GASCO Calibration Gas 650es-404S-18 20 PPM H2S, CO 100 PPM, Methane 2.5% (50% LEL) Oxygen 18% Balance Nitrogen Contained in a 650 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder Connection CGA 660, Calibration Certificate NIST, Made in the USA