Site owners buy LPG in litres but bill end users in cubic metres of vapour. The rate at which the liquid converts to vapour changes in relation to temperature.

Simplicity

A simple and straightforward method of calculating LPG bills is to use a constant conversion factor. The standard LPG industry conversion factor for converting litres of liquid to cubic metres of vapour is that 3.65 litres of liquid are required to give 1m3 of vapour. This is based on liquid and vapour at 15˚C, and vapour at atmospheric pressure.

Accuracy

The ideal gas law, a combination of Boyle and Charles’ laws, can be used to provide a more accurate conversion factor

LPG going through a meter is at a pressure of 37mbar at, for most of the year, a temperature lower than 15˚C (the temperature compensation figure when the bulk tank is filled). For most installations, at least 60% of the gas passing through the meter will be at a temperature at or below 10˚C as it is cooled both when it vaporises in the tank and at each stage of pressure reduction.

For calculation purposes a gas temperature of 10˚C is used.

The above calculations show that for every cubic metre of gas passing through the meter at 37mbar and at 10˚C, there is 1.055 cubic metres of gas at atmospheric pressure at 15˚C. An increase of 5.5% in the amount of gas to be billed.