GASCO Calibration Gas 900es-489S Mixture 40 PPM Carbon Monoxide, 20 PPM Hydrogen Sulfide, 0.5 % Methane (10 % LEL), 18 % Oxygen, Balance Nitrogen Contained in a 900 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder Connection CGA 330, Calibration Certificate NIST, Made in the USA