SLAM OCLT Chlorine level accuracy

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J

jay42k

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Jul 20, 2011
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Houston, TX
I'm SLAMing my pool. Although it was crystal clear, I noticed my salt cell required higher percentage than normal for this time of year. Ran OCLT @ CYA: ~55. 6.0ppm down to 3.5ppm the next morining. Not good. Started SLAM, target 24ppm. I use the TF-100 test kit.

Question: How accurate is the Chlorine test at 24ppm vs 6ppm. I ask because the 24ppm test (48 drops) seems like it would have a lot more accuracy issues than around 6ppm (12 drops). Example: Last night I started the OCLT and did a series of baseline samples at the same time and had the following variance (in DROPS): 49, 51, 47, 47, 49. I use a mixer. I have the bottle vertical. Let the drops "hang" before going in, about 1 second intervals, fresh chemicals, etc. Is this variance normal? For an OCLT, the result needs to be within 2 drops, but my variance is larger than that (not repeatable).

I'm sure if I was doing this same test at 6ppm, I bet my variance for 5 tests simultaneously would all be within 1 drop.

Right now OCLT continues as this morning, my result was 44 & 44 drops, so definitively different than last night. However, as I get closer to being "done" with the OCLT, I want to "know" that I'm done, not guessing due to variance.

Please advise
Jay
 
Jay, the higher the FC the more chance for some variance, that is true. There's a thread out there somewhere on it. In any case, you did the right thing to do an Overnight Chlorine Loss Test when something seemed off. I'm assuming you grab your water sample from the same location, depth, and when the water is well mixed, so that shouldn't be an issue. It all boils down to the test itself. That many drops and it's easy to see a larger variance. If you are concerned at some point that your high FC level is contributing to a larger than normal variance, you could let it fall a bit one evening. Maybe about halfway from your SLAM/shock level, just so you could compare the results. I wouldn't let it go all the way down to your regular FC level of 5-6 in case something is still brewing, but if you tried somewhere in the 24 range you should be able to receive a fairly consistent OCLT result.
 
In reality, one could do the OCLT at any level. The point of the test is to determine what the loss rate is when UV light is not a factor. So you are looking at organic contamination and biological contamination. Trying to achieve a <1ppm loss rate with an FC concentration above 20ppm is very difficult because there are decomposition reaction of chlorine that are concentration dependent and so at high concentration you’ll get faster decomposition.

Here’s what I do - get the FC to 10ppm and then do an OCLT. If the loss is less than 1ppm, then I call that good. Anything more than 1ppm, then it’s back up to shock level for 24 hours and I test again. A 10% loss of FC in under 12 hours is large enough to be a good measure of good versus bad. When I do it this way, I typically pass OCLT with about a 0.2ppm drop overnight (so a 2% loss of FC). That’s very clean water in my opinion.
 
JoyfulNoise said:
Here’s what I do - get the FC to 10ppm and then do an OCLT. If the loss is less than 1ppm, then I call that good. Anything more than 1ppm, then it’s back up to shock level for 24 hours and I test again. A 10% loss of FC in under 12 hours is large enough to be a good measure of good versus bad. When I do it this way, I typically pass OCLT with about a 0.2ppm drop overnight (so a 2% loss of FC). That’s very clean water in my opinion.
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That then implies 2ppm loss at 20ppm level ... So if my target is 48 drops can I safely assume 4 drop margin? I'm not trying to rush this but I also don't want to be chasing something unachievable due to the higher ppm chlorine.
 
jay42k said:
That then implies 2ppm loss at 20ppm level ... So if my target is 48 drops can I safely assume 4 drop margin? I'm not trying to rush this but I also don't want to be chasing something unachievable due to the higher ppm chlorine.
Click to expand...

There is no exact way to know if it scales line that...all I’m telling you is what I do. My pool may be very different than yours. So, in some sense, you have to just give it try at whatever level is easy to achieve and see what happens. You’ve said your pool is very clear and so if it were to cloud up again, you’ll know right away. You also said that your still seeing a large gap in the drop count between nightfall and day break, so that suggests that you’re not out of the woods yet.

The OCLT is not some exact science out of chemistry text book. It’s a guide at best and it requires you, the pool owner, to understand your own pool. This is why self-testing of your pool water and consistent testing is stressed so heavily - only you will know exactly how your pool behaves and only you will care about it.
 
What you’re seeing is a huge reason we say the user can see a 1ppm drop overnight and still pass the OCLT. In theory you should have a zero FC loss but there’s a 0.5 ppm “error” for the kit from Taylor plus a 0.5ppm “user error” built in.

IMHO if you’re still seeing a large change I’d keep slamming.
 
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