GASCO Calibration Gas 74-404S-18 20 PPM H2S, CO 100 PPM, Methane 2.5% (50% LEL) Oxygen 18% Balance Nitrogen 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