Parker A-Lok 6RU5-B Brass Compression Tube Fitting, Reducing Union, 3/8" Tube OD x 5/16" Tube OD (Pack of 10) Reducing union connects tubing of different diameters Fitting connects to tubing on both ends Brass for corrosion resistance, ductility at high temperatures, and low magnetic permeability Twin ferrule compression fitting for a leak-resistant seal in high-pressure applications For use with copper tubing