GASCO 309-18.5H Calibration Gas 200 PPM Carbon Monoxide, 2.5 % Methane (50 % LEL), 18.5 % Oxygen, Balance Nitrogencontained 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