GASCO Precision Calibration Gas 58es-421X Mixture 25 PPM Hydrogen Sulfide, 2.5 % Methane (50 % LEL), 18 % Oxygen, Balance Nitrogen Contained in a 58L Liter ecosmart Factory Refillable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA