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  #1  
Old 12-30-2006, 04:20 PM
kanicbird kanicbird is offline
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Why 15 psi max on pressure cookers?

All the pressure cookers I have seen have 15 PSI as their max pressure, which is the 'normal' pressure one would cook with in a pressure cooker. Many have no other setting. The ones that do have other choices usually have 5 PSI as a option.

My questions are:

Why 15 PSI?
Why don't they make ones that go to 25, or for that matter 90 PSI?
Does anyone make one that exceeded the 15 PSI limit?
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  #2  
Old 12-30-2006, 05:43 PM
Rick Rick is online now
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I would guess that it has to do with the amount of material that it would take to reinforce it to withstand a higher pressure. For example look at the clamps on this spray paint tank
Can imagine the potential danger if a pressure cooker at 90 PSI failed and sprayed shrapnel, and dangerously hot food all over your kitchen?
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  #3  
Old 12-30-2006, 06:27 PM
minor7flat5 minor7flat5 is offline
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Robustness of the vessel aside, the difference would be the saturation temperature of water at the target pressure.

Your 15psig (g=gage pressure, add 14.7psi for absolute pressure) pot has a saturation temperature (boiling point) of around 250F. Bumping the pressure up to 90psig gets you a saturation temperature of 331F.

I'm not sure what effect that would have on the food you are cooking, but that would be the physical difference.
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  #4  
Old 12-30-2006, 10:09 PM
Napier Napier is offline
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I bet it's historical tradition or lawyers.

Check out Autoclave Engineers in Erie, PA. When I visited them a few years ago they made stock vessels that handled up to 160,000 psi and custom vessels that went higher. I think they managed pretty high temperatures, too.

I remember working out one time conditions that made supercritical water miscible with silica.
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  #5  
Old 12-31-2006, 06:24 AM
casdave casdave is offline
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I would expect that you'll find that above a certain pressure (and also a certain volume), it would come under pressure vessel legislation, which requires regular maintenance inpections, such as safety valve testing and overpressure tests.

Differant countries have differant requirements, but when you take into account manufacturing tolerances, and the general srewup factor, the potential safety margin pressures might mean that although 15 p.s.i is hugely under the limits, this other factors might push it somewhat closer.

According to [Wikipeadia

The pressure was set by USDA way back in 1917, probably under the Food control and production acts

http://en.wikipedia.org/wiki/Pressure_cooker

In the UK pressure vessels means anything above around 7 p.s.i with a volume greater than 250 bar litres, but as the pressure goes up, the allowable volume decreases.

http://www.devonline.gov.uk/text/ind...ressuresys.htm

I would expect that to construct a pressure cooker that fell into these classifications would result in something prohibitively expensive, and realistically it not actually worth it in terms of pratical use, higher pressure+higher temperature with the accompanying risks of safe venting.
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  #6  
Old 12-31-2006, 08:46 AM
Czarcasm Czarcasm is offline
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It looks like this question calls for factual answers rather than opinions, so I'm moving this thread from IMHO to General Questions.
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  #7  
Old 12-31-2006, 09:17 AM
Harmonious Discord Harmonious Discord is offline
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I believe casdav has the reason that higher pressure ones are not in most retail stores. You would have to have regular inspections all the stuff that goes with it. When a high pressure vessel explodes everybody can end up dead.
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  #8  
Old 12-31-2006, 03:51 PM
rowrrbazzle rowrrbazzle is offline
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Quote:
Originally Posted by minor7flat5
Robustness of the vessel aside, the difference would be the saturation temperature of water at the target pressure.

Your 15psig (g=gage pressure, add 14.7psi for absolute pressure) pot has a saturation temperature (boiling point) of around 250F. Bumping the pressure up to 90psig gets you a saturation temperature of 331F.

I'm not sure what effect that would have on the food you are cooking, but that would be the physical difference.
Home canning was once widely practiced. According to this page http://www.bchealthguide.org/healthfiles/hfile22.stm 240F is the minimum temperature to destroy botulism spores, reached at 10psig. It's necessary when canning low acid foods like meat and most vegetables. This page http://muextension.missouri.edu/xplo...nut/gh1490.htm has a chart that says to use 15psig for weighted gauge pressure cookers for altitudes 1000-2000 feet.
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  #9  
Old 01-01-2007, 10:10 PM
t-bonham@scc.net t-bonham@scc.net is offline
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Quote:
Originally Posted by kanicbird
Why 15 PSI?
To answer this specifically:
Because there is no consumer demand for anything more than 15psi.

- all the recipes out there are designed for 5, 10, or at most 15psi.
- all the cooks out there have experience using these psi's.
- food cooks quickly at 15psi, without significant damage to flavor or texture. Higher psi's would tend to result in mush-like food.
- more than 15psi would require a sturdier cooker, thus more expensive.
- more than 15psi would be riskier, thus more expensive product liability insurance.
- more than 15 psi would be more expensive to use; you have to heat it up to a higher psi, all that is lost when you open it to take out a set of jars, then you have to get it back up to that high psi again for the next load.

So basically, a max of 15psi is what everybody's used to using, and it's good enough to do the job.

Such info is one of the benefits of having a mother who has done home canning all her life. And still is doing so in her mid-80's. For Xmas, I received a box of home canned items, mostly grown in her home garden.
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  #10  
Old 01-02-2007, 02:24 AM
David Simmons David Simmons is offline
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Quote:
Originally Posted by casdave
I would expect that you'll find that above a certain pressure (and also a certain volume), it would come under pressure vessel legislation, which requires regular maintenance inpections, such as safety valve testing and overpressure tests.
Maybe, but I have an air compressor that goes to 150 psi for which I don't need regular inspections. Government safey regulations don't seem to apply as long as the public and any employees are not involved with the compressor.

As a complete WAG I would go along with minor7flat5 and t-bonham@scc.net that 15 psi gets you to as high a boiling point as is needed or desirable for cooking purposes.
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  #11  
Old 01-02-2007, 07:57 AM
SuperNelson SuperNelson is offline
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Is there any relationship between 15 PSI in the pressure cooker, and 15 PSI being the ambient pressure at sea level? I would guess that the pressure cooker value is relative pressure, but is there a particular significance to the pressure inside the cooker being roughly double the pressure outside?
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  #12  
Old 01-02-2007, 11:08 AM
Napier Napier is offline
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>Is there any relationship between 15 PSI in the pressure cooker, and 15 PSI being the ambient pressure at sea level?

Good question, and this seems oddly significant, but I can't think of any relationship at all. Temperatures depend in a nonlinear way on pressures so it's not like this should let you cut cooking times exactly in half, easier to calculate. Nothing special happens at 15psig = 30 psia to my knowledge. I do a lot of work in thermodynamics, but nothing looks important about this pressure to me.
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  #13  
Old 01-02-2007, 11:48 AM
casdave casdave is offline
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The UK pressure vessel regulations certainly cover compressed air vessels, however here we are dealing with steam, which has the added ingredient of temperature.

Pressurised air vessels would still require safety valves, and above a certain volume and pressure, would certainly require testing.

Control of pressure could also be achieved by ensuring that the pump could not supply above a certain limit.

I can think of plenty of applications where fairly small steam pressure vessels come under the regulations, things such as instrument autoclaves, the total volume of some of these isn't a huge leap from that of a large pressure cooker.

Some industrial cooking systems also fall under the regulations, so all in all, it isn't too great a stretch.
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  #14  
Old 09-28-2011, 03:13 PM
wilderkind wilderkind is offline
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15psig safety

AMSE code states that any vessel exceeding 15 psig must be inspected periodically for safety reasons. This law is inacted to regulate boilers primarily.
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  #15  
Old 09-28-2011, 03:18 PM
Folacin Folacin is online now
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Quote:
Originally Posted by wilderkind View Post
AMSE code states that any vessel exceeding 15 psig must be inspected periodically for safety reasons. This law is inacted to regulate boilers primarily.
But I wonder if that level was set because most pressure cookers are 15 psi? Possibly a chicken/egg sort of thing?
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  #16  
Old 09-28-2011, 03:24 PM
Malacandra Malacandra is offline
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What temperature kills zombification spores, anyway?
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  #17  
Old 09-28-2011, 04:16 PM
Rocketeer Rocketeer is offline
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Quote:
Originally Posted by David Simmons View Post
Maybe, but I have an air compressor that goes to 150 psi for which I don't need regular inspections. Government safey regulations don't seem to apply as long as the public and any employees are not involved with the compressor...
Your air compressor isn't full of superheated steam, either. A steam explosion is much much worse than a compressed-gas explosion.
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  #18  
Old 09-28-2011, 06:36 PM
Jinx Jinx is offline
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Quote:
Originally Posted by Napier View Post
I bet it's historical tradition or lawyers.

