GASCO Precision Calibration Gas 58es-416-15 Mixture 200 PPM Carbon Monoxide, 20 PPM Hydrogen Sulfide, 2.5% Methane (50% LEL), 15% 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