GASCO 304CO2-17 Precision Calibration Gas Mixture 100 PPM Carbon Monoxide, 2.5 % Carbon Dioxide, 2.5 % Methane (50 % LEL) , 17 % Oxygen, Balance Nitrogen, for calibration and maintenance of gas detectors, NIST Calibration Certificate Included. GASCO Calibration Gas is ISO 9001 Certified, Made in the USA