Well, first things first, I'm in the trust-but-verify phase. After passing the OCLT twice before this, I'm not taking this latest Pass as gospel. I'm going to run an OCLT again at least one more night, and then watch things very closely.
Second, I honestly don't think this was an exception. I think the SLAM worked. Certainly after I cleaned out the niche behind the light and got the last of the algae, I think the second passing OCLT was accurate.
At this point, I'm attributing the extended duration due to subsequently failing OCLTs to testing error (and maybe sunscreen?? It's certainly in the pool in a high amount; I'm still undecided on its impact.)
Even though I was as diligent as I could be, my testing proficiency simply got better as I went on and realized a lot of testing mistakes I was making.
The opportunity for error at high FC with 10 ml water samples is very high. Getting the sample size right, with clean tools, and titrating carefully, is huge.
I also skipped doing OCLTs every night because I observed the high amount of chlorine loss during the day and figured (incorrectly) that of course there was still a lot of algae being fought off so why bother with an OCLT. The most frustrating part, that I think a lot of people don't realize because thankfully they don't need to SLAM very often (and now hopefully neither will I!), but
@NorCalX certainly found out as well, is that
the sun burns off huge amounts of FC at high SLAM levels. My empirical data, as well as his, confirms that losing something on the order of
5% of FC per hour is accurate. For large pools at high SLAM levels, that becomes a dishearteningly prohibitive amount of chlorine. I've gone through at least 64 gallons in a space of time that I probably should have gone through maybe 20. (3-5 gallons per day (12-20 ppm/day) over 3 weeks to hold SLAM levels compared to an expected 0.5-1 gal/day (2-4 ppm) to maintain an algae free pool at FC/CYA levels).
My main takeaways, and I what I would encourage everyone else to consider, are:
1) Most importantly, the SLAM successfully cleared what was unquestionably an algae bloom.
2) You will burn through Chlorine at prodigious rates during the day, not only due to algae, but also due to losses to the sun.
Maintaining an algae-free pool with as low a CYA as possible/recommended (in the event that SLAM levels become necessary) is critical. The FC losses during the day do not, themselves, indicate an algae problem, or a failing SLAM... just a very high cost of doing business this way, especially if you can't defeat it quickly. It also makes almost no difference (in terms of losses to the sun) whether you add the chlorine at frequent intervals, or in larger doses more infrequently. (See my previous post in this thread walking through the math on that).
3) TEST. VERY. CAREFULLY.
Clean your equipment. Be precise on water quantity. Use additional tools if necessary (e.g. a kid's medicine cup or syringe, sample sizer, etc.) to increase your accuracy and be even more precise. If you think you really should have passed an OCLT by now, and you just can't, re-examine your test methods even closer. +/-1 mL on a 10 mL sample size is 10%. That's 2 ppm at FC=20, 4 ppm at FC=40, etc. The ability to fail a test just based on inaccuracy, regardless of algae, is very real. Especially when the criteria for passing is an absolute 1 ppm, regardless of what percentage of your FC SLAM level that represents.
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