GASCO Calibration Gas, 100 PPM CO, 25% LEL Pent. (.35% vol.), 25 PPM H2S, 12% O2, 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