ORP drifts high regardless of chlorine concentration

[RIMOC]

Our basic problem is that the ORP will drift up, especially during a warm, sunny day even when the chlorine concentration is not increasing (and sometimes even dropping). And I have not been able to find any reference to such a problem either on the internet or from our local technician.
We have a 220,000 gal outdoor pool in Tampa FL. We had a Polaris Watermatic C660 but after the following problem developed and nothing else worked, we had installed a Hayward 2000. However, the problem persists. Starting last fall (2013), The ORP would drift upward during the day after the chlorine feed had stopped. It would reach its peak about midday and then drift down. All the time the chlorine level would remain constant at the desired level (2-3 PPM). It often seemed to be more pronounced on sunny days. After the new regulator was put in, the technician from our pool supplier tried to regulate it and let the CYA stabilizer drop to a very low concentration (<< 20 ppm) and it seems to work okay. However that was during the cool, and often cloudy weather. Lately our weather is warming up and the temperature has been getting into the 80s with sunny days. The last couple days, the ORP was set at 760. It dropped low enough during the night to feed chlorine to 1 ppm. But as the day progressed, the ORP rose above the set point and continued rising until the top cut-off of 840 even though the chlorine actually decreased because of the sun. This was similar to last fall, although then we kept the "CYA at about 30 ppm and the chlorine about 3 ppm. I have not been able to find anything about the ORP rising high like this without an accompanying rise in the chlorine, and was advised that it was probably a problem with the regulator, so we got the new one. However, the problem persists. The technician from our supplier is at a loss as to how to explain this. He did some testing and suspected a possible contamination of the CYA, but, with the warm sunny weather the situation is persisting/returning. I have a fairly good background in chemistry and can think of nothing else than some kind of contaminant that get oxidized in the sunlight, but cannot imagine what that would be. We get our water from a well on the property, but things worked ok during last summer. Any ideas, anyone?
Rimoc

 

[Patrick_B]

Welcome to the forum,

I only know about ORP controllers from a practical hands on experience, so I can't offer a technical explanation for upward drift during sunlight. That just seems really odd, and I can't imagine a reason for it. What I do know is that a lot of factors contribute to the accuracy of ORP readings and they are in my experience, extremely difficult to correlate directly to FC levels with any consistency.

I'm curious about a couple of things however. Did you try replacing the probe? pH and ORP probes often do not last and have to be replaced.

How did the technician, as you say, "let the stabilizer level drop" to less than 20 PPM? I'm not sure what you mean but that.

Also, did he mean the probe was contaminated with Cya, or that the Cya was contaminated as the statement reads?

 

[JasonLion]

Welcome to TFP!

Do you have a PH regulation system, and have there been any issues with it recently? ORP won't work reliable if the PH is fluctuating.

How do you feed chlorine? Do you pump sodium hypochlorite, or are you using a salt chlorine generator? There are some ways a salt system could cause what you describe, though that isn't what they commonly do.

If it isn't either of those, it sounds like something is broken in the ORP sensor system.

 

[chem geek]

When there is more sun, then the UV from sunlight breaks down chlorine into hydroxyl radicals (this post and this paper goes into more technical details). Your particular ORP sensor may be more sensitive to such hydroxyl radicals that have very high ORP, but these are very short-lived so unless your ORP sensor is close to water exposed to sunlight I wouldn't expect it to show up. Is your ORP sensor far along the circulation path or is it close to water exposed to sunlight?

 

[RIMOC]

Originally, we replaced the ORP sensor. That had no effect. Then we replaced the controller as described. I described the subsequent work with the technician from our suplier. The stabilizer concentration drops over time. We have an old DE filtration system and the pit (3200 gal) is emptied each time the filters are cleaned. The controller controls the pH and keeps it quite constant at pH 6 +/- 1. Both chemicals are fed by Stenner feeder pumps (peristalic). Chlorine is liquid hypochlorous acid from barrels.The chemical regulator/feeder is kept in a controller room constructed with concrete block which also contains the DE filter pit. The sensors feed from the pipe which comes directly from the filter pit.

 

[chem geek]

Are you sure the pH is 6 +/- 1 ? That doesn't make sense. First of all, that pH is too low and should be in the 7.0 to 8.0 range, usually near 7.5. Second, the variation allowed for pH is usually +/- 0.1 or lower.

So the location of the ORP sensor should be well away from water exposed to sunlight and it does not seem likely that the hydroxyl radicals would last long enough to reach the sensor, but that doesn't explain why the ORP rises around mid-day and more so when their is sunlight.

 

[RIMOC]

Sorry, typo. Meant pH is 7.6

 

[JamesW]

UV light makes chlorine bond to cyanuric acid more tightly, and can make orp levels drop in strong sunlight. This is the opposite of what you're experiencing. Can you list all of the chemicals you use?

Update: Based on new information, I think that the following references are likely to be inaccurate and not based on known chemistry.

With stabilized pools, the HOCl concentration and therefore the ORP readings can vary daily as a result of exposure to the UV rays in the sun. Due to the slow rate of chlorine release from the cyanurate compound, the HOCl concentration decreases during the day and increases after sunset, even though no chlorine is added. This does not show on test kits.
http://www.sbcontrol.com/orpuse.pdf

Another interesting observation was on our outdoor pools we could tell when the sun was rising and setting based on ORP values alone. When the sun was setting, the ORP level would begin to rise and by nightfall, the ORP had risen by 40 – 50 mV. When the sun started rising, the ORP level started dropping again. This would confirm the theory that HOCl attaches itself to cyanuric acid in the presence of sunlight.
https://www.poolwarden.com/cynuricacid

The bond between chlorine and CYA becomes stronger in the presence of UV rays, so this difference in readings is intensified during daylight hours, notes Ron Akin, vice president of sales at Chemtrol in Santa Barbara, Calif.

“If you look at data logs of controllers, you generally see the ORP go up at night because the bond is weakened,” he says.
http://www.poolspanews.com/bond-financing/acid-watch.aspx

Why does ORP go up during the night and down during the day?

Because during the night, with no UV light the chlorine does not bind to the cyanuric acid well, so more is available to sanitise the pool, and so the ORP will be higher. During the day, the UV light will cause the chlorine to bind strongly to the cyanuric acid and so the ORP will go down.
http://www.aasensors.com/pool-gem-q-a/

 

[RIMOC]

We use sodium hypochlorite solution, hydrochloric acid, sodium bicarbonate, and calcium chloride
We have used cyanuric acid, but to take that out of the variable, there is none in, at least not measureable.
We use diatomaceous earth on the filters (pre 1970 system) and the pH of our tapwater = 7.9 and is from a well.
This all started about November of last year. Before that it was OK. I just came here at the beginning of the summer last year. Could it have anything to do with just being idle in the winter? The water temperature got down to the mid 60s and the pool was maintained to meet the state standards through the winter.

 

[JamesW]

It's hard to say based on the available information. There is nothing obvious. It might be a faulty orp sensor. If you can give a more complete description of your system, including pictures, we might be able to help more. Without an on-site visit, it's going to be hard to diagnose.

Are you heating the water?

How are you testing the chlorine levels?

Is the pool getting high use or high chloramine levels?

Have you used thiosulfate or mps?

Can you describe how the orp sensor is installed?

 

[RIMOC]

Water is not being heated. Just what is natural for Tampa FL
Chlorine is tested by DPD.
There is no measureable chlodramine levels. During the winter months, the pool gets very little use. It is just maintained.
No thiosulfate or mps.
We had a Polaris Watermatic C660 but after the following problem developed and nothing else worked, we had installed a Hayward 2000. However, the problem persists.
Here is a photo of the controller on the wall above the outflow pipe and a photo just to the left showing the pumps and the edge of the filtering pit.

 

[JamesW]

Do you have anything added before the intake to the orp sample cell that could be throwing off the readings?

 

[RIMOC]

in the upper photo, the large pipe along the wall has the flow from the filter pit directly to the pool. The tubing on the far left goes to the flow cell. The next 2 feed the chlorine and the acid into the outgoing water. The water in the flow cell has always had concentrations of CHL & acid similar to that of the pool, whenever sampled.

 

[Bama Rambler]

How is the CYA being tested?

One thing I haven't seen is a full set of test results. Can you post a set?
Are there metals in the water? If so, have any sequestrants been used?
Does sunlight enter that room?
When you changed the controller did the probes and probe cables get changed too?
The picture shows the C660. Is the new one the HCC 2000 or the CAT 2000?

Do you have the well water tested at a water lab?

 

[JamesW]

Note: In one of my above posts, I said that sunlight causes chlorine to bind more to CYA. Based on new information, I no longer believe that that is true. Sunlight might have some sort of effect, however, I don't know if it does, or by how much, if at all.

RIMOC, your ORP does seem to be correlated to the sunlight. However, correlation does not necessarily mean causation. There are multiple reasons why sunlight might be correlated without being causative.

For example, during strong sunlight, FC would be depleted causing the ORP to go down, and the ORP to trigger the chlorine feeder. If the chlorine were to be fed into the return line, and if a floor return were near the main drain, the main drain might intake some of that chlorinated water, causing the chlorine level in the intake to be higher than in the main body of water, and therefore not representative.

It might be worthwhile to call in a service person who is familiar with ORP to see if they can help you figure this out.

It might also be helpful to contact the manufacturer to see if they can assist you with this. They might be able to send a representative for an on-site evaluation.

If you can document all of your processes and procedures, a service person or manufacturer's representative would be better able to assist you.

I would suggest details like:

1) FC and CC to a precision of 0.2 ppm at least once per hour.
2) How much chlorine has been used per hour.
3) ORP readings at least once per hour.
4) Document ORP when chlorine is feeding, and when it is not.
5) Document all other chemicals as they are used, how much, how added etc.
6) Bather load details.
7) Sun intensity levels.
8) Any other details that you think would be relevant.

 

[RIMOC]

I was busy over the weekend and also had to wait until today to see if we had a test of our well water. As it turned out, the testing which is done is for pathogens and chlorine.
I have been working with the service person from out pool supply for months. He has not been able to find out anything. I have asked around a lot and it seems no one has ever heard of a problem with the the ORP rising without chlorination. I suppose I will have to use my chemistry background to check out some things. I see that HHGreg has some pocket ORP testers for less than $100. If they can be read precisely enough, I might get one to check out some things.

Thank you for your dialogue.

 

[JasonLion]

Note: In one of my above posts, I said that sunlight causes chlorine to bind more to CYA. Based on new information, I no longer believe that that is true. Sunlight might have some sort of effect, however, I don't know if it does, or by how much, if at all.

Direct sunlight on the water, with CYA in the water, tends to lower ORP by abut 30. I am not sure of the chemistry.

 

[jumpturner]

I was busy over the weekend and also had to wait until today to see if we had a test of our well water. As it turned out, the testing which is done is for pathogens and chlorine.
I have been working with the service person from out pool supply for months. He has not been able to find out anything. I have asked around a lot and it seems no one has ever heard of a problem with the the ORP rising without chlorination. I suppose I will have to use my chemistry background to check out some things. I see that HHGreg has some pocket ORP testers for less than $100. If they can be read precisely enough, I might get one to check out some things.

Thank you for your dialogue.

Hi - old thread here but I'm trying to sort out the same thing.
Did you ever figure this out?

I have an ORP sensor (Scientific Atlantic) and an arduino. Sensor drifts higher up to 815 or so after almost 24 hours. No chlorine being added, and 55 degree water in Toronto. Sensor reads 235mv in reference solution and doesn't drift so the probe seems fine.
ph, stabilizer all 'normal'.

any further thoughts appreciated.