GASCO Calibration Gas 900es-401-18 250ppm CO, 2.5% Volume (50% LEL) Methane, 40ppm H2S, 18% Oxygen, Balance Nitrogen Contained in a 900 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder Connection CGA TBD, Calibration Certificate NIST, Made in the USA