GASCO Precision Calibration Gas 58es-437S Mixture 50 PPM Carbon Monoxide, 25 PPM Hydrogen Sulfide, 1.05 % Propane (50 % LEL), Balance Air Contained in a 58L 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