PETCUP112 Poly Insert Coupling, 1-1/2 Inch. A coupling is designed in order to connect two sections of piping together. It features barbed connections, which allow the poly pipe to be slid onto the fitting. The coupling design of this fitting lets you connect two sections of 1-1/2 inch poly pipe. Poly fittings are used in multiple water applications but should never be used in gas, steam or compressed air applications. The fittings are specifically designed to work with polyethylene tubing (aka Poly Pipe, Poly Tube). In order to create a secure and tight connection, hose clamps should be used. The barbed connections of the poly fittings work well to hold tight to the pipe, but water pressure can push the tube off the fitting, making the hose clamps a great choice for installation.