Does this make sense?

[Shoogle]

I’m feeling a little confused on the amount of chlorine my pool is going through for this SLAM session:

Important fact, my CYA is about 90ppm and will be about the same until I can do another partial drain/refill in the coming weeks.

pre SLAM my FC was 11ppm at 10 pm, 8.5ppm at 6 am, 6.5ppm at 5 pm, and 5.5 at 8 pm before I started to SLAM. Loss of 2.5ppm overnight, and an additional 3 ppm throughout the day. No chlorine added at all between 10 pm and 8 pm the following night. Total loss of 5.5ppm.

Last night, post SLAM my FC was 32 at 10 pm, and 25ppm this morning at 6:30 am. I’ve added 6 gal of 10% bleach since then to maintain ~35ppm, I just tested at 1 pm and I’m at 30ppm. Thats still a net loss of 2 ppm despite all the bleach today.

I guess I’m just curious about why, if yesterday there was still available FC, did I only lose 5.5ppm without adding bleach all day, but today I’ve seemed to have lost far more (7ppm overnight, and essentially all of the 6 gallons I’ve added since this morning)?

The only other factor is that UV was listed as “low” yesterday, and “high” today...but will that make up a loss that high (pool math says 6 gal of 10%=29ppm)?

Thanks in advance for clarifying any misunderstandings I may be having!

 

[Texas Splash]

You will lose more FC when at the SLAM level because the free chlorine is approaching a higher threshold of available stabilizer to protect it. In other words, when at your ideal (daily) range, the FC/CYA ratio is ideal for protection from the sun. When the FC goes higher, it's an easier target for oxidation from the sun. That's why in some cases we say more is not always better. But when doing a SLAM, you have no choice. You need to maintain that higher level with the understanding that some bleach will be consumed quicker.

 

[Shoogle]

Ahhhh, makes sense! My last test I didn’t lose near as much. I’m testing/adding hourly, and I’ve gone from 1-2 gallons per test, to .5 gals needed on the last test...so I’m hoping that tonight is the night it passes the OCLT. Thanks for the succinct explanation!