Higher than expected daytime FC losses?

[jblizzle]

Got a little lazy the last month or so and neglected the pool, since I was planning to replace a lot of the water anyway (I won a drawing to get free water 1Q 2013) to lower the CH. Anyway, the FC must have just dropped to zero and water was starting to get a bit cloudy when I started to drain.

Filled it back up and got the CYA 25-30ppm (pH 7.5, TA 80, CH 350)
Notes: I have not shown any CC throughout and the pump as been on high 24/7 and solar is running during the day.

Mon
7pm: Added 2 182 ox bottles of 6% measured FC of 8.5ppm ... I expected each bottle to add ~4.5ppm, so this seemed reasonable

Tues
7am: FC 6.5 ... clearly something in the water still, failed OCLT, added 1 bottle of the 8.25% should take me to ~11ppm
9pm: FC 6 ... hmmm seemed to loose ~5ppm during the day, added 1 bottle of the 6% bleach, should take me to 10.5ppm
10pm: FC 10.5 ... looks good

Wed
6:30am: FC 9.5 ... might have passed the OCLT, water looks great, had wife add 1/2 bottle of 8.25% @ 7AM and 1PM
9:30pm: FC 7 ... seemed to loose ~7ppm during the day, added 1 bottle 8.25%

Thurs
10am: FC 9 ... did not test after bleach addition last night & sun was out, so not true OCLT, verified CYA ~25ppm, pH 7.8, added 1 bottle 8.25% and 16oz MA
10:30pm: FC 6.5, ph 7.5 ... seemed to loose ~7ppm during the day, added 1 bottle of 8.25%

Fri
10am: FC 9 ... sun was out, so ignoring ~2ppm OCLT


I need to verify that I am actually getting ~4.5ppm rise from the 8.25% bleach and do a true OCLT tonight to see if I pass. The water is very clear and there is no debris in the pool.
I am surprised that I seem to be loosing 5-7ppm during the day which is about 50% of what I am starting at. Is that 50% loss pretty normal for the low CYA? I guess I was expecting to only loose 3-5ppm.

After the OCLT, I am going to start raising the CYA closer to 50ppm. I am thinking that maybe the CYA is a little lower than I think and that is why the daytime losses are so high. Pool looks great, so I am thinking my pseudo-shock process is complete ... OCLT should confirm.

Any thoughts or care to confirm my thinking?

 

[JasonLion]

That is not really surprising. I would expect roughly a 70% loss of chlorine over the course of a sunny day with CYA around 30 and the water around 80 degrees. If your water is colder, a 50% chlorine loss is very reasonable. And of course the CYA test is about +-15.

 

[mas985]

I think that is pretty similar to my experience. Another thing I noticed is that absolute FC loss is less at higher CYA levels than at lower CYA levels even though the FC level is higher. In other words, if you double the CYA level, the percentage loss goes down by more than 50%. So less chlorine is used to maintain the pool. I think everyone can benefit running at higher CYA levels (not just SWG owners) unless they are prone to algae outbreaks then it might be more difficult to kill.

 

[jblizzle]

Water is still about 70 degrees.

I had just read some older posts that seemed to indicate a lower FC loss with the lower CYA; although I thought more recent discussions trended toward the % loss per day ... which at 50% would be in the "normal" range ... as you state.

Do you recall a post or know where there may be a summary of the expect % loss for given CYA levels? Would be good to have for reference.

Guess I just need to confirm the OCLT and then get that CYA back up so I do not have to keep adding so much FC.

Thanks for the sanity check.

 

[chem geek]

I had just read some older posts that seemed to indicate a lower FC loss with the lower CYA; although I thought more recent discussions trended toward the % loss per day ... which at 50% would be in the "normal" range ... as you state.

I don't know what posts you are referring to since I don't think we've ever said that there is a lower FC loss with lower CYA. As for a rough guide of what you might expect, you can look at the table in this post, but realize that every pool is different (mostly due to differing sunlight exposure) and this just gives you a rough idea of what may be going on. It shows how the % chlorine loss per day drops as the CYA increases, possibly due to some CYA shielding effect (the chlorine bound to CYA by itself doesn't fully account for this effect). It also shows how the absolute FC loss is lower in spite of having proportionately higher FC at higher CYA to keep the FC/CYA ratio, and hence the active chlorine level, constant.

 

[jblizzle]

Not a lower FC at lower CYA per se. It just said a 3-5 ppm loss at a CYA around 30ppm iirc. But that likely was assuming normal initial FC levels. It just did not mention a % loss.

A little odd yesterday.

Passed the OCLT from 13.5 to 13. But then dropped all the way to 4 over the day which was more than dropping to about 6 the previous few days.

Anyway, starting to get the CYA up now and using a lower normal FC level which should lower the daily losses.

Posted from my Droid with Tapatalk ... sorry if my response is short