Algae in Umbrella holder

[Jeffnhelen]

Use 28 for your target. If you lost more than 1ppm for any reason overnight, continue slam.

Will do. Read in another thread about FC/CYA ratio that anything over the SLAM amount you end up with loss to UV (not my case for OCLT) or self-extinction. Is “self-extinction” potentially a good reason for why I kept losing the same amount both nights?

Post in thread 'CYA/FC Relationship during SLAM'
CYA/FC Relationship during SLAM

 

[PoolStored]

Slam level is set to be safe for people and equipment. Same loss can be very typical. I wouldn’t read anything into it.

 

[Jeffnhelen]

Slam level is set to be safe for people and equipment. Same loss can be very typical. I wouldn’t read anything into it.

Well decided to conduct an experiment to see if I could replicate the scenario after some things were known to be true. One truth is my pool is no longer “fighting” anything. I passed the OCLT two nights in a row. Another is my CYA is now down to 50 after I refilled the drained water. PH is not a “perfect” 7.5, but close at 7.7.
To make sure the water is sanitized for this experiment, I have been bringing the pool up to SLAM level (20FC) in the mornings and letting it use that up during the day before I test again in the evening. Again, based on the current CYA reading of 50 I should be SLAMing at 20FC. However, as I was 20CYA over in my early attempts I decided to go with that same amount of error and SLAM based on 70CYA which is 28FC. So at 10pm last night I added enough LC to get me to 28FC instead of 20FC I should have brought it up too. Tested this morning before sunrise at 6am and it resulted in a reading of exactly 20FC.
This wasn’t meant to be a scientific conclusion that even away from the sunlight and devoid of any sanitizing needs, chlorine will still be lost if there is not enough CYA to bond to. However, I believe the consistent results show something interesting is going on. Three separate events should move beyond coincidence and closer to evidence. I know there is a large and detailed thread, that I understood 1% of, going deep into chemical relationships by chem geek:
Thread 'Pool Water Chemistry'
Pool Water Chemistry

 

[Jeffnhelen]

With all that aside, I don’t think I have noticed this being advised, but maybe we should suggest to verify CYA levels again if only the OCLT test is failing out of the three SLAM tests. I believe the CYA “black dot” test is very picky and difficult to determine exactness compared to a drop-test. For daily chlorine additions, it’s not necessarily a big deal if you are 10 off in the CYA reading, but for SLAMing it seems it could prevent you from completing a successful OCLT and wasting a lot of chemical. Just my thought anyway after going through 20 gallons of bleach