GASCO Calibration Gas 900es-495 50 PPM Carbon Monoxide, 10 PPM Hydrogen Sulfide, 2.5% Methane (50% LEL), 12% Oxygen, Balance Nitrogen Contained in a 900 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder Connection CGA 330, Calibration Certificate NIST, Made in the USA