for calibration and maintenance of gas detectors, NIST Calibration Certificate Included. GASCO Calibration Gas is ISO 9001 Certified, Made in the USA 25 PPM Hydrogen Sulfide, 2.5% Methane (50 % LEL), 19% 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