GASCO Calibration Gas 116es-409BU Mixture 50 PPM Carbon Monoxide, 0.95 % Isobutane (50 % LEL), 25 PPM Hydrogen Sulfide, 12 % 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