GASCO Precision Calibration Gas 44es-409-17 Mixture 50 PPM Carbon Monoxide, 25 PPM Hydrogen Sulfide, 2.5% Methane (50% LEL), 17% Oxygen, Balance Nitrogen Contained in a 44L Liter ecosmart Factory Refillable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA