chlorine usage at different cya levels

LeeM2

Well-known member
Jul 3, 2010
96
I loose about 2-3pm per day when it's 90-95 F. My cc is 0.5.
Does it mean that I'd lose the same 2-3ppm on the same day regardless of the CYA level?

According to Chlorine / CYA Chart the difference between minimum FC and target FC is 2ppm (for cya between 30-60). Also, in order to add 3ppm I need to add the same amount of chlorine, regardless of the cya level

I see my CYA level at 30-35, while leslies stores see it at 60. I wonder if by keeping it at 7ppm instead of 5ppm I'm wasting chlorine (assuming I have 30-35, not 60)

Thanks.

edit - I use liquid chlorine only, 12.5%
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,880
Silver Spring, MD
The higher the CYA level, the less total chlorine you need to add to the pool each day. This is true even though you need to raise the FC level at higher CYA levels. If you are only losing 2 ppm from a starting FC level of 7, it is much more likely that CYA is around 60 than that it is around 30. With CYA around 30 you would expect to lose about 2/3rds of your chlorine over the course of a sunny day.

The amount of chlorine lost to sunlight goes as a percentage of the starting chlorine level. So a higher starting FC level means more chlorine lost to sunlight (all else remaining constant).
 

LeeM2

Well-known member
Jul 3, 2010
96
JasonLion said:
The higher the CYA level, the less total chlorine you need to add to the pool each day. This is true even though you need to raise the FC level at higher CYA levels. If you are only losing 2 ppm from a starting FC level of 7, it is much more likely that CYA is around 60 than that it is around 30. With CYA around 30 you would expect to lose about 2/3rds of your chlorine over the course of a sunny day.

The amount of chlorine lost to sunlight goes as a percentage of the starting chlorine level. So a higher starting FC level means more chlorine lost to sunlight (all else remaining constant).
I'm confused. With higher CYA level the starting FC level is higher. So, will I end up using more or less chlorine?
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,880
Silver Spring, MD
I said it as clearly as I know how to say it. At higher CYA level you use up less total chlorine (taking everything into account).
 

LeeM2

Well-known member
Jul 3, 2010
96
The amount of chlorine lost to sunlight goes as a percentage of the starting chlorine level. So a higher starting FC level means more chlorine lost to sunlight (all else remaining constant).

this sentence throws me off. I guess I need to go to bed now and re-read it in the morning :)

Thanks
 

zea3

Mod Squad
TFP Expert
LifeTime Supporter
Jul 10, 2009
11,065
Houston, Texas
The chlorine will remain longer (thus be effective longer) when the CYA is higher, however as the CYA increases, the volume of chlorine needed to reach the same effective level increases also. 3-7 FC at 30 CYA will sanitize as well as 5-9 FC at 70 CYA.
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,082
San Rafael, CA USA
While the amount of FC that is lost to sunlight is a percentage of the FC level, that percentage will vary depending on the CYA level. As two examples, let's use a loss of 60% per day at 40 ppm CYA vs. a loss of 20% per day at 80 ppm CYA. At the recommended FC levels for each we have

3 ppm FC at 40 ppm CYA loses 0.6*3 = 1.8 ppm
6 ppm FC at 80 ppm CYA loses 0.2*6 = 1.2 ppm

Every pool is a little different in this regard so one can always try out different CYA levels to see the effects, though unfortunately it's easy to raise the CYA level (by adding CYA or using stabilized chlorine) and much harder to lower it (by dilution). There are also additional chlorine losses not related to sunlight so the total chlorine loss is often somewhat higher than shown above and may make the difference less dramatic. For example, with additional losses of 10% of the FC level, we would have

at 40 ppm CYA: 1.8 + 0.1*3 = 2.1 ppm
at 80 ppm CYA: 1.2 + 0.1*6 = 1.8 ppm
 

Jersey Devil

Platinum Supporter
Aug 4, 2007
61
Jackson, NJ
So, since FC losses not due to sunlight are not related to CYA level, but are relatively fixed (assuming bather load and other conditions remain steady) you will see that the higher the non-sunlight loss is, the more independent of CYA level FC loss becomes. What it all comes down to is that, as has been stated many times, every pool is different. My pool is surrounded by trees and subject to nighttime critters (RIP) so I get FC chewed up by pollen, etc. Your pool is different and you need to become the expert on how your pool reacts. Although all (or most) of the advice you get on TFP is accurate, some of it is specific to individual experiences. The whole BBB thing is for you to take control by regular and accurate testing and following the basic principles outlined in the Pool School. Sounds like you are well on your way.
 

LeeM2

Well-known member
Jul 3, 2010
96
I read somewhere that free available chlorine should be between 2 to 4 ppm, and that too much chlorine can cause "corrosion of important pool components".

If my CYA is indeed 30 and not 60 (I tested it again and again...), I wonder if I can damage anything by keeping my FC between 5-7ppm?

Thanks.
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,880
Silver Spring, MD
You won't have any problems. What FC level can cause problems depends on your CYA level. Long term damage usually just starts to be detectable at a couple of times shock level for your CYA level, and that is only after months to years of exposure at that level. You have to go way above that to see any damage over days or weeks.
 

LeeM2

Well-known member
Jul 3, 2010
96
I got R-7065 to test the validity of my CYA testing and I still get around 30-35, but not even 40. I tested a few times with 7065 until I found the ideal spot where I constantly got 50ppm, so I'm assuming my testing is (and was) pretty accurate. Either Leslie's use different solution and that solution yields different results or I don't even know what to think. So, I guess I need to target 4-5ppm, but since I'm adding bleach at 9-10PM, I may just shoot for 5-6ppm, so in the morning I have over 4ppm. Does it sound right?
 

Other Threads of Interest