1. Cooking gas inside cylinder has approximately 85% of liquid and rest vapors of that liquid.

2. This liquid and vapor is in equilibrium with each other. The pressure in such a system is dependent only on temperature.

3. As the gas is released from the cylinder, the pressure and temperature inside cylinder decreases. To compensate this fall in pressure, phase transition from liquid to gas takes place and some of liquid changes into vapors.

4. For this to happen, system takes heat from surrounding and hence temperature of the system remains constant. And temperature constant means pressure also becomes constant.

5. If phase transition takes place, pressure inside the cylinder will remain same.

6. Hence because of continuous phase transition inside the cylinder, pressure of cooking gas cylinder does not fall appreciably.