Free Chlorine in the Deep End

Oly

Gold Supporter
Jun 28, 2017
470
Fresno, CA
#1
Why is it that when I test FC in my pool water I get different numbers depending on where I take the sample?

When testing from my filter return pipe located in the shallow end I get 5.5ppm FC and when testing 18" from the surface at the deep end I get 6.5ppm FC.

I am using a suction side pool sweep which is pulling 80% (estimate only) from the bottom of the pool and remainder from the skimmer box where my pool sweep hose is connected. I am using a Pentair adjustment valve to keep the sweep from climbing to the surface and breaking suction.

So why the difference? Any deep ideas?
 

YippeeSkippy

Mod Squad
LifeTime Supporter
Jan 17, 2012
8,994
Evans, Georgia
#2
I'd say it was circulation variables and possibly sun burn off more in the shallow end but the deeper water is shading some of that water from the UV rays?? Just like the water in the shallow end is warmer than the deeper end.

I dunno... that's my theory :geek:

Maddie :flower:
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
14,344
Tucson, AZ
#3
In a completely mixed volume of water, the chemical levels should be all the same. Pools are not very efficiently designed for mixing so there can be some stagnant areas. There is also the theory, as YS stated above, that CYA adds an anamolous screening effect to UV light - ie, not only does the fact that chlorine binds to CYA and gets protection from UV that way but also the cyanurate s themselves absorb some of the UV and down-convert it to less harmful radiation. This effect has yet to be proven as its difficult to measure UV absorbance differences at different pool depths. But it would explain why higher levels of CYA seem to reduce chlorine loss more so than just the chemical binding theory would predict.

Simplest answer - stagnant water + testing error yields a 1ppm difference.
 

domct203

Bronze Supporter
LifeTime Supporter
TFP Guide
Jun 4, 2015
3,959
CT
#4
Why is it that when I test FC in my pool water I get different numbers depending on where I take the sample?

When testing from my filter return pipe located in the shallow end I get 5.5ppm FC and when testing 18" from the surface at the deep end I get 6.5ppm FC.

I am using a suction side pool sweep which is pulling 80% (estimate only) from the bottom of the pool and remainder from the skimmer box where my pool sweep hose is connected. I am using a Pentair adjustment valve to keep the sweep from climbing to the surface and breaking suction.

So why the difference? Any deep ideas?
You are pretty much getting 6.0ppm +/- 0.5ppm, using a 10ml sample that’s +/- 1 drop. No worries there.

Between not-so perfect pool water mixing and standard testing tolerance, I would expect this.
 

Oly

Gold Supporter
Jun 28, 2017
470
Fresno, CA
#5
Yes a 10ml sample but initial color a deeper barbie pink and a two drop reagent difference to get back to clear. Pretty clear to me that the difference cannot be explained by testing error alone.

Now the mixing issue is where I believe the difference may be and it appears to be the bottom of the pool (less FC) verses the 18" deep (more FC) but I would need to actually test the deep water at depth not simply the return line.

So regarding layering of chemical additives in poorly mixed cold pool water, would you expect FC or CYA concentrations to be different at depth. In other words would these soluble materials naturally tend to accumulate at different depths if the pool water was not well mixed?