GASCO Precision Calibration Gas 74-465BU-12 Mixture 25 ppm H2S, 50 ppm CO, 50% LEL Isobutane, 12% O2, Balance N2 Contained in a 74L Liter Disposable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA