GASCO Calibration Gas 421-CO2-0.5 Mixture 2.5% volume Methane (50% LEL), 18% Oxygen, 100 ppm Carbon Monoxide, 25 ppm Hydrogen Sulfide, 0.5% Carbon Dioxide, Balance Nitrogen, for calibration and maintenance of gas detectors, NIST Calibration Certificate Included. GASCO Calibration Gas is ISO 9001 Certified, Made in the USA