GASCO Precision Calibration Gas 650es-433ESP-SO2-25 Mixture 25 ppm S02, 50 ppm CO, 50% LEL CH4, 12% 02, Balance N2 Contained in a 650 Liter Factory refillable cylinder, For Use with Any and All Gas Detectors, Cylinder Connection CGA TBD, Calibration Certificate NIST, Made in the USA