GASCO 402ES 100 PPM Carbon Monoxide, 25 PPM Hydrogen Sulfide, 1.25 % Methane, (25 % LEL) , (50 % LEL Pentane Equivalent) , 18 % 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