An apparatus for sampling landfill gas from a landfill flowing through a pipe, the apparatus comprising: an enclosure ; a gas inlet port coupled to a section of the pipe ; at least one calibration port configured for connection to at least one source of calibration gas of known composition, wherein the at least one source of calibration gas of known composition comprises a first gas sample of a first mixture of gasses of known composition and a second gas sample of a second mixture of gasses of known compositions, the second mixture of gasses being different than the first mixture of gasses; at least one sensor device disposed in a region of the enclosure , the at least one sensor device being coupled to the section of the pipe through the gas inlet port and to the at least one source of the calibration gas of known composition through the at least one calibration port ; and a gas outlet port coupled to the section of the pipe and the at least one sensor device; the apparatus being characterized by a controller configured to supply the calibration gas to a sensor chamber of the at least one sensor device for making calibration measurements by controlling at least one first valve of the apparatus to divert a sample path from a landfill gas stream to the at least one calibration port, the at least one first valve being coupled to the gas inlet port and to the at least one calibration port , and to calibrate the at least one sensor device by employing a two-point calibration to find a difference between an expected reading and an actual reading of a sensor at zero and a rated range of the at least one sensor to calculate a calibration offset and gain used for compensation an enclosure ; a gas inlet port coupled to a section of the pipe ; at least one calibration port configured for connection to at least one source of calibration gas of known composition, wherein the at least one source of calibration gas of known composition comprises a first gas sample of a first mixture of gasses of known composition and a second gas sample of a second mixture of gasses of known compositions, the second mixture of gasses being different than the first mixture of gasses; at least one sensor device disposed in a region of the enclosure , the at least one sensor device being coupled to the section of the pipe through the gas inlet port and to the at least one source of the calibration gas of known composition through the at least one calibration port ; and a gas outlet port coupled to the section of the pipe and the at least one sensor device; the apparatus being characterized by a controller configured to supply the calibration gas to a sensor chamber of the at least one sensor device for making calibration measurements by controlling at least one first valve of the apparatus to divert a sample path from a landfill gas stream to the at least one calibration port, the at least one first valve being coupled to the gas inlet port and to the at least one calibration port , and to calibrate the at least one sensor device by employing a two-point calibration to find a difference between an expected reading and an actual reading of a sensor at zero and a rated range of the at least one sensor to calculate a calibration offset and gain used for compensation <EOS>