GASCO Precision Calibration Gas 74-438 Mixture 50 PPM Carbon Monoxide, 25 PPM Hydrogen Sulfide, 0.55 % Propane (25 % LEL), 19% Oxygen, Balance Nitrogen Contained in a 74L Liter Disposable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA