GASCO Calibration Gas 900es-421-CO2-0.5 Mixture 2.5% volume Methane (50% LEL), 18% Oxygen, 100 ppm Carbon Monoxide, 25 ppm Hydrogen Sulfide, 0.5% Carbon Dioxide, 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