GASCO Precision Calibration Gas 34L-421SO2B Mixture 50% LEL Methane, 17% Oxygen, 100 ppm Carbon Monoxide, 10 ppm Sulfur Dioxide, Balance Nitrogen Contained in a 34 Liter Disposable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA