GASCO Precision Calibration Gas 116es-497BS-18 Mixture 50 PPM Carbon Monoxide, 10 PPM Hydrogen Sulfide, 0.35 % Pentane (25 % LEL), 18 % Oxygen, Balance Nitrogen Contained in a 116 Liter Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA