Chlorine/CYA chart vs 7.5% of CYA

azpooldude

Active member
May 14, 2019
30
Scottsdale, AZ
I believe I've seen it said here that FC should be 7.5% of CYA. In other words, if your CYA is 50 then FC should be 3.75 (let's say 4).

I've also been referred to the Chlorine/CYA Chart which says that for a CYA of 50, FC should be 6 to 8, with a minimum of 4.

So is the 7.5% calculation that is often mentioned a "minimum" FC but not ideal FC level?
 

Donldson

TFP Expert
LifeTime Supporter
In The Industry
Jun 12, 2009
3,950
NW Ohio
Correct, 7.5% is essentially the FC level where chlorine kills algae faster than it can reproduce in an otherwise balanced and clear pool. The chart is rounded up to a round number for simplicity. The target levels are because you will lose some FC every day to organics, sunlight, and oxidizing bather waste. While a few more ppm extra FC will not be noticed by a swimmer, even a fraction of a ppm FC below minimum will be noticed by algae.
 
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Richard320

TFP Expert
LifeTime Supporter
Jan 6, 2010
22,069
San Dimas, CA (LA County)
4 is the absolute minimum. How precise is that CYA test, anyway? 50 CYA could be anything from 40 to 60.

6-8 as the target is what you need to determine by your own testing. If you consistently lose 2 FC per day and minimum is 4, target 6. If you get a lot of use and sun and lose 4, target 8. Whatever it takes to get you through to the next time you test and dose without dipping below minimum.
 

Mrmsubeck

Member
May 6, 2013
15
Houston, TX
4 is the absolute minimum. How precise is that CYA test, anyway? 50 CYA could be anything from 40 to 60.

6-8 as the target is what you need to determine by your own testing. If you consistently lose 2 FC per day and minimum is 4, target 6. If you get a lot of use and sun and lose 4, target 8. Whatever it takes to get you through to the next time you test and dose without dipping below minimum.
Would you say that the CYA/Cl ratio would hold true if you use borates as well? I run my chlorine at around 3 in Houston with 38ppm borates and cya of around 40 and pool seems to be doing ok (digital tests from PoolWerx). I do have a spot of green algae once in a while on shelf and sometimes the bench but I admit I'm not brushing every day so I suspect that it's due to poor circulation in this areas. I just hit it with a puck and seems to take care of the situation. I do seem to be using about 4 gallons of HASA/week in my liquidator to maintain the Cl level pretty constant so I'm wondering if a higher CYA is needed or if 4 gal sounds right. I have a 42,500 gal pebbletec and run pump 24x7 on medium setting. Not sure of actula GPM/turnover. I know 24x7 is excessive but until I can get time to figure out actual pool turnover I'm sticking with it.
Thanks
 

Richard320

TFP Expert
LifeTime Supporter
Jan 6, 2010
22,069
San Dimas, CA (LA County)
Would you say that the CYA/Cl ratio would hold true if you use borates as well? I run my chlorine at around 3 in Houston with 38ppm borates and cya of around 40 and pool seems to be doing ok (digital tests from PoolWerx). I do have a spot of green algae once in a while on shelf and sometimes the bench but I admit I'm not brushing every day so I suspect that it's due to poor circulation in this areas. I just hit it with a puck and seems to take care of the situation. I do seem to be using about 4 gallons of HASA/week in my liquidator to maintain the Cl level pretty constant so I'm wondering if a higher CYA is needed or if 4 gal sounds right. I have a 42,500 gal pebbletec and run pump 24x7 on medium setting. Not sure of actula GPM/turnover. I know 24x7 is excessive but until I can get time to figure out actual pool turnover I'm sticking with it.
Thanks
No clue. Never added them.
 

Donldson

TFP Expert
LifeTime Supporter
In The Industry
Jun 12, 2009
3,950
NW Ohio
For the intents and purpose of TFP, which is to educate and teach a system that is repeatable and reliable, the FC/CYA ratio is independent of other factors. Following the chart will keep algae at bay (of course, circulation problems are another issue altogether, as you have noticed). It is simple and effective and what we have always and for the foreseeable future will teach.

In theory things like borates or phosphate removal could allow someone to run lower without an algae invasion, but this would require far more experimentation and understanding of water chemistry, not to mention would vary from pool to pool. In addition it is difficult to know if water is sanitary when the FC is below 7.5%. So in recognition of keeping things simple, safe, and repeatable we always recommend sticking to the FC/CYA Chart. Those who want to experiment do so at their own risk and any questions they might have regarding it will be met with "raise your FC".