The flame of an LPG (liquefied petroleum gas) stove decreases suddenly when the gas in the cylinder runs out due to the way LPG operates and its physical properties.
LPG is stored in a pressurized state as a liquid inside the gas cylinder. When you turn on the stove, the LPG is released through the valve and passes through a regulator. The regulator reduces the high pressure of the gas to a lower, constant pressure that is suitable for use in stoves and other appliances.
As the LPG flows from the regulator to the stove burners, it goes through a phase change. LPG at room temperature and atmospheric pressure is a gas, but inside the pressurized cylinder, it remains in liquid form. However, as it exits the regulator and enters the stove's gas lines, it expands and vaporizes into a gas again.
Now, when the gas in the cylinder starts to run out, there is less liquid LPG available inside the cylinder to provide a constant flow of gas. The pressure in the cylinder drops, and there may not be enough pressure to maintain a steady flow of gas through the regulator and into the stove.
As a result, the flame on the stove decreases suddenly or may even go out completely. This happens because the gas pressure is no longer sufficient to sustain a strong flame. The remaining gas in the cylinder might be in the form of vapor, but it is not enough to maintain a stable and strong flame.
To continue using the stove, you would need to replace the empty LPG cylinder with a new one. Always exercise caution while handling gas cylinders, and if you suspect a gas leak, turn off the stove immediately and seek professional assistance to address the issue.