Disappearing Chlorine?

TexasPool128

Well-known member
Apr 22, 2019
80
N. Texas
Using PoolMath app (CYA ~50, CC 0);
My FC will be 0. I'll add a gallon of 10% like it says. 2 days later it'll be 0 again. This has happened many times. So yesterday I added the regular 1gal. about noon. PoolMath says it should've taken it up to a FC of 7. I checked it last night around 9:30pm & it was down to FC 2.5. Can I really be rolling thru 7ppm of chlorine in 14 hours? Seems ludicrous!
If so, why?

Conditions: pool is in full sun, gunite, 3 months old, baja ledge running 10hrs day, single stage pump running 10 hours a day, nobody swimming.
 

frogabog

Gold Supporter
LifeTime Supporter
Jul 16, 2010
2,833
Portland, Oregon
Perhaps your problem is that you're not adding chlorine daily. The pool will lose FC every day. Dose to target at night, expect 2-4ppm daily loss to sun only. Then there's other factors such as dirt and pollen, leaves, etc. that are all organics. And swimmers. Chlorine is consumed by organics. Waiting 2 days to check FC is where you're losing all your progress.

Couple that with the fact that in losing FC below minimum, you're potentially allowing algae to bloom. If you're losing 7ppm in 14 hours chances are this is what is happening.

I'd suggest SLAMming. It sounds like you've got something a brewin. Target for 50ppm CYA is actually 8ppm FC, btw.
 
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Donldson

TFP Expert
LifeTime Supporter
In The Industry
Jun 12, 2009
3,952
NW Ohio
You have algae. As you saw, your FC has been dropping to zero within a day and then has sat there for over a day with no chlorine. That's a perfect situation for algae to grow.

The next step is to perform the SLAM Process. After completing that fully you are either going to have to start checking every day or dosing more than a gallon if you want to stretch it to every other day. The target FC is assuming daily testing and dosing. Your FC should never fall below 4 (given your CYA of 50 per the FC/CYA Chart ) so you need to add enough chlorine to make sure that it is 4 or greater when you next intend to test.
 
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frogabog

Gold Supporter
LifeTime Supporter
Jul 16, 2010
2,833
Portland, Oregon
The problem with dosing every other day is that the pool will lose more FC during the day to sun. Plan for about 50% loss to sun alone per day. More FC == more lost to sun. It's wasteful. If you dose to 8ppm, lose 4, you're at minimum 1 day in. If you dose to 15, then lose 7.5 the first day, the next day you lose 3.5, now you're below minimum. And that doesn't account for any swimmers or other organics.
 

TexasPool128

Well-known member
Apr 22, 2019
80
N. Texas
I was dosing every day (when I was on a prolonged vacation @ home) but only adding 1-2ppm each day so it seemed a bit of a waste of time.
If I had algae, wouldn't I have a CC higher than 0? Isn't CC chlorine that used up from eating whatever or chlorine that's attached to whatever it's eating?
I work 24 hour shifts. I can't add chlorine everyday as I'm not here every day. I only add with the pump running & it's not running @ 6am when I leave for work (but I could change the times on it).
Since I work 24 hours on/ 48 hours off, what would be my best course of action for keeping my chlorine levels @ a reasonable level?
 

Donldson

TFP Expert
LifeTime Supporter
In The Industry
Jun 12, 2009
3,952
NW Ohio
CC has nothing to do with algae. CC is created when chlorine reacts with another chemical, not when it kills algae.
 

TexasPool128

Well-known member
Apr 22, 2019
80
N. Texas
113362
"Best of luck with your pool. "
LOL. According to the section above titled "You are done when), looks like I'm 2/3's of the way done w/the SLAM!
I read the SLAM link you posted already, hence my question above ("My pool is crystal clear and CC is 0. Isn’t 2 of the 3 signs I’d need to SLAM is dirty looking water and CC >.05? "). Before I start the PITA process of SLAMming, shouldn't your advice have been do an OCLT? Once I make sure my FC is where it needs to be (~8ppm for CYA 50), if I have a loss of <1ppm, no need to SLAM.
 
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frogabog

Gold Supporter
LifeTime Supporter
Jul 16, 2010
2,833
Portland, Oregon
Water can look crystal clear but still be brewing algae. If you raise it to SLAM level and maintain SLAM level it for a day, chances are it'll go cloudy. That's a sure sign of algae. At that point you'll probably see CC in your testing. I would hazard a guess that you'll see some significant FC loss while testing regularly during your SLAM as well.

It will be a lucky day if you try this experiment and see no change in your water.

An easy way to see if you need to SLAM is to perform an Overnight Chlorine Loss Test.

To address your most recent post... You aren't at SLAM FC levels so SLAM guidelines aren't applicable yet. AND have dropped to 0 FC frequently. Going below minimum for your CYA level *ever* is a recipe for algae even if you don't notice it visually. Dropping to 0 is an algae haven. You're losing a lot of chlorine on a daily basis. You asked why. This has been answered. Something is eating it up. See above suggestions.
 

frogabog

Gold Supporter
LifeTime Supporter
Jul 16, 2010
2,833
Portland, Oregon
To clarify, if you aren't SLAMming currently, passing 2 of the criteria isn't applicable.

You have an additional problem that is applicable in the situation of clear water and <.5 CC. You're going through a lot of chlorine, more than expected. More than a pool without algae does. You will need to do an OCLT to prove this to yourself, so by all means do that.

When you do not need to SLAM, you will not lose more than 50% of your FC on a day without swimmers or other things that introduce organics. You lost over 50% in less than a full day of sun (unless noon is when sun hits your pool). That is excessive and indicative of algae. This is why we've recommended SLAM to you, in your specific situation.
 

TexasPool128

Well-known member
Apr 22, 2019
80
N. Texas
To clarify, if you aren't SLAMming currently, passing 2 of the criteria isn't applicable.

You have an additional problem that is applicable in the situation of clear water and <.5 CC. You're going through a lot of chlorine, more than expected. More than a pool without algae does. You will need to do an OCLT to prove this to yourself, so by all means do that.

When you do not need to SLAM, you will not lose more than 50% of your FC on a day without swimmers or other things that introduce organics. You lost over 50% in less than a full day of sun (unless noon is when sun hits your pool). That is excessive and indicative of algae. This is why we've recommended SLAM to you, in your specific situation.
My pool is in full sun from about 10am to 8pm, full sun, zero shade except for when the sun sinks lower then it gets partial shade from the grotto. I'm going to do an OCLT tonight. I added 1 gal of 10% chlorine @ 4pm today. @ 6:30pm it was 7ppm. I'm going to check it @ 10pm tonight then again @ 6am tomorrow morning & see where it's @. Thanks for your advice.
 
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TexasPool128

Well-known member
Apr 22, 2019
80
N. Texas
My pool is in full sun from about 10am to 8pm, full sun, zero shade except for when the sun sinks lower then it gets partial shade from the grotto. I'm going to do an OCLT tonight. I added 1 gal of 10% chlorine @ 4pm today. @ 6:30pm it was 7ppm. I'm going to check it @ 10pm tonight then again @ 6am tomorrow morning & see where it's @. Thanks for your advice.
OCLT was 1ppm. Went from 6 to 5. Still need to SLAM or just do a better job of addding chlorine every day? Thanks.