Check out Autoclave Engineers in Erie, PA. When I visited them a few years ago they made stock vessels that handled up to 160,000 psi and custom vessels that went higher. I think they managed pretty high temperatures, too.

I remember working out one time conditions that made supercritical water miscible with silica.
Actually, anything above 15 psig would be considered a pressure vessel by ASME Code and require to be regulated (inspected) under state and/or Federal law. Small vessels receive a "UM" stamp, IIRC, and large vessels over some threshold require a "U" stamp. Also, pressure vessels require a pressure relief valve which has its own cycle of required ASME certified inspections. Oh, and Napier, yes the pressure rating is for a specified temp as temps above the rated temp will weaken the pressure vessel wall material,

Last edited by Jinx; 09-28-2011 at 06:37 PM..
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  #19  
Old 09-28-2011, 07:42 PM
drachillix drachillix is offline
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Originally Posted by Rocketeer View Post
Your air compressor isn't full of superheated steam, either. A steam explosion is much much worse than a compressed-gas explosion.
Sure...until you have a tank of chlorine gas go....
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  #20  
Old 09-28-2011, 09:22 PM
AaronX AaronX is offline
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My friend managed to open a pressurized pressure cooker once, I heard there was food left on the ceiling. His hand and face got burnt, but he recovered ok (some scars, no loss of function).
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  #21  
Old 09-28-2011, 10:23 PM
nolonger lurking nolonger lurking is online now
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Originally Posted by Rocketeer View Post
Your air compressor isn't full of superheated steam, either. A steam explosion is much much worse than a compressed-gas explosion.
There is a reason for the backronym for BLEVE "Blast Levels Everything Very Efficiently"
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  #22  
Old 09-29-2011, 09:24 AM
Machine Elf Machine Elf is offline
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Quote:
Originally Posted by minor7flat5 View Post
Robustness of the vessel aside, the difference would be the saturation temperature of water at the target pressure.

Your 15psig (g=gage pressure, add 14.7psi for absolute pressure) pot has a saturation temperature (boiling point) of around 250F. Bumping the pressure up to 90psig gets you a saturation temperature of 331F.

I'm not sure what effect that would have on the food you are cooking, but that would be the physical difference.
Imagine a pressure cooker 12 inches in diameter. At 15 psig, the force trying to remove the lid is about 1662 pounds. Not only does the lid need to be strong, but the retention mechanism as well. Plus the bottom of the pot, which experiences the same total force, needs to remain relatively flat. bump up the gauge pressure sixfold, and you need some very thick material to ensure that the bottom remains flat and the lid stays on.

Given the safety concerns with respect to a potential BLEVE, a cooking vessel rated for 90 psi would be very heavy and very expensive, and will include a surcharge to provide the manufacturer with a hefty liability insurance policy against the inevitable fatal cooking accident that will occur when a user does something wrong or the cooker suffers a mechanical failure (sell 100,000 units, and someone somewhere somehow will have a problem).
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  #23  
Old 09-29-2011, 09:27 AM
muldoonthief muldoonthief is offline
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Quote:
Originally Posted by Malacandra View Post
What temperature kills zombification spores, anyway?
No kidding, I was shocked then saddened to see David Simmons post to this thread.

Last edited by muldoonthief; 09-29-2011 at 09:27 AM..
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  #24  
Old 09-29-2011, 09:51 AM
Borzo Borzo is offline
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Quote:
Originally Posted by AaronX View Post
My friend managed to open a pressurized pressure cooker once...
Why would someone do such a thing?
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  #25  
Old 09-29-2011, 10:10 AM
kunilou kunilou is offline
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Quote:
Originally Posted by AaronX View Post
My friend managed to open a pressurized pressure cooker once, I heard there was food left on the ceiling. His hand and face got burnt, but he recovered ok (some scars, no loss of function).
Another anecdote. My mother used to can using a pressure cooker. One time she did something wrong, I don't know what, but the safety cap blew and left a dent in the ceiling.

If that could happen with a standard kitchen pressure cooker, what kind of damage could have happened with a malfunctioning/user error high pressure cooker?
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  #26  
Old 10-04-2011, 09:02 PM
AaronX AaronX is offline
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Quote:
Originally Posted by Borzo View Post
Why would someone do such a thing?
He didn't know how pressure cookers work, or that you're supposed to depressurise them before opening.
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