GASCO 421-200 Calibration Gas, 100 PPM Carbon Monoxide, 50% LEL Methane, 200 PPM Hydrogen Sulfide, 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