GASCO Calibration Gas 650es-401B-17 Mixture 250 PPM Carbon Monoxide, 25 PPM Hydrogen Sulfide, 2.5 % Methane (50 % LEL), 17 % Oxygen, Balance Nitrogen Contained in a 650 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder Connection TBD, Calibration Certificate NIST, Made in the USA