ORP in lieu of FC for maintaining sanitizer level

[lucasaltic]

I read the great thread on Understanding ORP on here and it got me wondering: for those of us who don’t have automatic ORP controllers, is it still practical to maintain correct sanitization levels by working from a handheld ORP sensor in lieu of a FC drop test? As long as we are mindful of pH (and maintain it reasonably stably) what’s wrong with checking ORP daily with a handheld probe? And then adding an appropriate quantity of chlorine (or adjusting our SWG) to maintain a target ORP? That seems like what we’re ultimately after and by directly controlling ORP vs controlling a FC analog, it seems like we could get closer to optimal while benefiting from the convenience of a fast electrochemical test that uses no chemicals.

 

[mgtfp]

Most of us don't have automatic ORP controllers. ORP sounds great at first sight, but fails in practice, especially in the presence of CYA, which I consider crucial in residential pool care.

There are many posts explaining the reasons, for example this one:

ORP measures the the oxidation potential (in mV) of water which is affected by many factors - oxidizer level, pH, alkalinity, temperature, UV light, etc, etc. It does not measure the concentration of hypochlorous acid directly. As well, because cyanuric acid stabilizer reduces the overall level of hypochlorous acid to parts per billion levels while holding the vast majority of chlorine in reserve, the electrode is left to try to measure a low level voltage signal in a highly noisy environment. As an engineer you should be able to appreciate the concept of signal-to-noise ratio. Finally, CYA tends to foul up and deposit onto the delicate gold covered glass frit that separates the internal solution of the probe from the pool water. That build up causes even more signal degradation.

ORP sensors are great in principal but fail in practice to live up to the marketing hype.

Or that one:

Not quite...

ORP stands for oxidation-reduction potential and it is a measure of the tendency of any chemical species to either be oxidized (lose and electron) or reduced (gain an electron) in an aqueous environment. It is typically measured using a platinum or gold electrode that is compared to another reference electrode with a known potential (all of this is built into one small probe). So, an ORP probe doesn't measure any one chemical species specifically but rather the overall condition of the solution it is dipped into. When you place the probe into pool water it is telling you how easily an electron can be gained by the probe and thus how strongly "oxidizing" your pool water is.



It doesn't.

ORP is a general characteristic of your pool water. Any oxidizer present will cause a change in ORP. So adding chlorine to the water raises the ORP voltage...but so does adding hydrogen peroxide, or bromine, or ozone, or UV light, etc, etc, etc.. Any oxidizing species added increases the ORP. As well, the condition of the water, ie, pH, TA, salinity, etc., affects the ORP voltage as well.

Finally, Cyanuric Acid, or stabilizer, changes (or reduces) the amount of available hypochlorous acid (HOCl) in the pool water. HOCl is the primary oxidizing species of chlorine in pool water. Therefore it's concentration is what the ORP sensor will react to. If there isn't enough of it in the water, then the ORP voltage reading will fluctuate and be more sensitive to other parameters like UV light or the presence of chlroramine, etc, etc..

In short, there is no direct 1-to-1 correlation between chlorine levels and ORP. In pool water, it becomes a very complicated mixture of multiple parameters.



This is the problem with ORP control over your SWG running or not running - anything that affects ORP that isn't related to chlorine levels might cause the SWG to either run when it doesn't need to OR not run when it should.

A better, but far less desirable way to use ORP, is to treat it as a waring sign. You could run the SWG in a duty-cycle mode and measure your actual FC doing daily testing. Then, with the ORP simply as a readout, track ORP versus FC and look for a correlation. Then, once you understand your system better, you can test less often and use the ORP as a warning to either increase or decrease your SWG output manually.



You can only do this if your cyanuric acid levels are very low, typically below 20ppm. However, what will happen is your SWG will run constantly and you will wear out your salt cell faster than a typical SWG pool that simply uses a duty-cycle method and keep the CYA high.

Why??

Because SWG's add chlorine very slowly to pool water. SWG pools need high CYA levels to protect the chlorine generated from burning off in the sun. With no CYA or very low CYA, the chlorine put out by the SWG will almost instantly get used up. In the overnight hours, when pumps and SWG are not running, your pool water will be dangerously low in FC or not have any at all and that will lead to an algae bloom.

Sorry, but this is why TFP does not endorse the use of ORP as a control method for SWG systems nor the use of "low chlorine". There is nothing wrong with chlorine at any level as long as pool water chemistry is properly maintained. A swimming pool with 50ppm CYA and 3-4ppm FC has 17 times less hypochlorous acid concentration than a pool with 0 CYA and 1ppm FC. Pool's that follow TFP's FC/CYA ratio are far less harsh on the skin, eyes and bathing suits than pools that try to operate any other way.

 

[JoyfulNoise]

You have it backwards - knowing your free chlorine level is more important than knowing what the ORP level of the water is. pH has little to do with water sanitation and ORP is simply a measure of the oxidation (or reducing) state of the water. But oxidation is not as important as sanitation and disinfection. Many different chemicals can cause the ORP value of the water to increase and be over the point at which it is assumed sanitary. But there are oxidizers that are fast sanitizers (chlorine and bromine) and there are oxidizers that are slow sanitizer (peroxide and MPS) and there are even oxidizers that have very little disinfection ability because they cannot maintain a residual (ozone, for example). Yet all of those types of oxidizers can lead to high ORP values and allow you to think that the water is sanitary when it is not. ORP is also affected by pH and UV light levels as those can affect what radical species are generated from the breakdown of chlorine in water with dissolved oxygen. Chlorine and hydroxyl radicals are too short lived to be of any practical use in pool water sanitation but they can throw off ORP measurements.

ORP is not a direct measure for chlorine, it is at best a proxy value. People that have used ORP sensors in pools find that you must keep CYA levels very low to avoid fouling the probe tips and in order to get a good enough signal to noise ratio to allow for feedback control. But at no point should ORP be a substitute for measuring actual chlorine levels.

 

[mgtfp]

Looks like Matt was already busy typing while I was quoting his older posts...

 

[Lake Placid]

Matt, as one would expect, nailed it.

As a practical example from my professional life where I trained a number of pool managers on automated cal hypo erosion feeders to manage chlorine levels in pool water. The first step was teaching them to use a test kit properly and understand what the numbers meant in regards to pool chemistry. The second step to was reiterate that when calibrating the chemical automation, you always calibrate the automation controller to your test kit results. It was imperative I taught them not to look at an orp reading and assume that because it reads 745, the chlorine level is where it needed to be, but rather trust the data from the test kit and calibrate the automation to its results. (Yes we tested 3x/day minimum)

I’ve seen fouled ORP probes for any number of reasons, too high CYA, sodium bicarb got caught in the sample stream flow meter, calcium chloride pellets got caught in a sample stream, cya granules got caught in a sample stream, pine needles or hair clogged a sample stream inlet inducing a reading that did not correspond to what was in the actual body of water, dirty probes that were fouled, etc etc etc.

Bottom line orp probes for automation are the equivalent of digital test trips that are a “go, no go“ test for a computer to flip on a switch to apply a chemical in a body of water to a pre determined level. The probes need to be managed and calibrated to actual reliable test kit results that truly identify how much FC is in the water by someone who can apply the meaning of the data in a meaningful way.

For someone without automation why create double work. Test the water once with an appropriate accurate test kit and make chemical additions accordingly.

 

[lucasaltic]

Thanks gents. Appreciate the feedback and additional information.