LPG cylinders are especially made to store the fuel entirely in its liquid state under slight pressure. The liquid turns into a gas very easily, and gas fills the space above the liquid in the cylinder (when cylinders are filled, space left at the top to allow this to happen). When gas is drawn off, the pressure reduces slightly and more liquid turns to gas to maintain the pressure. Butane in liquid form in the cylinder exerts an approximate pressure of 1.72 bar (25 p.s.i.) at 15˚C and Propane gas 6.90 bar (100 p.s.i.) at 15˚C.

Changing from liquid to gas:

1. Valve Closed Gas Fills	2. Valve Open Some gas released reducing pressure resulting in some liquid changing to gas	3. Valve Closed Pressure same as 1. but less liquid left