GASCO Precision Calibration Gas 66es-437-19 Mixture 50 PPM Carbon Monoxide, 25 PPM Hydrogen Sulfide, 1.05 % Propane (50 % LEL), 19% Oxygen, Balance Nitrogen Contained in a 66L Liter ecosmart Factory Refillable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA