GASCO 409BU-100 Calibration Gas, 100 PPM CO, 50% LEL (0.95 vol.) Isobutane 25 PPM H2S, 12% Oxygen, Balance Nitrogen contained in a 650 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder Connection: TBD Gas Specific, Calibration Certificate NIST, Made in the USA