GASCO Calibration Gas 650es-481S 50 PPM Carbon Monoxide, 25 PPM Hydrogen Sulfide, 0.176% Hexane (16% LEL), 12% 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