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 900 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder connection: TBD Gas Specific, Calibration Certificate NIST, Made in the USA