GASCO Calibration Gas 74-401-22 250 PPM Carbon Monoxide, 40 PPM Hydrogen Sulfide, 2.5 % Methane (50 % LEL), 22 % Oxygen, Balance Nitrogen n 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