Shocking disappointment

[polyvue]

I thought I would share with this forum my first notable failure (in regard to treatment of swimming pool water, that is!)

Late last week, a routine chlorine test detected a low level of chloramines (0.2 ppm); this is a rare occurrence in my pool and I've always been able to dispense with such annoyances quickly by throwing in a 10 oz. bag of TriChlor powder (67% available chlorine) or setting my SWG to "superchlorinate" for 20-30 minutes. It works so well, I don't even bother to test the outcome until the next regularly scheduled test.

This time proved different. Some will ask themselves why I even bothered to shock for chloramines given that many test kits won't even register a reading of .2 ppm. I hope you won't think my answer lame (or arrogant) but it's simply this: Because I can.

Anyway, pride goeth before a fall. Here is a summary excerpted from a two page log I created over the weekend that details my folly. If you're tempted, feel free to read this inglorious account (Excel spreadsheet) of how my beautiful Labor Day Weekend was swim-free but not stress free, and the result of which will likely be a partial draining of my pool. Enjoy.

Summary - Though never an issue before, numerous shocks have failed to clear chloramines from the pool. First noted on Sept 3, 2009, these events unfolded following the months-long slow increase of Total Dissolved Solids (to > 5000 ppm by 8/25/09) and then several purposeful additions of CYA over the course of the summer, including two in the month of August, in the hope of raising CYA from the 40-50 range into 60-80 territory, as recommended by nearly everyone. Chlorine levels also climbed precipitously throughout the month of August, from an average of about 2 ppm held since January. CYA prior to August had been low 30-40ppm. Efforts to shock have failed. I can drain the pool or hang myself. I choose to swim again....

 

[PaulR]

With increasing CYA, I'd expect to see decreasing FC loss in daytime, therefore your FC level will climb unless you turn down the SWG. That part all makes sense. Turning down the SWG (percentage and/or runtime) will extend the cell life.

The prideful part seems to be the insistence on zero CC.
--paulr

 

[chem geek]

A Combined Chlorine (CC) reading of 0.5 ppm implies that you are using a 10 ml water sample and 0.5 ppm does NOT mean 0.5 ppm but rather <= 0.5 ppm. When one drop turns the sample from pink to clear, you don't know if the CC is 0.1 or 0.5.

If you use a 25 ml sample, then you can get a resolution of 0.2 ppm for each drop. In my own pool, I typically get a slight pink for the CC test, but I use a 25 ml sample and one drop almost always gets rid of the pink meaning the CC is <= 0.2 ppm. It is unrealistic to target a CC of zero and also unnecessary.

Also note that since the CC test comes after the FC test using the same sample, if the last drop used in the FC test extinguished a very faint pink, then there is a fractional drop "excess" of FAS-DPD titrating reagent in the test sample such that the CC test won't show any pink. If on the other hand the FC test last drop is extinguishing a stronger pink to just barely clear, then the CC test will be more sensitive. The accuracy, therefore, can't be better than that of one drop and certainly using a 10 ml sample just increases the measurement error to +/- 0.5 ppm vs. +/- 0.2 ppm for a 25 ml sample size.

As for the CYA, note that for every 10 ppm FC added by Trichlor, it also increases CYA by 6 ppm.

 

[polyvue]

if the last drop used in the FC test extinguished a very faint pink, then there is a fractional drop "excess" of FAS-DPD titrating reagent in the test sample such that the CC test won't show any pink.

But, of course, it did. Why didn't this occur to me? It's always faint. If I have any doubt at all, I add another drop. That's the excess that's been leading me to think, rather uncritically, that I never have chloramines. My error resulted from the nature of the test and my finicky insistence to be right on the nose.

It is unrealistic to target a CC of zero and also unnecessary.

My enthusiasm for extinguishing the very last molecule of chloramine was driven no doubt by some sense of the virtue of "purity" (Lake Tahoe's mesmerizing beauty circa 1910) and the grecian swimming pool "ideal" (thinking of the ever-sparkling waters of the Neptune pool at Hearst's castle.) In this case I admit that the enemy of the good (enough) is my foolish drive to perfection.

Many thanks to PaulR and Richard for their perceptive and diagnostically pertinent comments.

- Greg