GASCO Calibration Gas 74-487TBS-M Mixture 50 PPM Hydrogen Sulfide, 0.9 % Butane (50 % LEL), 18 % 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