Chlorine/Cyanuric Acid Ratio Recommendations for SWG Pools

Jump to latest Follow Reply
T

Tom ONeill

Active member
Sep 3, 2018
30
Tucson, AZ
I have noticed that for some years now it has been recommended for Saltwater Generator Chlorine (SWG) pools that the Chlorine/Cyanuric Acid (FC/CYA) ratio should be a minimum of .05 with a recommended 70 to 80 PPM CYA level, vs. the traditional .075 FC/CYA ratio.

For the moment, set aside the advisability of the .05 FC/CYA ratio recommendation.

My principal question is: Why would 70 – 80 PPM Cyanuric Acid be recommended for SWG pools?

But first, some data generated from an EPA online application: https://usepaord.shinyapps.io/cyanuric/, Wahman 2017 derivative of O’Brien 1974.

CYA FC HOCl OCl- Av. Cl2
70 3.5 0.0203 0.0185 0.0388
60 3 0.0202 0.0185 0.0387
50 2.5 0.0202 0.0184 0.0386
40 2 0.0201 0.0183 0.0384
Avg = 0.0386

All of the above are PPM values at .05 FC/CYA ratio and 7.5 pH.

Av. Cl2 Definition: Chlorine not bound to Cyanuric Acid or as Combined Chloramines. And Av. Cl2 is what’s available for any reactions (HOCl + OCl-).

Given that the Av. Cl2 is virtually identical across all .05 FC/CYA instances:
Why not use 30, 40 or 50 PPM CYA rather than 70 or 80 PPM CYA, as the resulting Av. Cl2 will be virtually identical in all instances?
 
Because there are non-linear UV shielding effects at play that haven’t been fully explored but are easily seen in almost any outdoor pool with an SWG and intense UV. SWGs generate very small quantities of chlorine that get added slowly over the course of hours. If the CYA is too low, the extinction effects are too strong and the FAC gets used up very quickly.

In my own pool I will immediately see a huge change in chlorine demand once the CYA goes below 60ppm. I see this almost every spring and early summer when my CYA is low. As soon as I bump my CYA back up to 90ppm (yes, that’s where I keep it), my chlorine demand drops to less than 1ppm per day.
 
  • Like
  • Love
Reactions: jgofnj, mgtfp, Jimrahbe and 4 others
Tom ONeill said:
I have noticed that for some years now it has been recommended for Saltwater Generator Chlorine (SWG) pools that the Chlorine/Cyanuric Acid (FC/CYA) ratio should be a minimum of .05 with a recommended 70 to 80 PPM CYA level, vs. the traditional .075 FC/CYA ratio.

For the moment, set aside the advisability of the .05 FC/CYA ratio recommendation.

My principal question is: Why would 70 – 80 PPM Cyanuric Acid be recommended for SWG pools?

But first, some data generated from an EPA online application: https://usepaord.shinyapps.io/cyanuric/, Wahman 2017 derivative of O’Brien 1974.

CYA FC HOCl OCl- Av. Cl2
70 3.5 0.0203 0.0185 0.0388
60 3 0.0202 0.0185 0.0387
50 2.5 0.0202 0.0184 0.0386
40 2 0.0201 0.0183 0.0384
Avg = 0.0386

All of the above are PPM values at .05 FC/CYA ratio and 7.5 pH.

Av. Cl2 Definition: Chlorine not bound to Cyanuric Acid or as Combined Chloramines. And Av. Cl2 is what’s available for any reactions (HOCl + OCl-).

Given that the Av. Cl2 is virtually identical across all .05 FC/CYA instances:
Why not use 30, 40 or 50 PPM CYA rather than 70 or 80 PPM CYA, as the resulting Av. Cl2 will be virtually identical in all instances?
Click to expand...

Be careful with the Wahman calculator for pool typical FC/CYA ratios. The calculator is based on measurements made at FC/CYA ratios as they would appear when using di- or trichlor for disinfection of drinking water. Wahman didn't do measurements at pool typical FC/CYA ratios.

The Wahman model seems to show good agreement with the O'Brien calculation (at 25°C) as long as the FC/CYA ratio is above about 1. But at ratios relevant for swimming pools (FC/CYA around 10%), the model deviates about 30%-35% from the O'Brian calculation (at least that was the case when I tried the calculator about 3 years ago).

