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Mostly, all Pressure cookers are made of two types of metals namely stainless steel and aluminium.  A stainless steel pressure cooker that is of high quality is made of three ply or heavy copper bottom which helps to evenly spread the heat across the cooker and Aluminium pressure cookers are generally anodized.

A pressure cooker is not a utensil that is made of aluminium only, but it has many other attachments that complete it. Between the pot and the lid of the cookers, there is a sealed ring also known as gasket that does not allow steam or air to pass through the pressure cooker.  Then on the top of the lid there is regulator, which is the only passage from which the steam can be passed if the pressure is high. Sometimes, the regulator on the lid becomes blocked due to high pressure in the cooker or damaged regulators. Because of this reason, there is a small safety valve which acts as a backup gateway for steam to pass through.

The regulator and the safety valve act as the tools that control the pressure in the cooker. The lid and the pot of the cooker become airtight only when it is placed and twisted.