RO Membrane, Inline Filter/Tubing Sizing & Upgrade

This Guide is intended for customers who have a Standard type Reverse Osmosis System like the one pictured here.

If you have a proprietary system such as Hydrotech, Microline, etc., the basic way to size your membrane and how to upgrade apply, it's just the Drain Flow Restrictor you use is different from the one we describe here.

Some Reverse Osmosis Systems are available with different capacity RO Membranes and we are often asked by customers "Which membrane does my system use?" and "How do I upgrade to a higher capacity membrane?". On this page we explain the mysteries of the RO Membrane and how you can quickly determine which membrane your system currently uses.

Which Membrane Does My System Use?
Many times, the membrane type and capacity are written on the membrane housing itself.

Type of Membrane
Membranes come in two types, CTA (Cellulose Tri-Acetate) and TFC (Thin Film Composite) sometimes labeled as TFM (Thin Film Membrane). CTA Membranes can tolerate chlorine and TFC Membranes cannot tolerate chlorine. TFC Membranes are more efficient and are the standard type used in today's RO Systems, although systems using CTA membranes are still being manufactured.

CTA membranes are usually have a capacity of 20 GPD (Gallons Per Day) or less. TFC (TFM) membranes for residential use are now available at up to 150 GPD.

If your system does not have a label which tells you which type membrane you have, you may have to remove the membrane and look on the label.

Membrane Capacity
Most RO Systems list, on the membrane housing, the capacity of the membrane in GPD (Gallons Per Day). Of course, this makes it easy to find the replacement but if you're reading this, your system most likely does not tell you the capacity.

Why is this important? To operate efficiently, RO Membranes are matched to the proper size Drain Flow Restrictor. The RO Membrane and Drain Flow Restrictor must be matched to keep the proper balance between waste water and pure water. Mismatching can either cause excessive waste to the drain or premature membrane fouling.

There are two ways to determine the membrane capacity of your system. One is to remove the membrane from the RO housing and look at the label. The label should tell you the membrane Type and Capacity.

The other is to look at the membrane Drain Flow Restrictor. Most standard RO Systems use an inline Drain Flow Restrictor like the one pictured here.

Although not shown in this picture, the Drain Flow Restrictor has a number on it such as 150, 200, 250, etc.

As the Drain Flow Restrictor is sized to the Membrane Capacity, all you need to do is match this number to the chart and you will know the capacity of the membrane your system is currently using.

Number On Flow Restrictor Membrane Capacity
150 16-18 GPD
200-250 24 GPD
300-350 36 GPD
400-500 50 GPD
600-750 75 GPD
800-1000 100 GPD
1200 150 GPD

How Do I Upgrade To A Higher Capacity Membrane?

This is the easy part. As we mentioned, the RO Membrane and Drain Flow Restrictor are matched. To upgrade to a higher capacity membrane, all you need to do is make sure you also replace the Drain Flow Restrictor to match the new membrane.

Sizing The Inline Polishing Filter and Tubing

Inline Polishing Filters are available in two configurations. One is with Female Pipe Thread (FPT) and the other is Quick Connect Fittings.

In this picture, the filter on the Left has FPT fittings (The Blue Cap is a plug to keep debris out of the filter). With this type of filter, you have a fitting that screws into this filter and the tubing connects to this fitting.

The filter on the Right has Quick Connect fittings. With this type, the tubing is simply "Pushed In" to the filter and is held in place by the Quick Connect Collar.

If your inline polishing filter has FPT threaded fittings, the most common size is 1/4".


As you can see in this picture on the right, 1/4" FPT is approximately 1/2" wide if measured with a tape measure.

Tubing Size
The tubing on the LEFT is 1/4" tubing. The tubing on the RIGHT is 3/8" tubing. Most times, the tubing is labeled with the size. Just look on the tubing and it will have writing on it telling you the type and size.

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