GASCO 427A Calibration Gas 100 PPM Carbon Monoxide, 25 PPM Hydrogen Sulfide, 1.25 % Methane (25 % LEL) , Balance Air (20.9 % Oxygen balance Nitrogen) Contained in a 900 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder Connection CGA 330, Calibration Certificate NIST, Made in the USA