GASCO 303EX Calibration Gas, 50 PPM CO, 1.62% vol. Methane (50% LEL Prop. Equiv.), 18% Oxygen, Balance Nitrogen contained in a 552 Liter Disposable Steel Cylinder, For Use with Any and All Gas Detectors, Cylinder connection: TBD Gas Specific, Calibration Certificate NIST, Made in the USA