GASCO Calibration Gas 34L-416BS 250 PPM Carbon Monoxide, 25 PPM Hydrogen Sulfide, 2.5% Methane (50% LEL), 18% Oxygen, Balance Nitrogen Contained in a 34 Liter Disposable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA