What happens inside the LPG gas bottle?

In South African we use a mixture or Propane and Butane to give us LPG - liquid petroleum gas..

How much pressure is in an LPG gas bottle?

The average force per unit of area applied by the LPG gas on the internal walls of the LPG gas container is referred to as LPG gas pressure. As a result, the LPG pressure within the cylinder will remain constant from the time the cylinder is full to the time the last of the liquid LPG is vaporised. When the remaining LPG vapour is used, the LPG pressure will drop.

LPG Pressure Units

PSIG, kPa, and bar are the three most common units of measurement for LPG gas cylinder pressure (LPG gas bottle pressure). LPG gas pressure is measured in pounds per square inch gauge (PSIG). PSIA stands for pounds per square inch absolute pressure of LPG gas.

1 atm = 14.7 psia = 0 psig = 101.325 kPa is a metric pressure unit that is part of the International System of Units (SI).

How much pressure is in a LP Gas tank?

LP gas bottles have the same LPG gas cylinder-bottle pressure as big gas bottles, such as a 200kg LP gas tank or a 9kg gas bottle at a constant external temperature. The pressure in the LPG gas cylinder-bottle depends entirely on the ambient temperature. 

So the warmer a bottle or tank is the more pressure is inside, conversely the lower the temperature the less pressure.  That is why sometimes in winter your appliance might struggle to work, like a gas water heater, in the morning, but after the ambient temperature has increased in the afternoon, thus raising the tank temperature and pressure the appliance works just fine. 

So does a pressure gauge tell you how full your tank / bottle is? No, if there is 4kg pr 0.5kg of Liquid Petroleum on the bottle the pressure will be the same. The only real way to see how much gas is left is to weight the bottle and subtract the bottle TARE weight.