GASCO 421-18.5 Calibration Gas, 100 PPM Carbon Monoxide, 50% LEL Methane, 15 PPM Hydrogen Sulfide, 18.5% 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