Normal Chlorine loss question

mikemo

Member
Jun 28, 2010
18
Palm Harbor, FL
Greeting everyone. First let me say thanks in advance for this great resource. I've been reading for the past week and have taken control of my pool back from the Pool Store.

The pool is 7 years old, 10k gallons pebble-tec surface with a DE filter and a Goldline T-15 cell SWCG. This time of year, at least for the past 5 years, I struggled with keeping chlorine in the pool. I realize now that I never would get all the organics out of the water, which would then consume all the chlorine the salt cell would produce. The typical process would be
1) take a sample to the pool store
2) they measure zero chlorine and tell me to shock
3) put salt system in "super chlorinate" and run overnight
4) water looks better
5) 3 or 4 days later, take a water sample back in and measure zero chlorine
6) repeat until October :)

I have come to the conclusion that my pool was never properly shocked, since I would never get the FC level high enough or keep it long enough to remove the organics. I have the FAS-DPD test kit (thanks tftestkits.net) and can now measure my FC properly. This past week I've been trying to measure levels above 5ppm by diluting the sample with distilled water and multiplying the result (believe it or not it worked within 2 or 3 ppm). I ran an overnight test last night and went from FC 8.5ppm CC 0.5ppm to FC 5.5ppm and CC 0.5ppm this morning. It looks like I'm not done shocking, so I added liquid to get back to 17ppm. Depending on which pool store you believe, my CYA is 40 or 60ppm.

Back to my original question. How much FC loss is normal during a sunny day in Florida? If I turn my salt cell off and start with 17ppm, at my CYA level, how much should be left in the evening?

Thanks again
Mike M.
 

mikemo

Member
Jun 28, 2010
18
Palm Harbor, FL
Thank you for the reply. We had rain last night, so I assume it invalidates my overnight drop test. No matter, I'll keep the level up to shock and try again. It's funny, but in the last 3 days that I've been trying to get my pool in order we've had more long, consistant rain than we've had all summer... Go figure! We did have enough rain that my salt system is reporting 2600ppm and complaining about low salt, but it is still generating.

I'll post results as soon as I can run a good test. Hopefully tomorrow.
Thanks again
Mike M.
 

johnd13

LifeTime Supporter
Jun 23, 2009
67
I may be wrong, but if your CYA is really 60, then you need to get your FC up to 24 to shock the pool. Taking it only up to 17 is just throwing money away...
 

mikemo

Member
Jun 28, 2010
18
Palm Harbor, FL
I'm still fighting with the rain at night messing up my drop tests. I'll bring it up to 24ppm today and see what happens tonight (if the rain stops).
I need salt now too...

Mike M.
 

shacke

Platinum Supporter
LifeTime Supporter
Jul 5, 2010
134
Philadelphia
mikemo said:
I'm still fighting with the rain at night messing up my drop tests. I'll bring it up to 24ppm today and see what happens tonight (if the rain stops).
I need salt now too...

Mike M.
I am curious to know what happens overnight. My chlorine loss tends to be slow and I am raising my CYA a bit. I would think coming down from 24 would take quite a while!

Evan
 

mikemo

Member
Jun 28, 2010
18
Palm Harbor, FL
Ok, it didn't rain last night, and my FC only dropped one ppm from 10ppm to 9ppm overnight, with only a trace of CC. I turned my salt cell back on at 50% and the pump will run 12 hours today.

I will keep a close watch on my FC level to make sure I don't drop too low.

I still "really" don't know what my CYA level is. Two tests at two different pool stores late last week showed 60ppm and 40ppm. We had enough rain that my salt level dropped from 3000ppm to 2500ppm so I got my water tested again when I picked up 40lbs of salt. They reported 20ppm CYA. I don't believe it. If I lost the same percentage of CYA as salt it would only drop from 60 to 50, unless both the tests last week were way off. I just don't have any faith in these guys. My FAS/DPD test showed 17ppm and the pool store reported 3ppm. WRONG!

Anyway, I'm curious as to how much chlorine I will lose on a normal day. I'm starting today at 9ppm. If my SWCG can generate 1.45 lbs of FC in 24 hours, I figure 12 hours at 50% should yield 6oz of chlorine. According to the pool calculator that is about 5ppm for my pool. So that's about 1ppm for every 10% of the SWCG. Does this sound right, or am I thinking too hard?

I'll check the chlorine mid-afternoon and again this evening and see where I'm at.
Thanks again!
Mike M.
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,880
Silver Spring, MD
Getting incorrect test results from pool stores is extremely common. The kind of varied test results you are seeing is exactly why we always recommend that you get your own top quality test kit.

For adjusting the SWG, starting at 50% is fine. But instead of trying to calculate things, it is much better to test it out empirically. Test the FC level in the early evening, and if it is high you lower the percentage setting, if it is low you raise the percentage setting. Right at first you can make these adjustments every day or two, but as you get closer to the ideal value, you should give it several days to stabilize at the new level and then test to see where you are.

The amount of chlorine you lose in a day depends on your CYA level and your starting FC level. More CYA means less chlorine lost. More starting chlorine means more chlorine lost. Chlorine loss over the course of a sunny day is a percentage of your starting level. The percentage is determined by your CYA level and how much direct sunlight the pool gets in a day. Additional chlorine can be lost if you have lots of people swimming for a long time, but for the typical residential pool, loses due to sunlight are the dominant effect.
 

mikemo

Member
Jun 28, 2010
18
Palm Harbor, FL
Thank you all so much for your advice so far. The pool looks fantastic :cheers:

I have the chlorine under control and don't have any appreciable loss overnight. I used to run my cell between 70% and 80% and constantly had to add cal-hypo to get the chlorine up. Now I'm running at 40% and it is keeping up

I do have one more question. My best estimate of my CYA (from multiple pool-store tests) is 35ppm. My FC is between 7.5ppm and 9ppm, depending on when I test, and CC is near zero. I'm running my pump 14 hours per day, 6am to 8pm. We have family here from Connecticut and the little ones will be in the pool daily for the next 4 weeks, so I don't want to fall behind. Is my FC too high? Should I scale back on the SWG percentage? Any help will be appreciated. Here are estimates from my last test and my own testing. The pool just went into the evening shade when I took the FC numbers.

FC 7.5
CC 0
PH 7.5
TA 100
CH 300
CYA 35
Salt 3300

Thanks again!
Mike M.
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,880
Silver Spring, MD
Yes, you can cut bacl on the SWG percentage a little, which will let the FC level come down. Aim for an FC level around 4.

With a SWG, you really should bring up your CYA level to around 70. Once you do that, you will be able to turn down the SWG percentage even more.
 

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