You can try Richard's Pool Equations spreadsheet:

 
Last edited:
  • Like
Reactions: Newdude
JoyfulNoise said:
Because there are non-linear UV shielding effects at play that haven’t been fully explored but are easily seen in almost any outdoor pool with an SWG and intense UV. SWGs generate very small quantities of chlorine that get added slowly over the course of hours. If the CYA is too low, the extinction effects are too strong and the FAC gets used up very quickly.

In my own pool I will immediately see a huge change in chlorine demand once the CYA goes below 60ppm. I see this almost every spring and early summer when my CYA is low. As soon as I bump my CYA back up to 90ppm (yes, that’s where I keep it), my chlorine demand drops to less than 1ppm per day.
Click to expand...
Question for you.

When the CYA is low, will you see a very noticeable drop in salt level because the generator is working harder and using more?

I'm still getting my CYA back up, but my salt level went from 3200 to 2200. I do need to check and clean the cell, so maybe the reading in the system us off?
 
Bbp said:
will you see a very noticeable drop in salt level because the generator is working harder and using more?
Click to expand...
No. The water that is returned to the salt sensor has stabilized after the chlorine has done its oxidizing job and been grabbed by the Na to form NaCl, salt.
 
As Darin said, chlorine turns back into chloride. But in solution chloride and sodium don't recombine, both remain separate ions in water.

We often say that chlorine turns back into salt, that's just a sloppy way of saying the above. The "salt" drop-test tests by the way only for chloride ions, it can't distinguish if it got into the water by adding for example NaCl, KCl, CaCl2, liquid chlorine turning into chloride, or whatever.
 
It is a common misconception, and probably a result of the very poor state of what is considered highschool education in the US, that salt gets "used up" when an SWG converts the chloride ion into chlorine gas by the process of electrolysis. In a perfectly closed system, there is no consumption of the chlorine atom at all - it is sim ply converted from one oxidation state (electron configuration) to a different oxidation state. The process of killing pathogens or oxidizing bather waste simply causes the chlorine atom to accept an electron from whatever it is that it is doing and thereby revert back to a lower oxidation state. So, in effect, there is no change in salt level.

No pool would ever be a perfect system or a closed system and so it is possible to lose chlorine by the offgasing of a highly volatile combined chlorine compound, say for example nitrogen trichloride. But, the amount of chlorine that would be released from the water is so infinitesimally small that there would be no way to even detect it except in a highly controlled lab experiment.

When you see changes in salt levels, it is almost always operator/testing error, not chemistry.
 

Enjoying this content?

Support TFP with a donation.

Give Support
Thanks "mgtfp" for the reference to Richard's Pool Equations spreadsheet. I have been using it for quite a long time. It's a very comprehensive piece of work. I've used it often to "game" alternative operating profiles and really appreciate the ability to drill down to the fine details when desired. And I truly appreciate that it calculates the CSI. Saves having to calculate it myself - tedious.
The millimolar concentrations (mM) generated with the Pool Equations spreadsheet closely pattern the EPA (Wahman) generated values throughout the typical operating range of most pools. I'm not likely ever to intentionally operate at 1 ppm or 10 ppm FC. However, I find that the numbers gen'd by Pool Equations appear to "drift" more than the EPA app, so the EPA app is the one I use the most for the dissociation states of chlorinated cyanurates and HOCl - OCl- through the desired pH range, and it's much quicker to compile the results. With Pool Equations, you have to drill down to the 320 to 328 lines in the spreadsheet and then piece it together. I use the EPA values principally to model Chlorine loss due to photolysis under varying FC/CYA and pH values, paired with the seasonal and daily solar UVA-UVB patterns and Peak Hours equivalents. I use Pool Equations for nearly everything else.

We operate two semi-public open-air in-ground plaster pools, each at approx. 32,500 gallons and open for use year-round in our community in Tucson, AZ.
 
As a p.s., Richard's info from 2007 is actually more accurate than the results today from Pool Equations. (2014 last update, I believe). In the latter rendition Richard is calculating the loss rate against the Entirety of chlorine, not just what's exposed to photolysis. The entirety Is Not Exposed. Only the chlorine not bound to CYA.

Robert Lowry was the King. His work developing the essential nature of the FC/CYA ratio is dead bang on. Richard followed that as well, until more recently. Lowry was, and still is Correct. The FC/CYA Ratio totally controls how much HOCl and/OCl- is available to do anything. At a given constant Ratio: .05, .075, .115, or whatever you like, the available chlorine to do anything is very nearly exactly the same. That's why Lowry was Right and still is.
 
