GASCO Calibration Gas 34es-433BD-17 | 50% LEL Methane | 100 ppm CO | 10ppm SO2 | 17% Oxygen | Balance Nitrogen | Contained in a 34 Liter GASCO ecosmart Factory Refillable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA