Three generations of pressure cookers

First generation

Also known as "old type" pressure cookers, these operate with a weight-modified or "jiggly" valve which release pressure during operation. They are very loud because the weight-modified valve operates under the same principle as a piston in a steam engine. Often, they only offer one pressure level - with the exception of some newer "old style" pressure cookers which allow the operator to change the weight of the weight-modified valve.

Second generation

Also known as "new" or "latest generation" pressure cookers, these operate with a spring-loaded valve which is often hidden from view in a proprietary mechanism. Some of these pressure cookers do not release any steam vapour during operation, but only when the pan is opened, or as a safety precaution if the heat source has not been reduced enough when the pan reaches the required cooking pressure. Two or more pressure settings are also a mark of this generation. Some second generation pressure cookers use a rising indicator with marks to show the pressure level, others use a dial, which allows the operator to turn the dial a few clicks (which adjusts a spring underneath) in order to change the pressure setting or release the pressure; steam vapour is released during operation.

Electric pressure cookers

Called "third generation" pressure cookers by their manufacturers, these include an electric heat source which is automatically regulated to maintain the operating pressure. They also include a spring-loaded valve as described above. Two or more pressure settings are also available on this type of pressure cooker. However, this pressure cooker type cannot be opened with the cold water quick-release method and should be operated with caution when releasing vapour through the valve, especially in the case of foaming foods and liquids (beans, grains, milk, gravy, etc.).