GASCO Calibration Gas 650es-416-18.5 200 PPM Carbon Monoxide, 20 PPM Hydrogen Sulfide, 2.5% Methane (50% LEL), 18.5% Oxygen, Balance Nitrogen Contained in a 650 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder Connection CGA 330, Calibration Certificate NIST, Made in the USA