Clarification:
4 FC/50 CYA, .08 Ratio vs. 2 FC/25 CYA, .08 Ratio.
The PPM Chlorine not bound to CYA is Identical. At the same Ratio, all combinations of FC/CYA that have the same Ratio will have essentially the same unbound Chlorine level.
 
The true originator of the FC and CYA relationship was Ben Powell. Richard Falk then determined the FC/CYA ratio rule after that. Of course, the 1974 O'Brien paper gives the detailed equilibrium constants to figure this all out, but they didn't propose standards based on the ratio -- they just cautioned on using too much CYA. As for Bob Lowry, he took the 7.5% from Richard (a good portion of his article came from Richards posts he wrote on this and other forums, and Richard also reviewed what he wrote and gave him comments), but he did a long time ago propose a 10% ratio (as the inverse so have your CYA at 10 times your FC level and his context was the CYA level needed for protection).
 
Interesting that Richard recommended a FC/CYA ratio of 0.1. And I agree, as it's much better at maintaining sufficient disinfection of pathogens and also for better control against any algae growth.
What stumps me is why, after all these past years, everyone is saying you really only need a 0.05 FC/CYA ratio for SWG pools?
There is absolutely nothing magic about pool chemistry and SWG pools. SWG produces the direct equivalent of Sodium Hypochlorite, but without the added hydroxide of the bottled sort. I wish there were some Magic - but No.
Was this decrease in the recommended FC/CYA ratio to 0.05 an accommodation to the SWG industry? Frankly, that's my guess. Perhaps the SWG system manufacturers have been over-selling their system capabilities?
 
There is an ongoing debate about minimums. The point of minimums was around the longevity of cells. Reducing the amount to the minimum required to ensure algae death, while prolonging the cell life.

That is for the gear heads and OCD. Not for the populace, that don't want to be bothered with the chemistry. Also, pools are not static. The amount of UV and bather load changes with the season, and day to day.

If you have a pool, that has consistent FC demand (same bather load daily, same UV, in an enclosure to keep out organics, etc.), fine, run at minimum. That is NOT how the VAST majority of pools work. Some have covers, some don't, sun changes daily, I swim today, don't swim tomorrow. FC demand is all over the board. A pool is not a closed system.

.05 is a pipe dream in a perfect pool world. A world we don't live in.
 
Tom ONeill said:
There is absolutely nothing magic about pool chemistry and SWG pools
Click to expand...
True. But they come on everyday like clockwork and never are too tired or too busy to add the FC today. Particularly in the beginning everyone had small cells with long runtimes. With FC topped off around the clock, it could be run a lower FC and still remain algae free.

Fast forward a bunch of years and now people are using 6X units and only producing at night for off peak electricity savings. Those pools are swinging like LC pools.

For this reason, we added a range to the SWG chart two winters ago because chasing a 5 was a fools errand. You'll notice the old single targets are now the low target.

swcg_chart.jpg
 
A couple of fundamental examples at 50 ppm CYA if I may:
Chlorine loss to photolysis is the Big Bad Wolf.

At 3.75 FC/50CYA, 0.075 Ratio, photolysis ppm loss is 1.56 times that of 2.5 FC/50 CYA 0.05 Ratio.
At 5 FC/50 CYA, 0.1 Ratio, photolysis ppm loss is 2.16 times that of 2.5 FC/50 CYA 0.05 Ratio.

That's what it's about. These are very significant chlorine photolysis "burn-off" differences based on the FC/CYA ratios.
0.075/.05 = 1.5
0.1/.05 = 2
The variances in the above from the computed values are way more than within "good enough for Government work" tolerances.

Then look at the implications for neutralization of pathogens.
CDC CT value for neutralization of E. coli is less than a minute at 1 ppm FC and 0 CYA.
At 5 FC/50 CYA 0.1 Ratio it would take about 12 times as long.
At 3.75 FC/50 CYA 0.075 Ratio it would take about 16.5 times as long.
At 2.5 FC/50 CYA 0.05 Ratio it would take about 26 times as long.
And E. coli is the easiest pathogen to kill.

My day-to-day operating target ratio for our two community pools is 0.075 - 0.08, which I believe to be a reasonable compromise position.

 

Enjoying this content?

Support TFP with a donation.

Give Support
You must log in or register to reply here.
Thread Status
Hello , This thread has been inactive for over 60 days. New postings here are unlikely to be seen or responded to by other members. For better visibility, consider Starting A New Thread.
Top Bottom
Back
Jump to latest Follow Reply