GASCO Precision Calibration Gas 900es-421X Mixture 25 PPM Hydrogen Sulfide, 2.5 % Methane (50 % LEL), 18 % Oxygen, Balance Nitrogen Contained in a 900 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder Connection CGA TBD, Calibration Certificate NIST, Made in the USA