CYA and Chlorine loss

red-beard

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May 27, 2019
936
Houston, TX
Pool Size
25000
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Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite Pro (T-15)
Fire retardant underwear is now on!

Being an engineer (with a minor in Chemistry) and a computer geek even longer. I decided to look at the sensitivity of the chlorine loss to CYA. I need to download the CHEM GEEK spreadsheets, not to mention get a 1/2 life Excel function, and do a build up from there. But in the meantime, I'm using the CYA half-life charts and an on-line 1/2 life calculator.

One of my "hats" is a solar professional. I know more than a bit about calculating incoming solar radiation and the effect of off angle sun absorption. Thinking of the pool as a big solar collector that is fixed and flat, I take half-life numbers as if on the equator at solar noon. I ran hourly numbers for solar strength. I then used this to adjust the half-life each hour. For the most part, once you get under 5 ppm, the half-life doesn't change that much.

I used 6 ppm FC, since this is what I raise my chlorine to each night and I have a feel for my losses. I ran the calcs using 30 ppm CYA, again this is my pool, adjusting the half life based on the off-angle of the sun over 13 hours (specifically for August 5th). The final number is about 2.4 ppm FC or a loss of 3.6 ppm over the 13 hours I have sun on my pool. This matches fairly close on a full day of sun, no clouds, on my pool (2.8 ppm FC for a loss of 3.2 ppm). I'd attribute this to the humidity/haze in Houston blocking some of the UV and my pool sail shade.

I re-ran the numbers using 6 ppm FC, but with 40 CYA. The difference was less than 5% lower chlorine loss. I re-ran the numbers at 20 CYA and the chlorine loss was only 8.8% more than 30 CYA. It looks like there is little Chlorine savings with CYA over 30 PPM.

Flame away!
 
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JoyfulNoise

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May 23, 2015
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Tucson, AZ
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There is an unresolved issue with CYA that is simply no where to be found in the chemistry literature - some additional anomalous screening of UV that reduces the chlorine loss further with concentration. Richard’s spreadsheets only take into account the “known” mechanism of chlorine loss which is the photolysis of hypochlorite by UV breaking it down into oxygen and chloride. That is by far the strongest loss. However we know from pool owners that losses are reduced even further when CYA is higher which wound suggest loss reduction mechanisms not accounted for by the standard UV absorption data.

As a reference, my pool typically runs at 90ppm CYA and I have had FC losses less than 1ppm per day. If my CYA drops below 70ppm, my chlorine losses go way up and I have to run my SWG at much higher output and for longer hours to simply keep up.

Even Richard, if you contact him, will tell you that the full extent of how CYA blocks UV is not fully known. There are experiments that could be done to elucidate those mechanisms, but no one I know of has done them.
 
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red-beard

Gold Supporter
May 27, 2019
936
Houston, TX
Pool Size
25000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite Pro (T-15)
There is an unresolved issue with CYA that is simply no where to be found in the chemistry literature - some additional anomalous screening of UV that reduces the chlorine loss further with concentration. Richard’s spreadsheets only take into account the “known” mechanism of chlorine loss which is the photolysis of hypochlorite by UV breaking it down into oxygen and chloride. That is by far the strongest loss. However we know from pool owners that losses are reduced even further when CYA is higher which wound suggest loss reduction mechanisms not accounted for by the standard UV absorption data.

As a reference, my pool typically runs at 90ppm CYA and I have had FC losses less than 1ppm per day. If my CYA drops below 70ppm, my chlorine losses go way up and I have to run my SWG at much higher output and for longer hours to simply keep up.

Even Richard, if you contact him, will tell you that the full extent of how CYA blocks UV is not fully known. There are experiments that could be done to elucidate those mechanisms, but no one I know of has done them.

How high a FC do you run with your SWG? I ask, because I will shortly (2 or so weeks) be switching to a SWG. I purchased a standalone Hayward for 40K gal. I have enough CYA to jump from 30 to 70. I'd prefer to cautiously stay on the lower side of the CYA level, due to my mustard algae fight. With 30 CYA and running the FC to 6 every night, the Mustard is under control. The SWG will further reduce my "workload".
 

JoyfulNoise

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May 23, 2015
17,709
Tucson, AZ
Pool Size
16000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
I run my pool so that the SWG maintains about 3.5 to 4.5ppm FC with the CYA starting at 90ppm early in the season. My pool consumes almost 15-20ppm CYA during the very hot months (Jun-Aug) so I am usually down to around 60-70ppm by early August. At the height of the season I run my SWG at 45% output for 6hrs/day which roughly equates to 1.2ppm/day. Beginning and End of season is typically lower pump run times, usually only about 4 hours/day (sometimes less). I will occasionally throw a half a gallon of liquid chlorine into the pool here and there if I anticipate any large bather load beyond just my kids OR I’ll often use trichlor to bump my CYA up when it falls. In those cases I just let the SWG run as-is and let the FC drift up and back down again. This season saw a bunch of high bather load days and a lower than usual CYA, so I’ve added more “external” chlorine products than I have in previous years.

When you add the SWG you can try to leave your CYA at 30 and see if it works for you. If not, then you can bump it up to see if you can find a sweet-spot. My guess would be that you’re going to need at least 60ppm CYA to get to a reasonable run time / loss rate but there’s no harm in trying especially if you want to keep the FC on the high side for the mustard algae issue.

I also treat for phosphates and have borates in my water which gets me plenty of eye-rolling around here but it works fine for me. If you have mustard algae issues, starving the water of nutrients (phosphates) and adding a mild algaecide (borates) can only help you out. Borates also act as a high pH buffer and will hold down the pH rise inside the cell when it is generating chlorine. Lower pH inside the SWG cell helps to prevent scaling issues.
 
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red-beard

Gold Supporter
May 27, 2019
936
Houston, TX
Pool Size
25000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite Pro (T-15)
I'm running 30 ppm borates. I like the way it feels and the additional buffering. I expect I could lower the FC a bit and be OK. OTOH, I hate changing what works...but the SWG seems like so much less work.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,709
Tucson, AZ
Pool Size
16000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
I tend to run higher on borates, about 60-80ppm.

Once dialed in, and SWG can be a lot less work. If you have any questions or concerns, I’d suggest starting a new thread in the SWG subforum to see what others have done. Plenty of people to help with ideas if you’re on the fence about brands or if you want to get honest opinions about the experience.
 
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