GASCO Calibration Gas 650es-310NO2-17 Mixture 50% LEL Methane, 17% Oxygen, 100 ppm Carbon Monoxide, 10 ppm Nitrogen Dioxide, Balance Nitrogen Contained in a 650 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder Connection CGA TBD, Calibration Certificate NIST, Made in the USA