GASCO 416-18 Calibration Gas, 200 PPM Carbon Monoxide, 20 PPM H2s, 2.5% Methane (50% LEL) 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