GASCO Calibration Gas 650es-409SO2-5 Mixture 50 PPM Carbon Monoxide, 25 PPM Hydrogen Sulfide, 2.5 % Methane (50 % LEL), 5 PPM Sulfur Dioxide, 12 % Oxygen, Balance Nitrogen Contained in a 650 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder Connection TBD, Calibration Certificate NIST, Made in the USA