A: Some gas sensors are sensitive to more than one type of gas. Methane sensors, for example, measure hydro-carbon bonds, which are present in all hydrocarbons, such as Ethane, Propane, Butane and others. So, if these other gases are present, your GEM will read them as Methane, and therefore report more Methane than is actually present. Because landfills are known to contain large amounts of Methane, and don’t normally contain these other hydrocarbons, this cross-gas contamination is not usually a problem. The Oxygen and Carbon Dioxide sensors in the GEM instruments are relatively free of cross-gas effects. The H2S and CO sensors in the gas pods and the GEM™2000 Plus do show some cross-gas effects. The GEM™2000/GEM™2000 Plus manual has a cross-sensitivity table for these sensors.