GASCO 409BU-100BS Calibration Gas, 100 PPM Carbon Monoxide, 50% LEL (0.95 vol.) Isobutane, 25 PPM H2S, 18% Oxygen, Balance Nitrogen contained in a 44L Liter ecosmart Factory Refillable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA