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5 RO Parameters You Should Log Compulsively

When it comes to keeping reverse osmosis (RO) systems running reliably there’s absolutely nothing you can do that’s more important than keeping good logs. Since we keep meeting clients who don’t know what they should be logging or who don’t fully understand WHY they should be logging certain parameters, we thought we’d write this post to clear things up.

Feed Water Parameters

Ok, so this is really more of a category than it is a single parameter. These parameters are critical to log for the reasons we explain below.


pH is one of those parameters people don’t bother to check very often. In many applications, pH remains quite stable and so it’s not as necessary to check it regularly. In others, where feed water is drawn from a surface source (lakes, rivers, etc.) or where other processes may interfere with feed water quality, pH variations can spell trouble. In addition to the possibility of damaging RO membranes, pH variations have a serious impact on water chemistry. pH increases can lead to increased membrane fouling as certain impurities become less soluble while decreases in pH can have a negative impact on permeate quality.

Log pH daily if your application doesn’t have a very stable feed water source. This will help significantly in troubleshooting and system optimization and will help spot extreme conditions more quickly. Certain clients should be measuring pH continuously with in-line pH probes.

Total and Free Chlorine

Chlorine will kill standard RO membranes. Any reading for free chlorine above 0 mg/L (ppm) should be making alarm bells go off.

If you’re lucky enough to have a carbon filter in your pre-treatment train you’ll only want to measure and log total and free chlorine on a weekly basis. This is just to make sure that your carbon filter isn’t at the end of its useful chlorine removal lifetime.

If, however, you’re dosing sodium metabisulfite or some other chemical to neutralize free chlorine in the feed water you’ll want to measure more frequently. In fact, we’d strongly recommend installing an ORP probe and configuring an alarm condition when ORP deviates significantly from your baseline ORP value corresponding to 0 free chlorine. Daily measurement and logging are a strict minimum here.

Note that we also recommend testing total chlorine because chloramines don’t show up when testing for free chlorine but can still damage your membranes.


Most clients should measure feed water temperature daily. Water temperature has a profound impact on the quantity and quality of pure water produced by your system. Of course, high temperatures can also damage your membranes, but this is a rather uncommon issue.

Log temperature daily to allow for proper system performance tracking and optimization.


Ok, this one is a no-brainer… sort of. Everyone knows that they should expect a certain permeate flow out of their RO machine. Keep in mind that it’s important to log permeate, concentrate and recycle flows every day to get a clear picture of how your RO system is behaving. It’s not enough to know that the machine is producing the right amount of pure water. The maintenance of appropriate membrane cross-flow (to avoid fouling) and system recovery (to avoid fouling and wasting water) are absolutely critical for RO operation.

Measure and log permeate, concentrate and recycle (if available) flows every day.

Pre-filter, Primary, Final and Permeate Pressures

RO machines operate in a delicate balance of flows, pressures and water chemistry. Operating pressures give us lots of information on the health of the RO membranes as long as we also have flow and water quality information available. For example, a noted increase in the differential between primary and final pressures could indicate membrane fouling.

It’s important to log the pressure before and after your system pre-filters so that you can know when to change those cartridges. We won’t go into the details of when to change your pre-filters here, but know that this is critical to proper system maintenance, optimization and stock planning.

Log your system’s primary, final and permeate pressures on a daily basis and your water treatment partner will have a much clearer picture when optimizing your system. Keep track of pre-filter pressures and you’ll be able to optimize your pre-filter replacement frequency and prevent any system downtime.


Conductivity is a measurement we use to quickly estimate the concentration of impurities in water. Not everything gets measured by conductivity but it gives us a good, fast approximation. We can then use conductivity to track how well an RO system is performing. Conductivity helps us make sure that the membranes are performing well and it also serves as a cross-check for system flow meters. Conductivity is often used as a first indicator that a membrane cleaning or replacement will be required soon.

Measure conductivity of the feed water, permeate and concentrate streams daily and you’ll be gathering lots of useful information on the health of your system.

High Pressure Pump Frequency

If your system’s high pressure pump has been equipped with a variable frequency drive (VFD) you’ll want to make sure to log the frequency, in Hz, it’s operating at when you take all of your other readings. The motor frequency obviously has a huge impact on all the other system parameters so you’ll want to know if your parameters are changing appropriately as the frequency varies.

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