GASCO Calibration Gas 44es-400X 2.5% Volume Methane (50% LEL), 15 PPM Hydrogen Sulfide, 18% 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