CALGAZ 2A310041 Calibration Gas Mixture Equivalent by GASCO 25 PPM Hydrogen Sulfide, 50 PPM Carbon Monoxide, 2.5% Methane 20.9% Oxygen, Balance Nitrogen contained in a 34 Liter Disposable Aluminum Cylinder, For Use with Any and All Gas Detectors, Cylinder Connection C-10, Calibration Certificate NIST, Made in the USA