CYA and SWG

[tnmasterplan]

Hello all - So I have been using the TFP method for 2 yrs with bleach and have been FLOORED how well it has worked compared to years of PS chemicals. So first, I want to say THANKS for all the valuable info on this site

Now - I have finally decided to take the plunge and purchased a SWG (Pentair IC-40) to my IG gunite pool - 18k gallon.

After so many years of keeping the CYA down - I see with the SWG the CYA should come up (60-80). My question is why is a higher CYA needed in SWG than in Bleach method?

2nd question - I have added Clorox brand CYA 100% (approx 3lbs) and my test kit still shows no rise - any ideas?

 

[JoyfulNoise]

You are still using the same FC/CYA ratio (more CYA means a higher target FC). The point is that the higher CYA provides more protection against FC degradation from sunlight. This allows the SWG to be run at a lower output % or total run time. SWG's have a finite chlorine production lifetime; most manufacturers quote 10,000 hours of chlorine production time as the limit (your mileage may vary). So since the cell has a finite lifetime, you want to run it as little as possible. This is also why TFP recommends getting a cell that is sized 1.5-2X larger than your pool volume as this allows the cell to run less. The incremental cost difference between an IC40 and IC60 is fairly small but one can produce more chlorine (the IC-60 produces ~ 40% more chlorine than the IC-40 but is only 30% more expensive).

The CYA test has a tolerance of +/-15ppm. You added 20ppm so you should have seen a small rise but it may not be totally perceptible. How did you perform the test? Bright outdoor lighting with your back to the sun and the tube held at waist height in your shadow? How did you add the CYA? Sock method? In front of a return or in the skimmer basket? How long has it been since you added it?

 

[pooldv]

Copied from JasonLion, Why high CYA is a good thing with a SWG

Despite what a great deal of the available literature says, higher CYA levels reduce the total amount of chlorine required to sanitize your pool. Even though a higher CYA level requires a higher FC level, the amount of chlorine you need to add each day to maintain that higher FC level is actually lower than what you would need at a lower CYA and FC level. This happens because CYA protects FC from sunlight, reducing the amount lost to sunlight enough to more than make up for the higher FC levels required.

Since less total chlorine is required at higher CYA levels, the SWG cell doesn't need to run as long each day. Since the cell in a SWG has a lifetime based primarily on how long it is on, a lower daily run time results in longer cell life.

 

[tnmasterplan]

Thanks for the info, that makes sense. I did add the CYA to the skimmer and tested today and did notice an increase in CYA. I used my Taylor test kit - tested both indoors and outdoor with sun to my back, not waist high, but probably mid stomach high