400ml@60 psi, 1/4" FNPT ; 400cc / min

Every reverse osmosis unit has a device called a flow restrictor that determines the amount of water that flows to drain. In general, the flow restrictor must be sized to fit the membrane production size. The flow to drain is usually about four times the membrane production rating, although the drain flow ratio often decreases a bit as membranes get larger.

Part of the difficulty in understanding this ratio is that membrane production is usually given in gallons per day (gpd), while flow restrictors are usually sized in milliliters per minute (mlm). To convert, multiply mlm by .38 and the result, roughly, will be gallons per day.

For example, the standard flow restrictor used for an 18 gallon per day membrane is a 180 or 185 mlm restrictor.

180 times .38 is about 68. Thus, an 18 gpd membrane needs about a 68 or 70 gallon per day flow restrictor. 4 times 18 = 72. That's about the normal ratio.