GASCO Calibration Gas 650es-413-18BS 50 PPM Carbon Monoxide, 10 PPM Hydrogen Sulfide, 2.2% Methane (44% LEL), 18% 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