GASCO Calibration Gas 74-421SO2-17 Mixture 100 PPM Carbon Monoxide, 35 PPM Sulfur Dioxide, 2.5 % Methane (50 % LEL), 17 % Oxygen, Balance Nitrogen Contained in a 74L Liter Disposable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA