GASCO 409BU-100 Calibration Gas, 100 PPM CO, 50% LEL (0.95 vol.) Isobutane 25 PPM H2S, 12% 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