GASCO 355BS-15 Precision Calibration Gas 300 PPM Carbon Monoxide, 2.5% Methane (50% LEL), 15% Oxygen, Balance Nitrogen contained 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