I just increased my calcium hardness from 150 to 200 and noticed a marked increase in chlorine usage. Chlorine actually went to zero by days end. Which has only occurred once before, early this year, when I also added calcium (pool store's Hardness Plus). Thought the first time was just a coincidence, maybe I had the beginnings of an algae bloom. But, now that I've seen it again, I'm beginning to wonder. The first time, I just raised the FC back to 5 and all was good (never shocked it). Trying that again now.

Chlorine has been above the minimum (keep it around 5, my CYA is ~40), so doubted I was fighting algae (but hard to know for sure). Full results:

FC: 5.5 before adding CH in the morning, 0.0 days end
CC: 0
pH: 7.4
TA: 90
CH: 150 before, 200 after
CYA: 40

I did a quick search, and found some similar topics:
raised-calcium-hardness-and-free-chloring-went-to-zero-t33518.html
calcium-chloride-might-contain-bromine-t5744.html

So, guess I'm wondering how many others have seen this. From the search, doesn't seem to be a common problem, but figure I'd throw it out there.
Thanks.

FC going down after adding CH is extremely rare. It is assumed that it is caused by organic contamination in the calcium being used. But it is so rare that we have never been able to pin down exactly what is happening.

Thanks for the reply. Guess I'll just keep a close eye on the FC next time I add calcium. Must be some contamination in the calcium I have. Not a big deal. Raised the FC back today and it's holding fine now.