Pressure Cooker PSI FAQ: the stuff you didn’t think to ask about pressure


What are “standard” and “non-standard” pressure cookers?
How do “non-standard” pressure cookers affect cooking time?
Do all pressure cookers actually cook at full 15 psi?
Is there an electric pressure cooker that cooks at 15 psi?
How are pressure cooking times affected by high altitude?
Why do American, European and Asian manufactured pressure cookers all reach different pressures?
How can I find what pressure my pressure cooker reaches and cooks?

What are “standard” and “non-standard” pressure cookers?

To facilitate the writing of pressure cooker cookbooks and sharing recipes, there is an un-official standard.  This standard includes the maximum operating pressure for American Pressure Cookers (15 psi)  and the maximum operating pressure for most modern European Cookers (which is about 13 psi for spring-valve type cookers).

Any pressure cooker that does not fall within this range  – or used in a high-altitude situation (see below) – is “non-standard.”

There is no international pressure cooker  organization that sets a global standard.  Pressure cooker UL Rating, which is an American Appliance Testing standard, only states that a domestic pressure cooker “operate at a nominal pressure of 15 psi (103 kPa) or less.” While in Europe the CE rating, the equivalent to the American certification, state that a “simple pressure vessel” can be above .5 bar (7.2 psi) and below 50 bar (720 PSI) . American manufactured pressure cookers adhere to a standard for pressure canners set by USDA in 1917 – 15 psi.

While some European pressure cookers are sold world-wide many of these manufacturers make a separate model specifically for the American market that reaches 15 psi.  Some European manufacturers are switching to a single model distributed world-wide that reaches 15 psi. At the writing of this article,  American pressure cooker manufacturers only sell their pressure cookers in the United States.

How do “non-standard” pressure cookers affect cooking time?

There is no set rule or formula. Pressure cooking time really depends on the size and density of the food. Most refined grains and quick-cooking vegetables do not need pressure cooking time adjustments, while tough legumes, whole grains and thick roasts will need several minutes more cooking time. In other words, the denser or larger the food, the more time a cooker operating at a lower pressure (thus lower temperature) will need to achieve the same results as a standard cooker.

All recipes and cooking time charts on this website are written to accommodate both “standard” and “non-standard” pressure cookers.  When necessary,  times are written in a range – standard pressure cookers should use the shorter cooking time (13 minutes) and non-standard pressure cookers the longer (20 minutes).

More Info: Pressure Cooking Times for standard and non-standard pressure cookers

Do all pressure cookers actually reach 15 psi?  What is the relationship between pressure and temperature?

This comes down to the difference in measuring systems between the United States (imperial) and the rest of the world (metric).  Pressure in the rest of the world is measured  in kilopascals (kPa) and bars while the U.S.it is measured with pounds per square inch (psi).

European manufactured pressure cookers are designed to cook at 1 bar or 100 kpa (metric pressure measurements) and that translates to 14.5 psi (this is rounded up to 15 psi) but American manufactured pressure cookers are designed to reach a full 15 psi (see below, for information on electric pressure cookers).

The rise in pressure inside the pressure cooker directly correlates to the rise in boiling point – the maximum cooking temperature that can be achieved at a given pressure.

Using the Antoine Equation we  also produced more detailed charts. 

More Info: Pressure and Boiling Points of Water – The Engineering ToolBox
More Info: Water Vapor Pressure and Boiling Points (Antoine Equation)

How are pressure cooking times affected by “high altitudes”? Isn’t atmospheric pressure already 15 psi?

The pressure cooker adds pressure above the current atmospheric pressure. Since there is a pressure difference in the atmosphere between one altitude and another, the pressure cooker’s pressure will vary accordingly.

At sea level, the atmospheric pressure averages 14.7 psi – add 15 psi of pressure generated by the pressure cooker and the food in the cooker is cooking at 29.7 psi of absolute pressure.

Moving up in the atmosphere, or going to higher altitude, the atmospheric pressure decreases.  So in Denver Colorado (about 5,000 feet) the atmospheric pressure averages only 12.2 psi- add 15 psi of pressure generated by the cooker and there the food is cooking at just 27.2 psi of absolute pressure -almost 3 psi less pressure than pressure cooking at sea level!

