GASCO Precision Calibration Gas 428X-20 Mixture 20 PPM Hydrogen Sulfide, 1.45 % Methane,(29 % LEL,(58 % LEL Pentane Equivalent), 15% 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