Calcium chloride and chlorine usage

[barefooting]

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.

 

[JasonLion]

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.

 

[barefooting]

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.

 

[TableRock]

Bumping this topic. My CH was 250, and Pool Math instructed me to add 150 lbs of calcium chloride. My FC was 7.5. I added 125 lbs 3 days ago, and my FC went to 1.0 the next day. I added 6 gallons of 12.5% bleach, and tested the next day to find FC was 0. I then added 9.5 gallons of 12.5% bleach. (Which is what Pool math instructed). Water is crystal clear, and my CH went up to 450.

 

[pgershon]

I had something similar happen twice. My Diamondbrite finish was deteriorating and I decided to research why. Bought a testing kit and found CH at 70 (and low in fill water too). Phased out the pool service company and added calcium- first to about 200. My FC dropped from 5 to zero. Ultimately I decided to SLAM pool for 6 days. As I was bringing FC down to 8, I decided to up the CH to 375-400 (my target). Suddenly the FC dropped again to 1.5 and I added 1.5 gallons of liquid 10% chlorine to my 30,000 gal pool. Been pretty stable since. Water crystal clear throughout, but stains been forming on surface (I suspect metals).

What I don’t know is if there was algae the first time. Coming out of SLAM I’m pretty sure no algae the more recent time. But it’s possible that the sun was the cause as well. My SWG was at 50% the day FC dropped, and sun was strong with no clouds for 8 hours on pool. Been much less sun since, SWG at 100%, and slight FC increases daily since. Now at 8% a week post drop.

so could be contaminated CaCl? Could be sun and lower SWG output, or some cross reaction

 

[TableRock]

FC has leveled for me back at 7.5, which is where I want it. That was an interesting 3 days. My pool is in direct sunlight 14+ hours a day, so I have to deal with that. But I wonder if I inadvertently contaminated the calcium chloride, I used a 5 gallon bucket to dissolve the Calcium in the bucket with pool water and a drill agitator, before dumping it in the pool. (It took ~10 iterations of this to add the full 125lbs) I do this because I wanted to make sure the calcium was dissolved before putting in the pool. The bucket and/or the drill bit/agitator may have had some residue on them. Who knows.

 

[pgershon]

I used two empty 5 gallon buckets from my calcium purchases to dissolve the calcium chloride. I put in about 1 bucket worth at a time. My FC has been stable since I added the calcium a week ago, but we have not had any bright and sunny days since. I am paying careful attention on the next sunny day to see if the FC falls again. I have CYA=70 and FC running around 7 with my SWG at 100% and pump running 15 hours. Temperature of the water ranges 79-84 this time of year.

I see no reports of sun causing FC to drop by 6 PPM more than drop on non sunny day (especially with good CYA) so I really believe it was teh calcium addition process involved in FC loss.