No matter where you are, a pressure cooker will always add pressure to the current atmospheric pressure.

The same 15 psi pressure cooker will cook 15 psi in San Francisco, California (sea level) but only 12.5 psi in Denver, Colorado (5,000 feet).  Now, “standard pressure cooker” has become “non-standard” in Denver.  This means that the recipes will need the same timing adjustments used  for non-standard pressure cookers (see pie chart, above).

More Info:  Air Pressure and Altitude Above Sea Level – The Engineering ToolBox

Why do American, European and Asian pressure cookers all reach different pressures?

“The manufacturing facilities of the makers of PRESTO COOKERS are now devoted to war production. Once victory is won – there will be Presto Cookers for everybody. Until then, if you own one, share it, won’t you? It’s a good neighbor policy.”

The transformation form a Renaissance “bone digester” invented by French scientist Denis Papin (in 1689) to pressure canners (1905)  and finally to the pressure cooker we know today began in 1926. The Home Exhibition in Paris introduced the first model for home use. The pressure cooker made it out of Europe and into the United States via the 1939 New York Fair where the National Pressure Cooker Company launched the first U.S. model.

Aluminum pressure cookers took off in the U.S. and many companies began producing them. Then, America’s involvement in WWII halted the production of pressure cookers and their factories were dedicated to producing munitions for the overseas war.

Once the war ended, European and American pressure cooker manufacturers began to develop and produce pressure cookers independently from each other. While in America unscrupulous factories made and sold sub-standard pressure cookers – that eventually went on to mar the cooker’s reputation and halt innovation- European manufactures continued to develop, perfect and innovate their designs adding multiple redundant safety mechanisms, selectable pressure levels and more features.

The 90’s started the trickle of European manufactured pressure cookers, and their features into America. It’s also when the patent for the first electric pressure cooker was filed by Chinese scientist, Mr. Yong Guang Wang.  The electric pressure cooker was developed independently from stovetop pressure cookers in that they were based on the ever-popular electric rice cookers (hence the resemblance) and are manufactured in a range of pressures – depending on the manufacturer or design team.

Is there an electric pressure cooker that cooks 15 psi?

At the time of the writing of this article, most electric pressure cookers reach 15 psi but they do not cook at 15 psi.  As illustrated by the graphic below – electric pressure cookers reach 15 psi briefly during the warming process.

Electric pressure cookers build pressure up to 15 psi but then maintain a lower pressure during the cooking.  In the  graph below the “operating pressure” is 11.6 even though the cooker reaches 15 psi while it’s building pressure. “Operating Pressure” is the true pressure at which an electric pressure cooker cooks.

More Info: How Electric Pressure Cookers Work – Instant Pot

How can I find out what pressure my pressure cooker reaches?

Most instruction booklets have this information written in them. The number may be written in kPa, bar or PSI (use the table in this page to decode the approximate PSI).  There may be several terms used in conjunction with those numbers and they include:

  • Operating Pressure –  the pressure at which the pressure cooker cooks the food.
  • Valve Release Pressure – the pressure at which the main regulating valve releases pressure (2 to 4 psi more than the operating pressure depending on the manufacturer).
  • Warping Pressure – the pressure at which the base or components of the pressure cooker deform.

Instant Pot pressure valve with “105kp” (15.2 psi) valve release pressure. But the “Operating Pressure” at which this cooker cooks food is 11.6 psi.

Stove top pressure cookers may also have this information written on the base, and the maximum valve release may be written on or near the valve.

Electric pressure cookers will have the “valve release pressure” written in very small text on the underside of the pressure release valve either on the plastic housing, or the metal part of the valve.

More Info: Free Pressure Cooker Manual Library

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Summary
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Pressure Cooker PSI FAQ: the stuff you didn't think to ask
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The facts about pressure cooker pressure (PSI, kpa and bar) and how it's affected by the type of cooker being used, where it's being used and where it was made.