A: You probably have other hydrocarbons in your gas. The methane detector is sensitive to all hydrocarbons, so the presence of ethane, propane, butane and others in your gas stream will cause your methane reading to be too high. LANDTEC sells "Charcoal Absorber Filters (1-00000-5084)", which will reduce the contamination from non-methane hydrocarbons. For more information, see section on "Cross Gas Effects" of the GEM™2000 Manual
A: Readings, especially Methane, can be affected by cell phones. Do not use your cell phone while you are taking field readings.
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.