GASCO Precision Calibration Gas 650es-497BS-18 Mixture 50 PPM Carbon Monoxide, 10 PPM Hydrogen Sulfide, 0.35 % Pentane (25 % LEL), 18 % Oxygen, Balance Nitrogen Contained in a 650 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder Connection CGA TBD, Calibration Certificate NIST, Made in the USA