GASCO 404S Calibration Gas, 100 PPM Carbon Monoxide, 50% LEL Methane, 20 PPM Hydrogen Sulfide, 20.9% Oxygen, Balance Nitrogen contained in a 66L 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