Chlorine Use per day as a function of CYA ppm?

[John Nelson]

I searched the forum and google scholar and couldn't find an answer to this. Is anyone aware of a study or personal experiment with data that shows the chlorine usage rate per day in ppm (or percentage) as a function of different CYA levels? For example, at 10ppm CYA you lose 2ppm FC per day, at 30ppm CYA you lose 1ppm FC per day, at 50ppm you lose .75ppm per day, at 100ppm CYA you lose .5ppm FC per day. Just making those numbers up of course. In Bob Lowry's writing he suggests that you lose around 1ppm per day at 30ppm CYA, but I can't find him talking about how that chlorine loss rate changes with different levels of CYA.

I know the decay rate changes with other factors besides CYA (temp, Crud in the pool, etc), but all else being equal, am trying to find data showing the relationship between CYA level and chlorine loss rate, before I go and try to do my own experiments.

 

[ajw22]

Welcome to TFP.

The answer may be within this thread...

Pool Water Chemistry

This thread presents my findings so far on pool water chemistry including the following: More Accurate Calcite Saturation Index (CSI) to replace Langelier Saturation Index (LSI) Calculation of ppm HOCl (disinfecting chlorine) at various levels of Total Free Chlorine (FC) and Cyanuric Acid (CYA)...

www.troublefreepool.com

This chart in the thread sort of answers your question...

The following is a graph showing that a large amount of the benefit of CYA protection of chlorine from UV (sunlight) is already there at around 20 ppm. This data is approximate, not only because it is dependent on the amount of sun exposure, but because the rate constants themselves change with FC level (because there is a mix of two different rates of destruction -- one from HOCl and the other from the chlorinated cyanurates which are more stable, but still breakdown from sunlight). The limiting half-life for HOCl/OCl- is 35 minutes which is consistent with pool studies, but some experimental studies give 11.6 minutes. The limiting half-life of the chlorinated cyanurates is 8.4 hours though some other data shows it could be 6 hours.

Happy hunting.

 

[JamesW]

The best correlation is the FC/CYA ratio.

All else being equal, the same FC/CYA ratio will create about the same FC loss per day.

So, an 8 ppm FC and an 80 CYA will have about the same loss as a 4 ppm FC and a 40 ppm CYA because the ratio is the same.

Higher CYA will tend to have lower overall loss even with a correspondingly higher FC level .

Higher pH will slightly increase the loss due to more OCl^- (hypochlorite ion) vs. hypochlorous acid (HOCl), but not enough to make it worthwhile to try to maintain a lower pH.

 

[AUSpool]

Maybe this graph from Wahman might help.

Chlorinated Cyanurates: Review of Water Chemistry and Associated Drinking Water Implications - PMC

Since 1958, cyanuric acid and two chlorinated cyanurates, commonly referred to as dichlor (anhydrous sodium dichloroisocyanurate or sodium dichloroisocyanurate dihydrate) or trichlor (trichloroisocyanuric acid), have been added to outdoor swimming ...

pmc.ncbi.nlm.nih.gov

 

[JamesW]

Maybe this graph from Wahman might help.

That uses a single level for FC, which is not really useful since we go by the FC/CYA ratio, which requires a higher FC for a higher CYA.

 

[mas985]

I did these experiments several years ago:

CYA Testing Update

I thought I would give those who are interested an update on some testing that I have been doing regarding CYA levels and chlorine. First a summary of the testing that I did in this thread. - Running my spa with the SWG at CYA levels from 45-80 ppm showed no change in chlorine production. -...

www.troublefreepool.com

CYA vs UV Chlorine Loss Test Update

Many years ago, I did a test of chlorine loss vs CYA level and published the results here: https://www.troublefreepool.com/threads/cya-testing-update.2984/ There were a few issues with the test that I wanted to resolve with some additional testing but never got around to doing it...until now...

www.troublefreepool.com

What I found was that with a constant FC/CYA ratio, higher CYA levels resulted in less absolute FC loss.

These are the measured losses in a 2 hour window near solar noon.

1:0 FC/CYA 7.2/80 --> FC/CYA 6.8/80 - 0.4 ppm FC Loss
1:1 FC/CYA 3.6/40 --> FC/CYA 2.6/40 - 1.0 ppm FC Loss
1:3 FC/CYA 1.8/20 --> FC/CYA 0.4/20 - 1.4 ppm FC Loss

 

[AUSpool]

That uses a single level for FC, which is not really useful since we go by the FC/CYA ratio, which requires a higher FC for a higher CYA.

True, but the OP simply asks for chlorine use or loss as a function of CyA. A good starting point maybe.

 

[JamesW]

If you do not maintain adequate FC for the CYA, you begin to have other problems like algae, which consumes chlorine.

So, the question cannot really be answered because it is oversimplifying the nature of the problem.

There is no formula or equation that can describe the loss of FC per day given the single variable of the CYA level.

 

[AUSpool]

If you do not maintain adequate FC for the CYA, you begin to have other problems like algae, which consumes chlorine.

So, the question cannot really be answered because it is oversimplifying the nature of the problem.

There is no formula or equation that can describe the loss of FC per day given the single variable of the CYA level.

Agreed. And that kind of sums up where the rest of pool industry is. Kind of profound if you think about it.

 

[John Nelson]

Sorry for the delayed response (was traveling), and thank you for the input. I do understand I need adequate FC for the CYA used, and that the question I asked is only one part of the system. A little more context:

I own a pool company that provides weekly service and I want to train my guys on how these dynamics work and also develop practical rules for when we should use liquid only vs when trichlor may be needed in the hot summer weeks. I want to show my team a few simple graphs during training about the role of CYA and how it affects chlorine decay at different CYA levels (the purpose of my original question), and how it's necessary to stay above the min FC/CYA ratio during the entire week to avoid algae growth.

Basically, was hoping to show them that to be at the 7.5% FC/CYA ratio 7 days after prior chlorine dosing means that we need to put in x ppm of liquid chlorine if we're at y ppm of CYA. And was hoping that knowing the half-life of chlorine at various levels of CYA would get me closer to doing that and would also illuminate why the 30-50ppm CYA level is "optimal" for economic use of chlorine.

Seems like it may be more complicated than that after reading the conflicting experiments that Mark's done though.

 

[ajw22]

Basically, was hoping to show them that to be at the 7.5% FC/CYA ratio 7 days after prior chlorine dosing means that we need to put in x ppm of liquid chlorine if we're at y ppm of CYA. And was hoping that knowing the half-life of chlorine at various levels of CYA would get me closer to doing that and would also illuminate why the 30-50ppm CYA level is "optimal" for economic use of chlorine.

Liquid chlorine dosing every 7 days does not work well.

You should have an idea of the average chlorine consumption in your area for the time of year. It can be 2ppm to 4ppm per day.

FC target for 30-50 ppm of CYA is 4 to 6 ppm.

7 days of FC consumption can be 14 - 28 ppm.

So you want to raise the pool to 18 ppm at CYA 30 to 34 ppm at CYA 50.

But safe FC is 40% of CYA or 12 ppm at CYA 30 to 20 ppm at CYA 50.

If the pool loses more than 2ppm per day of FC the FC will drop below minimum and you can end up with a green pool. That is likely to happen in the mid-summer months.

Best advice would be to raise the FC to 40% of CYA with liquid chlorine every 7 days and hope for the best. That is as high as you can safely go for a customer.

Or change your schedule to visit pools twice a week in peak summer.