GASCO Calibration Gas 58es-401-18 250ppm CO, 2.5% Volume (50% LEL) Methane, 40ppm H2S, 18% Oxygen, Balance Nitrogen Contained in a 58L 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