A mixture of hydrocarbon gas and air cannot ignite unless its composition lies within a range of gas in air concentration known as the “flammable range”. The lower limit of this range, known as the “lower flammable limit” is any hydrocarbon concentration below, with insufficient hydrocarbon gas to support combustion. The upper limit of the range, known as “the upper flammable limit” is any hydrocarbon concentration above, with insufficient air to support combustion.