Chlorine settling to the bottom?

toadlife

Well-known member
May 9, 2009
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Lemoore, CA
#1
So I pulled out my Pool Health spreadsheet after getting my shiny new FAS-DPD test in the mail a couple of days ago, and got some perplexing results. A graph and the corresponding measurements...




I brought my Chlorine up to 10.8 and the next morning at 9:45AM it was still 10.8. After my three kids and I swam for about two hours, I measured at 3pm and it was down to 9.4. Hours later at 8pm after the sun was no longer touching the pool I measured again and got 8.6. All of this seemed normal. The next morning however, I got a reading of 7.5 which seemed way too low. The CC reading was still almost zero so I went ahead of left it alone. After a whole day of 107 degree heat with intense sun and three hours of five people swimming, I measured again and got the exact same reading I got in the morning - 7.5.

At first I had no idea what the heck could have caused these readings, but then I remembered that I accidentally left the pump in the "off" mode last night which caused it to miss its normal 9PM to 9AM scheduled run. With the water not being circulated overnight, could the chlorine have settled to the bottom and caused the 9:45AM reading to be abnormally low?
 

lightingguy

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Jun 18, 2010
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Glendale, CA
#2
You are probably monitoring your pool a bit too closely - although I'm a little jealous :) The answer to your question probably involves a much larger data set before you can accurately infer that you have a problem at this resolution.

1 technical question - Your FC numbers include measuring 10.8ppm, 7.7ppm and 7.5ppm

Are you really using a 50ml pool water sample size to generate these numbers at a 0.1ppm resolution?
 

toadlife

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May 9, 2009
58
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Lemoore, CA
#3
lightingguy said:
You are probably monitoring your pool a bit too closely - although I'm a little jealous :) The answer to your question probably involves a much larger data set before you can accurately infer that you have a problem at this resolution.

1 technical question - Your FC numbers include measuring 10.8ppm, 7.7ppm and 7.5ppm

Are you really using a 50ml pool water sample size to generate these numbers at a 0.1ppm resolution?
My first several measurements were with a 25ml sample. I switched to a 10ml sample after the novelty wore off and I realized that I would run out of reagent in no time at that rate.

On the test that shows 7.5fc and 7.7cc I think I used the 10ml sample and made a guess that the cc was only 0.2 due to the color being extremely close to clear after adding the second reagent.

As for monitoring my pool to closely...bahh!! UNPOSSIBLE! :-D

Actually I use my pool health chart at the beginning of the season to get a feel for exactly how much chlorine I need to add and go by the results rest of summer. I'm using a higher CYA level this season, so I'm trying to see how many days I can go without adding chlorine.
 

duraleigh

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#4
Virtually any test you perform on pool water should be done with the water thoroughly mixed. Chlorine doesn't "settle", nor do any of the other parameters, but unmixed pool water will give uneven results.
 

toadlife

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May 9, 2009
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Lemoore, CA
#5
duraleigh said:
Virtually any test you perform on pool water should be done with the water thoroughly mixed. Chlorine doesn't "settle", nor do any of the other parameters, but unmixed pool water will give uneven results.
Sure enough, a test this morning before heading out for work and I gave me an expected result - 7.0 FC and 0 CC. The overnight pump schedule had been running so the water was mixed.
 

chem geek

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Mar 28, 2007
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San Rafael, CA USA
#6
Though chlorine doesn't "settle", it does outgas, albeit slowly. So if the pump is off and you measure too close to the surface, you can read an abnormally low reading that doesn't represent the true FC in the bulk pool water.