GASCO 404M-18 Calibration Gas, 100 PPM Carbon Monoxide, 2.2% Volume Methane, 25 PPM H2S, 18% Oxygen, Balance Nitrogen contained in a 650 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder Connection: TBD Gas Specific, Calibration Certificate NIST, Made in the USA