GASCO Calibration Gas 900es-489-S 40 PPM Carbon Monoxide, 20 PPM Hydrogen Sulfide, 0.5% Methane (10% LEL), 18% 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