure cooker so that a chunk of food blocks the vent. Then the pressure and temperature build up until the emergency relief plug pops and a plume of BLEVE-atomized stew paints the entire kitchen ceiling.
No, you can have stable supersaturation conditions. In particle physics, a cloud chamber is supersaturated. In general, the air inside atmospheric clouds is supersaturated. If there is a flat water surface, supersaturated air will keep condensing water out at that surface. But if all the surfaces are something else condensation may not happen. It depends on the affinity of the surface for water. If the surface is solid sodium chloride water will condense out if the RH is over something like 70% or 80% (I forget the exact number), and if it’s lithium bromide the RH only has to be over something like 25%. If the surface is something like polyethylene, for which water has little affinity, the RH can certainly be over 100%.
There has to be a place for condensation to start. In the atmosphere it’s typically a “condensation nucleus” made of sea salt from ocean spray evaporation, though in extremely clean conditions (for example in a clean room) you might have to rely on air ions to nucleate condensation. Look up “condensation nuclei counter” to see an instrument based on this.
That is the same as assuming a “spherical cow” on a frinctionless road …
Water always gets superheated when you are using normal home cooking utensils - that is a part of the water. One common assumption all physicists will make is that water boiling in a container (even a pressure cooker) is homogenous and at constant temperature - it is decidedly not so**. There are parts - expecially in contact with the lower surface that gets superheated - for more see this (Page 2 - Figure on Bottom).