GASCO Precision Calibration Gas 34es-405X Mixture 2.5% Volume (50% LEL) Methane, 25 PPM Hydrogen Sulfide, Balance Nitrogen Contained in a 34 Liter Factory Refillable ecosmart Disposable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA