GASCO 309-18.5 Calibration Gas 200 PPM Carbon Monoxide, 2.5 % Methane (50 % LEL), 18.5 % Oxygen, Balance Nitrogencontained in a 600 Liter Disposable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection: TBD GAS Specific,Calibration Certificate NIST, Made in the USA