GASCO 34es-311-18 Calibration Gas 100 PPM Carbon Monoxide, 0.35% Pentane (25% LEL), 18% 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