GASCO Precision Calibration Gas 900es-465BU-12 Mixture 25 ppm H2S, 50 ppm CO, 50% LEL Isobutane, 12% O2, Balance N2 Contained in a 900 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder Connection CGA TBD, Calibration Certificate NIST, Made in the USA