Chlorine demand?

gregsfc

Bronze Supporter
May 27, 2014
204
Cookeville,TN
Did I learn something new this year? It started with overshooting my cya level goal in the Spring. My test back in May after adding stabilizer and an extra week of mixing was 60. I began keeping a higher-than-ever fc level in my pool. I dose with LC, aka bleach, daily. From the previous six years history, I know that the chlorine demand will start out at about 1 and work it's way up to 2 1/2 ppm for my pool beginning its peak in late June until mid August and work it's way back down.

This year, however, I got thrown for a loop. Beginning mid July, I started losing 4 ppm per day. I first thought it to be stale KIK brand 10% bleach (I like to call all sodium hypochlorite bleach); but that tested out okay. Late July, I restested cya and it was still at 60. I then became convinced it was organics of some sort but passed all three SLAM-passing criteria even before starting. Started to SLAM in late July when I got a cc reading of 1 on a test; but got interrupted and never got to go beyond one adjustment.

In early August, I tested cya and it showed 50. I had been trying to keep fc between 7-11 due to my fear of some sort of organic.

Since I suspected some sort of weird organic, I was keeping fc way up there; maybe 8-10 most times, but would regularly get a test result down around 6-7 before dosing due to excessive fc loss.

In early August, right after my cya test, I conducted SLAM. I had to add chlorine twice during the day; once had to add 6 ppm and the other time, only 3, but the nightime test for OCLT came out to be 23 and the morning result at 22; I passed and ended SLAM. But the next evening, I had gone from 22 to 10 and it was a cloudy day. Excessive chlorine demand continued.

I started following threads of SLAMs. Mknauss was helping someone who was passing OCLT but having larger-than-expected daytime loss in fc. He suggested that the cya may be actually lower than the test result causing higher-than-expected loss.

I had just recently recently rechecked my cya. Friday, it showed 35 (40). Over the weekend, I've got my fc down to 5-7 targets, and suddenly, my fc seems to be losing it's normal loss of 1 1/2 to 2 ppm daily for this time of year. So is it possible that, from the time when I first started experiencing extra fc demand until now, that a combination of trying to keep fc a little higher, along with an overestimated cya level; I was actually experiencing more chlorine use due to more uv loss by trying to keep it above "goal" levels for the actual cya level and it was never organics?
 
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mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
37,338
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Sounds like a reasonable conclusion ------
 
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gregsfc

Bronze Supporter
May 27, 2014
204
Cookeville,TN
I've learned as much about pool care following threads from experts and even OPs on those threads as any other resource on this site. Thanks for what y'all do!
 
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revitup

Gold Supporter
Bronze Supporter
Nov 30, 2019
439
Pawleys Island, SC
Pool Size
8500
Surface
Fiberglass
Chlorine
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SWG Type
Hayward Aqua Rite (T-15)
I have to say that I generally have kept CYA and chlorine at 70 and 10-13 respectively and have not experienced any excessive chlorine loss. SCG output has been set variably at about 1.7 to 2.6 ppm per day during this season and chlorine level remains pretty steady. Check my logs. I too was concerned that I might see excessive chlorine loss at those levels but it has not been the case.
 
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gregsfc

Bronze Supporter
May 27, 2014
204
Cookeville,TN
I wonder if swg verus manual addition makes any difference at the levels we're both at?

From May until mid July, everything was like normal, except that I had never operated at cya levels that high before and don't really like to, because it makes pH testing more tricky and will force me to use more chlorine amd reagent if by chance I have to SLAM; and since I don't use an SWG, I don't really need cya way up at 60 to protect chlorine. I usually prefer 30-40; and after this ordeal, I'll be more careful to try and maintain it; but I'm still watching for a possible organics problem, because I've had several .5 cc results and one that came up at 1. Nothing but crystal clear water though; not a single day of low fc; and not a speck of any kind of algae seen anywhere.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,757
Tucson, AZ
Pool Size
16000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
Chlorine loss rate is proportional to FC - the higher the FC level, the faster the rate of decomposition irrespective of CYA. This is why TFP carefully chose the FC/CYA ratios it did - the ratio provides the best level of chlorine for sanitation while minimizing the decomposition loss. This is also why we tell people NOT to overdose during a SLAM as it is only a waste of chlorine and will not generally make the SLAM go any faster.

My guess is you were keeping your FC/CYA ratio way too high and you were simply experiencing a greater level of hypochlorite decomposition. For a manually dosed pool, a 7.5% FC/CYA ratio is where the pool should be. You can dose a little bit higher than that to compensate for daily loss, but going a lot higher than the recommended level is ill-advised.
 
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gregsfc

Bronze Supporter
May 27, 2014
204
Cookeville,TN
@JoyfulNoise. What an honor to have you answer my post. I can take it to the bank.

One question, I've been PM'ing someone in the middle of SLAM. (Frank in FL). He has been keeping his fc level super high. I think he mistook Mknauss's advice to add more chlorine to mean raise it higher. He is at 40 cya with fc from 25-35 he says and was talking about going to 50 (concrete pool). Still not passing OCLT but down to 2 1/2 loss last night OCLT. I sort of gave him the same summary as what you told me about it not speding up SLAM and just wasting chlorine without the chemistry in my answer. I told him that I hoped that you would come on to his thread.

He has been asking on the thread why someone who started with a clear pool should take so long to pass. He's on day 10. Had only a chlorine loss issue. Never seen algae or cc's or anything but a clear pool. He has been posting on his help inquiry thread but sort of got left alone on the thread.

Anyway, does this chlorine loss situation ever have any relevance for anything except UV? In other words, if he had fc at 37 when he tested at night, would any of his 2 1/2 loss be anything except organics?
 
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JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,757
Tucson, AZ
Pool Size
16000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
@JoyfulNoise. What an honor to have you answer my post. I can take it to the bank.

One question, I've been PM'ing someone in the middle of SLAM. (Frank in FL). He has been keeping his fc level super high. I think he mistook Mknauss's advice to add more chlorine to mean raise it higher. He is at 40 cya with fc from 25-35 he says and was talking about going to 50 (concrete pool). Still not passing OCLT but down to 2 1/2 loss last night OCLT. I sort of gave him the same summary as what you told me about it not speding up SLAM and just wasting chlorine without the chemistry in my answer. I told him that I hoped that you would come on to his thread.

He has been asking on the thread why someone who started with a clear pool should take so long to pass. He's on day 10. Had only a chlorine loss issue. Never seen algae or cc's or anything but a clear pool. He has been posting on his help inquiry thread but sort of got left alone on the thread.

Anyway, does this chlorine loss situation ever have any relevance for anything except UV? In other words, if he had fc at 37 when he tested at night, would any of his 2 1/2 loss be anything except organics?

It's likely a contaminant of some kind. UV mostly interacts with hypochlorite anion (OCl-) to decompose it into chloride (Cl-) and oxygen gas (O2). In the absence of UV, only organics matter. There is some spontaneous decomposition of hypochlorite that will occur, but it's usually not measurable with the testing kits most pool owners have. Raising the FC too high will make passing the OCLT harder. In reality, pool owners should stick to SLAM levels and not venture too much higher or else losses will just consume the chlorine with no benefit. Even if a pool owner raised their FC to shock levels in the morning and the FC fell to half that amount by the end of the day, that's still plenty of sanitation and oxidation. Going overboard doesn't accomplish any purpose other than wasting chlorine.
 
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Hello , There was no answer in this thread for more than 60 days.
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