GASCO Calibration Gas 498 Mixture 50 PPM Carbon Monoxide, 25 PPM Hydrogen Sulfide, 1.62% Methane,  (32.4% LEL),  (50% LEL Propane Equivalent), Balance Air (20.9% Oxygen balance Nitrogen), For Use with Any and All Gas Detectors, Calibration Certificate NIST, Made in the USA