Pressure Settings
Most pressure cookers have a working pressure setting of 15 pounds per square inch (psi), sometimes expressed as "lb" or "lbs" (pounds), which equates to 103 kPa, 1.03 bar, or 1 kg per square centimetre (kg/cm2 or kgf/cm2) above atmospheric pressure. This standard was determined by the United States Department of Agriculture in 1917. At this pressure, water boils at 121 °C (250 °F) (described in vapour pressure of water article).
The higher temperature causes food to cook faster; cooking times can typically be reduced to 1⁄3 of the time for conventional cooking methods. The actual cooking time also depends on the pressure release method used after timing (see #Pressure release methods for details) and the thickness and density of the food, since thicker foods take longer to cook. Meat joints and some other foods like sponge puddings and Christmas puddings are typically timed according to their weight. Frozen foods need extra cooking time to allow for thawing.
When pressure cooking at 15 psi, approximate cooking times are one minute for shredded cabbage and three minutes for fresh green beans, assuming that the pressure is immediately released after timing and not left to drop naturally.
If the pressure is allowed to slowly drop after timing (see #Pressure release methods for details), cooking times are even shorter. Examples of approximate cooking times using this method: potatoes cut into 1-inch pieces, seven minutes (potatoes suitable for boiling or steaming will not break up if the pressure is left to drop slowly after timing); a whole chicken, which easily fits into all but small pressure cookers, cooks in about twenty minutes. Stews can be cooked in under one hour, compared to several hours in a slow cooker or oven.
Some pressure cookers have a lower maximum pressure than the industry standard 15 psi or can be adjusted to different maximum pressures for some recipes; cooking times will increase or decrease accordingly. This is typically done by having different regulator weights or different pressure settings. If the pressure cooker is not capable of cooking at the industry standard 15 psi and the recipe is intended for cooking at 15 psi, the cooking time will need to be extended (e.g. the cooking time for 15 psi multiplied by 1.3), but this can overcook food. The manufacturer's instruction booklet should state the maximum operating pressure, possibly using different pressure units instead of the traditional "psi" rating (these pressure units are mentioned above). Below 15 psi, the alternative pressure units and values are: kPa below 100, bar below 1, kgcm2 (or kgf/cm2) below 1. As an example, a pressure cooker with a maximum operating pressure of 0.8 bar equates to only 11.6 psi. Increasing the heat under pressure cookers designed for pressures below 15 psi will not make the food cook faster; it will only waste energy, possibly spoil food, increase the risk of boiling dry, and reduce the life of the gasket and other rubber parts. Virtually all electric pressure cookers operate below 15 psi.
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