Reverse Osmosis Flow Restrictor Available in 300cc, 450cc, 500cc, 600cc, 800cc & 1000cc / ML Per Minute Sizes Flow Restrictors or Capillaries are required for all reverse osmosis systems. The size of the Flow Restrictor required depends upon the size of the membrane being used. Flow Restrictors create back-pressure throughout the reverse osmosis membrane element and maintain a ratio of drain water to pure water at around 3 or 4:1. It is recommended that you replace the Flow Restrictor each time you replace your reverse osmosis membrane to keep your system operating at peak efficiency. The number on the flow restrictor indicates the flow rate, in milliliters per minute. The flow restrictor should match your reverse osmosis membrane's production rate.Inline Flow restrictors are easy to install. They are inserted into the 1/4" Drain Line Tubing. Simply push your 1/4" poly tube firmly into the quick connect fittings located on both the inlet and outlet ends of the flow restrictor and you are done. Flow restrictors are rated by a flow rate expressed as milliliters/minute (ml/min) Inline flow restrictor for reverse osmosis (RO) systems Features 1/4" push-fit / quick connect fittings Various sizes Easy to change or add to RO system The following table gives a guideline for selecting an inline flow restrictor for your RO membrane.In order to pick the correct flow restrictor suited for your membrane capacity please consult the list below 300cc ml/min - use with 50 - 75 GPD RO membranes. 450cc ml/min - use with 75 GPD RO membranes. 500cc ml/min - use with 75-100 GPD RO membranes. 600cc ml/min - use with 100 - 150 GPD RO membranes. 800cc ml/min - use with 150 - 200 GPD RO membranes. 1000cc ml/min - use with 200+ GPD RO membranes.