GASCO Calibration Gas 34L-401D Mixture 200 PPM Carbon Monoxide, 20 PPM Hydrogen Sulfide, 2.5 % Methane (50 % LEL), Balance Air Contained in a 34 Liter Disposable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA