GASCO 34es-413-18 Calibration Gas Mixture: 50 PPM Carbon Monoxide, 50% LEL Methane, 10 PPM Hydrogen Sulfide, 18% Oxygen, in Nitrogen contained in a 34 Liter Factory Refillable ecosmart Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder connection C-10 5/8"-18 UNF, Calibration Certificate NIST, Made in the USA