The flow restrictor easily inserts into 1/4-inch OD tubing located on the reverse osmosis membrane drain line. Many RO units have small insert style flow restrictors that are inserted into the 1/8-inch X 1/4-inch elbow that connects the drain tube to the membrane vessel. This insert flow restrictor is compatible with quick-connect and compression fittings. This insert flow restrictor is white and used with 100 GPD RO Membranes. Internal (capillary) flow controls work as brine water flow is restricted by forcing it through a thin capillary tube inside the regular 1/4-inch drain tubing. Do not confuse the Insert Flow Restrictor with a tube insert that is only designed to help the tube hold its shape. INSERT flow restrictors are typically less expensive than INLINE flow restrictors. However, INSERT flow restrictors must be situated correctly to perform correctly and are not as easy to use. We recommend INLINE flow restrictors if possible. If you install a new inline flow restrictor, you MUST remove the original INSERT restrictor.