GASCO Calibration Gas 34es-416-18.5 200 PPM Carbon Monoxide, 20 PPM Hydrogen Sulfide, 2.5% Methane (50% LEL), 18.5% Oxygen, Balance Nitrogen Contained in a 34 Liter Factory Refillable ecosmart Disposable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA