GASCO Precision Calibration Gas 66es-416X Mixture 200 PPM Carbon Monoxide, 20 PPM Hydrogen Sulfide, 18% Oxygen, Balance Nitrogen Contained in a 66L Liter ecosmart Factory Refillable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA