GASCO Calibration Gas 900es-441-19 100 PPM Carbon Monoxide, 5 PPM Sulfur Dioxide, 2.5% Methane (50% LEL), 19% Oxygen, Balance Nitrogenn Contained in a 900 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder Connection CGA 330, Calibration Certificate NIST, Made in the USA