GASCO Calibration Gas 900es-413-18-ISO Mixture 50 PPM Carbon Monoxide, 2.5% vol. (50% LEL) Methane, 100 PPM Isobutylene, 10 PPM Hydrogen Sulfide, Oxygen 18% vol. balance Nitrogen 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