GASCO Precision Calibration Gas 44es-433ESP-SO2-25 Mixture 25 ppm S02, 50 ppm CO, 50% LEL CH4, 12% 02, Balance N2 Contained in a 44L Liter ecosmart Factory Refillable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA