GASCO Calibration Gas 66es-413-18BS 50 PPM Carbon Monoxide, 10 PPM Hydrogen Sulfide, 2.2% Methane (44% LEL), 18% 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