How to Size a Reverse Osmosis Flow Restrictor (+ Sizing Chart)
Written by: Gene Fitzgerald // Last Updated: Jul 12, 2023
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Sizing a reverse osmosis flow restrictor correctly is crucial for its optimal functioning.
Still, it can be confusing to know what size flow restrictor works with your particular system and RO membrane. Thankfully, there is an easy formula to figure it out.
And, if you don’t like math, you can use the handy sizing chart we have for you in this article.
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Key Takeaways
Here is how to size a reverse osmosis flow restrictor:
- Take the gpd rating of your reverse osmosis membrane and divide it by 0.38. That will give you the number in milliliters per minute. For example: 75 gpd / 0.38 = 197.34 mL/min
- Your flow restrictor number should be 3 to 4 times the rating of the RO membrane. So, multiply 197.34 mL/min by 3 and by 4: 197.34 mL/min x 3 = 592.11 mL/min. 197.34 mL/min x 4 = 789.47 mL/min.
- For a 75-gallons-per-day reverse osmosis membrane, you should use a 600 to 800 mL/min flow restrictor.
How to Size a Reverse Osmosis Flow Restrictor
So, how to size a reverse osmosis flow restrictor?
There will be numbers on most flow restrictors like 200, 300, or 450. These numbers tell you how much water the flow restrictor allows to pass through in a minute, measured in milliliters (mL).
Your RO membrane also has a number. But this number is in gallons per day (gpd), not milliliters per minute. 1 gallon is about the same as 3.78 liters of water.
Usually, the flow restrictor’s number should be 3 to 4 times bigger than the RO membrane’s number, so to match them up, we need to convert the membranes number into mL/min. Here’s how:
Take the RO membrane rating (in gallons per day) and divide it by 0.38. That will give you the number in milliliters per minute.
For example, if you have a membrane rated at 75 gpd, you do the math like this:
75 gpd / 0.38 = 197.34 mL/min
Remember, the flow restrictor’s number should be 3 to 4 times the RO membrane’s number. So, multiply the membrane’s number by 3 and by 4:
197.34 mL/min x 3 = 592.11 mL/min
197.34 mL/min x 4 = 789.47 mL/min
So, if your reverse osmosis membrane is rated at 75 gpd, you should use a 600 to 800 mL/min flow restrictor.
RO Flow Restrictor Sizing Chart
However, if you do not want to do complicated math, you are welcome to use this handy chart we have made below!
| Membrane Rating in gpd | Membrane Rating in mL/min | Smallest Flow Restrictor Size | Largest Flow Restrictor Size |
|---|---|---|---|
| 15 | 39.5 | 118.4 | 157.9 |
| 18 | 47.4 | 142.1 | 189.5 |
| 25 | 65.8 | 197.4 | 263.2 |
| 36 | 94.7 | 284.2 | 378.9 |
| 50 | 131.6 | 394.7 | 526.3 |
| 75 | 197.4 | 592.1 | 789.5 |
| 100 | 263.2 | 789.5 | 1052.6 |
| 150 | 394.7 | 1184.2 | 1578.9 |
The Importance of Flow Restrictor Sizing
Oversizing and undersizing refer to selecting components that are either too large or too small for the system’s needs. Oversizing or undersizing an RO flow restrictor can lead to performance issues that can affect the quality of your water and the lifespan of your RO components.
Therefore, it’s crucial to correctly size the components of a reverse osmosis system to ensure it operates efficiently and effectively.
What If I Under-Size My RO Flow Restrictor?
Undersizing means choosing a flow restrictor that is too small or has a lower capacity than what’s needed. This can lead to the component being overworked, reducing its lifespan and possibly causing system failure. For example, if a flow restrictor is undersized, it might create too much backpressure, damaging the RO membrane and reducing the amount of water the system can process.
Over-Sizing
Oversizing means choosing a flow restrictor or membrane that is bigger or has a higher capacity than what your system actually requires. While it might seem like a good idea to have extra capacity, oversizing can lead to inefficiencies, more wasted water, and even damage over time. For example, an oversized flow restrictor may not create enough backpressure for the membrane to work properly, which could reduce the quality of the filtered water.
When Should I Change My Flow Restrictor?
You should consider changing your reverse osmosis flow restrictor when too much or too little water flows into the drain. This can be determined by measuring the amount of water that comes out of the drain tube. If the flow restrictor is too loose, you may waste water and the unit may not produce enough water or shut off properly. On the other hand, if the restrictor clogs up, it could lead to poor water quality and eventually stop the unit from producing water altogether.
While it’s rare for flow restrictors to go bad, they can clog due to dirt or scale or become loose due to mechanical failure. Some manufacturers recommend replacing the restrictor each time you replace the membrane; however, most people tend to replace them only when a problem arises.
Further Reading:
How Does a Reverse Osmosis Flow Restrictor Work and What Are Its Benefits?
Flow restrictors function by opening and closing a two-port valve in response to changes in the supply line’s volume. This regulation occurs regardless of water pressure fluctuations, making the devices versatile for various household applications. By managing water flow, flow restrictors can prevent pressure build-up that could harm pipes and appliances. They also conserve water, particularly in high-pressure lines, reducing utility costs.
In reverse osmosis systems, flow restrictors are essential. They limit the amount of water that can flow down the drain line. This reduces wastewater production and maintains membrane pressure. The latter contributes to higher filtered water quality and output.
If you have any questions about RO system flow restrictor sizing please don’t hesitate to leave a comment below!
About the Author Gene Fitzgerald
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