Adding Calcium = 5 ppm FC Drop

ntpd

Active member
Apr 27, 2020
37
NJ
Hi
My CH was low, so as per Pool Math I added 12 lbs of Calcium Chloride. Prior to doing so I tested my FC and it was high at 6.0.

I have a WaterGuru which just alerted me my FC is at 0.8 ppm. I tested again with my Taylor and it is in fact ~1 ppm. What happened? And any tips on what to do? It has only been 7 hours.

My SWCG had not been adjusted and seems fine.
 

ntpd

Active member
Apr 27, 2020
37
NJ
If it helps to know at all, CC is 0 and the water is crystal clear. There’s some debris in the pool (just from wind blowing leaves, etc off trees).
 

Richard320

TFP Expert
LifeTime Supporter
Jan 6, 2010
23,930
San Dimas, CA (LA County)
What kind of Calcium Increaser?

I've been suspicious of some of the pelletized icemelt. I suspect it uses something like cornstarch as the binder. Could be that reacting and using up the chlorine.
 

ntpd

Active member
Apr 27, 2020
37
NJ
What kind of Calcium Increaser?

I've been suspicious of some of the pelletized icemelt. I suspect it uses something like cornstarch as the binder. Could be that reacting and using up the chlorine.
GLB Calcium Hardness Up
 

mgtfp

Bronze Supporter
Mar 5, 2020
477
Melbourne, Australia
I just had a look into your PoolmMath logs. Looks like your SWG is on 10%, that sounds quite low to me. I am wondering if that is sufficient to justify the lower saltwater FC levels, as you don't get much of the superchlorination effect within the cell. You were at times below the min for freshwater pools,maybe you are just about to have an algae bloom which might explain the FC drop.

Did you add pure calcium chloride? Is there a chance that there was something in there that used up your chlorine?

Edit: Just saw that you already answered my second question while I was typing over my breakfast.
 

ntpd

Active member
Apr 27, 2020
37
NJ
I just had a look into your PoolmMath logs. Looks like your SWG is on 10%, that sounds quite low to me. I am wondering if that is sufficient to justify the lower saltwater FC levels, as you don't get much of the superchlorination effect within the cell. You were at times below the min for freshwater pools,maybe you are just about to have an algae bloom which might explain the FC drop.

Did you add pure calcium chloride? Is there a chance that there was something in there that used up your chlorine?
I hadn’t updated my log for my SWCG. It’s been running at 33% for about a week.

I should also add I just looked at water guru. I received an alert at 1245pm that FC dropped to 1.0 PPM. That’s 2.5 hours after I tested at 6.0 and added the Calcium Chloride.

According to the label it’s 94% Calcium Chloride
 

mgtfp

Bronze Supporter
Mar 5, 2020
477
Melbourne, Australia
And the FC=6 measurement was a FAS-DPD test, not a WaterGuru test?

As far as I understand, WaterGuru uses an optical FC measurement method. With the water getting a bit cloudy when adding calcium chloride, I could understand an interference with WaterGuru. But you confirmed the low reading later with FAS-DPD, so that doesn't seem to explain it.

My best bet still is that there was some sort of bio load. You mentioned that it was a very rainy day, so maybe that introduced some contamination. Or it did come with the 6% of added material from the hardness increaser, even though that does sound unlikely to me, given that you were using an actual pool product. I'd be more suspicious with an ice melt product where the manufacturer doesn't really care about a potential bio load for a pool (I have never tried an ice melt, they are hard to source here, frozen roads and footpaths are not really an issue in Melbourne).

Is there a chance that you are underestimating your CYA?
 

ntpd

Active member
Apr 27, 2020
37
NJ
Yes FAS DPD test. I just use waterguru for fun / spot check.

My CYA is definitely 60-70. I had it tested independently and it came in at 58 prior to some minor adjustments.
 

mgtfp

Bronze Supporter
Mar 5, 2020
477
Melbourne, Australia
Looks like the waterguru worked out well here and gave you an early warning about low FC and might have helped to prevent an algae bloom just in time. Just out of curiosity: How often does water guru test your water? Was it just luck that the test was done so close after your last manual measurement?

If I understood correctly, then the timing of events was: You tested your water at FC=6 and added calcium chloride. 2.5 hours after that, waterguru tested FC=0.8, which you confirmed by FAS-DPD another 4.5 hours later (7 hours after your FC=6 test).

The drop of 5ppm in just 2.5 hours does sound extreme, especially with CYA in the water and no direct sun on that day. I am still tending that there might be an interference of the optical waterguru test with the cloudiness of the water after adding calcium chloride. There might also have been a locally increased salt concentration after adding calcium chloride. Waterguru seems to limit the use in their spec to salt concentrations below 5000ppm - not sure whether that's due to corrosion concerns (basically a loophole for them to get out of warranty claims) or an actual interference with the testing principle.

And after 7 hours there might have been the real FC drop due to a coincidental bio load. And you were just lucky that waterguru accidentally picked that up so early and made you retest. But I am just guesstimating here.

I have never heard of a real chemical interference of calcium chloride with FC. Just last week, I increased my CH after heavy rains to make sure my CSI doesn't drop too low with the colder water temperatures in winter, and I didn't see any influence on my FC.
 

ntpd

Active member
Apr 27, 2020
37
NJ
Yeah water guru alerted me to do a real test.

WG tests at the same time each day. You can also manually ask it to test additional times. I was working when it told me 0.8, and assumed it was wrong due to the extreme change and the fact I just added a bunch of chemicals.

I ran it again after work and got the same result. At that point I confirmed with FAS DPD.

I bumped up my SWG and got it up to 1.9 and it’s holding. Adding some bleach as well to get it up, just to be safe. Expecting a lot more rain overnight so I’m going to bring a sample inside to test in the am as well.
 
Last edited:

ntpd

Active member
Apr 27, 2020
37
NJ
Update

pulled a sample of water last night and left it inside to test OCLT (it was due to rain, so I didn’t want that to affect anything) and I didn’t lose any FC.

Additionally, I ran my pump and SWG overnight and I’m now back to 3 ppm (I expected to be in this neighborhood). I will check again half way through the day after some sun.

Appreciate everyone’s help.