GASCO Precision Calibration Gas 900es-438 Mixture 50 PPM Carbon Monoxide, 25 PPM Hydrogen Sulfide, 0.55 % Propane (25 % LEL), 19% Oxygen, Balance Nitrogen Contained in a 900 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder Connection CGA TBD, Calibration Certificate NIST, Made in the USA