Originally Posted by Catanzaro 
Is there a testing error when the FC becomes that high with the TF-100? I believe in the past I read the higher you go on the ladder, the probability of testing error increases. With that being said (if accurate), does it make sense to allow the FC to drop to around 10 and then perform the OCLT test? Please see comment below as I would like understand this better. For me, luckily I found TFP early enough and have never had any issues with passing the overnight test.
The official accuracy for the Taylor tests, and the TF-100 is based on Taylor reagents, is +/- 1 drop or 10% of the reading whichever is greater. This is for absolute accuracy. The OCLT is more about relative accuracy since one uses the same reagents for measurements at night and in the morning. So it's more about how accurately you can measure the sample size and create consistent drop sizes. You might be able to do better than 10%, but probably not better than 5%. With a 33 ppm FC and a 5% error that would be 1.65 ppm FC error. For sample size, 5% of 10 ml is 0.5 ml which is half way between two lines on the tube when measuring the meniscus. It would be hard to do any better than that. Also, when squeezing the FAS-DPD drops, one would need to create drops that are consistent (on average) within 5%. Actually, both of these would need to be better than 5% for their combined error to be less than 5%.
So you can see that having a 1 ppm FC criteria for OCLT at high FC levels doesn't make sense. This is why it should never be done with a high yellow/mustard algae SLAM and for a regular SLAM it is a questionable criteria when the CYA is higher since the FC would then be higher. If you are very careful with measurements, then at a 33 ppm FC I'd call a 2 ppm FC or lower drop "good" for passing the OCLT, especially if one is bouncing around this sometimes lower or no loss or even gaining in FC (indicative of the limits of testing). If one wants further verification assuming other SLAM criteria are met, then lowering the FC to more normal levels closer to 10 ppm FC (or certainly not higher than 20 ppm) would be reasonable.
Algae should not come back at a half-SLAM level. The worst that could happen is that you find there's still a chlorine demand that has not been exhausted and you need to go back up to try and finish it off.
Originally Posted by Crackerjack4u
I read a really good article on here the other day that I'll try to relocate for you, if I can, that explains that even with the margin of error at the higher shock levels, if the Slam is complete, the overnight FC loss will be 1.0ppm or less, regardless of the FC level the OCLT is being checked at.
Where did you read that? It's true you don't want to end a SLAM prematurely, but if the other criteria are met with no visible algae and minimal CC then if the FC drop is low but not meeting the OCLT criteria but within test error, then it should be safe to lower the FC and for reassurance redo the OCLT at a lower FC, particularly if we are talking about SLAM FC levels of 33 ppm as in this case.
It is also not true that the actual (i.e. no measurement error) loss will always be 1 ppm or less regardless of FC level. That is only roughly true for the same FC/CYA ratio, but at a given CYA if you increase the FC level it's overnight loss will increase if for no other reason that the chlorine will oxidize the CYA faster even if there were nothing else in the pool to oxidize. This is again why one should not have a 1 ppm criteria for the yellow/mustard algae SLAM level, for example, since that is 2.3 times higher in active chlorine level and can push chlorine loss from oxidation of CYA to at least 2 ppm over 12 hours or 1.3 over 8 hours.
