GASCO 421-15CO2-2% Calibration Gas Mixture: CO 100 PPM, CO2 2% volume, H2S 15 PPM, CH4 2.5% Volume (50% LEL), O2 18%, Balance Nitrogen, for calibration and maintenance of gas detectors, NIST Calibration Certificate Included. GASCO Calibration Gas is ISO 9001 Certified, Made in the USA