A gas metering system comprising a gas meter and contaminant detection apparatus, the apparatus comprising a mechanism for determining a value indicative of the amount of contaminant, such as carbon dioxide, present in the gas. Preferably the mechanism comprises an infrared light source for emitting infrared light of preferably 4200nm and a receiver for receiving the light, the light source and receiver arranged within a housing which receives natural gas. The apparatus further comprises processing means for comparing the level of contaminant present and volume of gas measured by the gas meter in order to determine an accurate value for the gas consumption by analysing the level of light received by the receiver.