GASCO Precision Calibration Gas 58L-459X Mixture 20 PPM Hydrogen Sulfide, 2.5 % Methane (50 % LEL), 15 % Oxygen, Balance Nitrogen Contained in a 58L Liter Disposable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA