GASCO Calibration Gas 34es-3044-2.2 | 500 PPM Carbon Monoxide | 50 PPM H2S | 2.2% Methane | 18% Oxygen | Balance Nitrogen | Contained in a 34 Liter GASCO ecosmart Factory Refillable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA