SWG with only pH probe - Ideal CYA Level Changes?

[AlexHush]

Hi all,
I've been reading that for SWG pools, the ideal CYA is 70-80. For bleach pools, ideal CYA is 40-50.

I have a SWG (Viron EQ 25) pool with pH probe only. There is no chlorine or ORP sensor.

Does this affect the CYA ideal level for me?

My concern with raising CYA to a high level is that according to the following link, achieving adequate HOCI levels for disinfection become more difficult.
HOCI/CYA: 2.0 to 4.0 Free Chlorine May Not Work • Pool Chemistry Training Institute

I also read that because of the law of diminishing returns, CYA over approx. 30 ppm only helps by a very small percentage. I read this on the following site: Five things to know about Cyanuric Acid (CYA)

Any advice and information would be appreciated.

Pool details:
13,200gal (50,000L)
Pebblecrete
Outdoor w/ no cover

 

[mknauss]

Welcome to the forum.

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

Have a read through the above thread for discussion on CYA levels, etc.

 

[AlexPowell]

What you said is correct, however you can just keep your FC at 8ppm instead. Your SWG will hold a FC level of 8 more comfortably with 70ppm of CYA than it will 4ppm with a CYA of 30

 

[AlexHush]

Thanks for that.

How can I calculate the HOCl value if I have FC level of 8 with 70ppm of CYA? Is there a formula I can use where I can adjust different variables such as pH, FC, CYA?

My aim is to have HOCl level of 0.05 ppm as well as "balance" the water according to CSI.

 

[AlexHush]

Hi all,

My aim is to constantly have a HOCl level of 0.05 ppm (to kill bacteria and algae) as well as to "balance" the water according to CSI.

I have a Pebblecrete pool with SWG (with pH probe only).

If I keep a pH level of 7.6 and CYA of 70, what Free Chlorine level do I need to achieve a HOCl level of 0.05 ppm and how do I work it out incase any variables change?

Thank you,
Alex

 

[Texas Splash]

My aim is to constantly have a HOCl level of 0.05 ppm (to kill bacteria and algae) as well as to "balance" the water according to CSI.

Hello Alex. I may be misunderstanding the question, so others will reply to further breakdown your number scenario, but I'm curious as to why/where you derived that number of 0.05 ppm for free chlorine? Here at TFP we balance the FC to the current CYA per our FC/CYA Levels. Most saltwater chlorine generator (SWG) pools do well to prevent algae at a minimum FC level of 4.5% of the CYA level as compared with the roughly 7.5% of the CYA level shown in the "Min FC" column for manually chlorinated pools. When it comes to algae prevention and sanitation, balancing refers specifically to that FC/CYA Levels to ensure the FC is balanced to the current CYA.

 

[Donldson]

Keeping your FC between 10-15% of your CYA level will keep you in that ballpark. The CYA reduces the effects of pH enough that it's not worth worrying about as long as it's within range.

 

[JoyfulNoise]

50ppb HOCl is the very lower limit for good sanitation. You’ll have a hard time maintaining that exactly and you’ll constantly be walking the line of inadequate sanitizer levels. Once an algae bloom starts, it can easily overwhelm 50ppb and then go green.

You can search any of the pool water chemistry threads here on TFP written by @chem geek where he has formulae for HOCl as a function of pH and FC. But it’s not necessary. If you follow TFP’s FC/CYA ratio recommendations or simply target approximately 7.5% as your FC/CYA ratio, then you will have an HOCl concentration roughly around 100ppb and adequate chlorine in reserve to protect against algae blooms. Trying to “skirt the line” is never a good idea for pool sanitation.

 

[mgtfp]

HOCL 0.05ppm is roughly equivalent to an FC/CYA ratio of 12.5%.

Approximate formula is here:

The following is an approximate formula you can use so long as your CYA ppm is at least 5 times your FC (the formula really falls apart terribly below a ratio of CYA/FC of 3).

(HOCl as ppm Cl2) = (FC as ppm Cl2) / ( 2.7*(ppm CYA) - 4.9*(FC as ppm Cl2) + 5 )

and if you are interested in the FC for a given HOCl (to construct the equivalent of Ben's table, for example), you can use the following which just solves for ppm FC from the above.

(FC as ppm Cl2) = ( 2.7*(ppm CYA) + 5 ) / ( 4.9 + 1/(ppm HOCl) )

The constants in the above formulas are for a pH of 7.5 (which is the only parameter that significantly affects these constants).

The two factors in the formula for different pH is given here:

The following table gives the factors on CYA and FC for the first formula as a function of pH (I got these from the end of my spreadsheet near lines 571 and 572 for factors "A" and "B"):

pH ..... CYA factor .. FC factor
7.0 ........ 2.0 .............. 3.6
7.1 ........ 2.1 .............. 3.9
7.2 ........ 2.3 .............. 4.2
7.3 ........ 2.4 .............. 4.4
7.4 ........ 2.6 .............. 4.7
7.5 ........ 2.7 .............. 4.9
7.6 ........ 2.8 .............. 5.1
7.7 ........ 2.9 .............. 5.2
7.8 ........ 2.9 .............. 5.4
7.9 ........ 3.0 .............. 5.5
8.0 ........ 3.1 .............. 5.6

The recommendations in the FC/CYA Levels (which is in reality all you need to follow, anything else is really just of academic interest) seem to be based on calculations with pH 7.8. At lower pH, you need slight less FC at the same CYA, but it's really negligible.

Exact calculations can be done with Chem Geek's Pool Equations spreadsheet (that's where you really enter geek territory):

https://www.troublefreepool.com/~richardfalk/pool/PoolEquations.xls