GASCO 309-18CO2-4% Calibration Gas Carbon Monoxide (CO) 200 PPM, Carbon Dioxide (CO2) 4%, Methane (CH4) 2.50%, Oxygen (O2) 18%, Balance Nitrogen, contained in a 116 Liter ecosmart Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA