GASCO Calibration Gas 74-421X-17 Mixture 25 PPM Hydrogen Sulfide, 2.5 % Methane (50% LEL) , 17% Oxygen, Balance Nitrogen 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