FC dropped from 5.2 to .4 in 8 hours

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R

Randy698

Active member
May 12, 2022
44
Downingtown Pennsylvania
Pool Size
18500
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Jandy Truclear / Ei
Tested my water this morning at 9:00am and FC was 5.2. Had 3 kids and two adults in the pool for 3 hours or so. Tested water again at 5:00 and it was .4. Tested it again to be sure and again it was .4.

I did add some stabilizer, calcium carbonate, and salt this morning as well.

No clue why the big drop as I’ve had more people in the pool for longer without such a huge decrease.

Any thoughts?

I’ll try to attach my numbers from Pool Math

==========================================
Barcelona
------------------------------------------
Build Type: Fiberglass
Volume: 17500 gallons
------------------------------------------
Latest Test Result Summary:
FC: 5.2 (8 hours ago)
CC: 0.2 (8 hours ago)
pH: 7.6 (8 hours ago)
TA: 60 (8 hours ago)
CH: 250 (8 hours ago)
CYA: 50 (8 hours ago)
SALT: 2600 (8 hours ago)
TEMPERATURE: 80° (8 hours ago)
CSI: -0.44 (8 hours ago)
==========================================
------------------------------------------
Chemical Addition 08-09-2023 @ 11:08 AM
------------------------------------------
+ 1 Pounds of DryStabilizer

------------------------------------------
Chemical Addition 08-09-2023 @ 11:07 AM
------------------------------------------
+ 40 Pounds of Salt

------------------------------------------
Chemical Addition 08-09-2023 @ 09:35 AM
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+ 8 Pounds of CalciumChloride

------------------------------------------
Test Results 08-09-2023 @ 09:11 AM
------------------------------------------
Free Chlorine: 5.2
Combined Chlorine: 0.2
pH: 7.6
Total Alkalinity: 60
Calcium Hardness: 250
CYA: 50
Salt: 2600
Temperature: 80°F
CSI: -0.44

------------------------------------------
Test Results 08-07-2023 @ 11:17 AM
------------------------------------------
Free Chlorine: 5.6
Combined Chlorine: 0.0
pH: 7.6
Salt: 2600
CSI: -0.30

------------------------------------------
Test Results 08-05-2023 @ 12:53 PM
------------------------------------------
Free Chlorine: 5.5
Combined Chlorine: 0.0
pH: 7.8
CSI: -0.11

------------------------------------------
Test Results 08-03-2023 @ 11:34 AM
------------------------------------------
Free Chlorine: 5.0
Combined Chlorine: 0.2
pH: 7.6
Total Alkalinity: 60
CSI: -0.30
 
  • Wow
Reactions: plnewb
What's your runtime and % ? Your swg makes about 6 ppm per 24 hours in a perfect situation.

Targeting 5ppm leaves very little room for unplanned events such as a particularly high UV day or a big storm. I've caught my SWG shutting off for 12 hours 5 times this season after a big rain that hit during the cells self check.

Overnight Chlorine Loss Test to rule out algae and then we can blame the kids. :ROFLMAO:
 
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I run the pump and SWG 16 hours a day at 70% and was trying to keep a minimum of 5ppm. I did recently reduce it to 60% two days ago or so.

That’s just a crazy drop of chlorine.

I’ll do an OCLT tonight.
 
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Randy698 said:
I run the pump and SWG 16 hours a day at 70% and was trying to keep a minimum of 5ppm. I did recently reduce it to 60% two days ago or so.

That’s just a crazy drop of chlorine.

I’ll do an OCLT tonight.
Click to expand...
Some people have reported that adding calcium seems to drop the chlorine level, but there doesn’t seem to be any explanation for why. Maybe contaminated calcium?
 
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Randy698 said:
I did recently reduce it to 60% two days ago or so.
Click to expand...
16 hours at 60% makes 2.5 FC with a truclear in 17500 gallons.


Use effects of adding in the upper left menu of poolmath. Select swg from the scroll menu. Your cell produces .92 lbs of chlorine per 24 hours. Then you can play with hours and % to see what it'll produce for you.

Regardless of how this plays out, I highly suggest you raise your target to 8. Many of us 'run hot' and it affords us more time to catch a possible problem.

swcg_chart.jpg


Also as a possible cause, I'd like to suggest option 4, none of the above. Many of us also realize our CYA dipped to low when we see the sudden FC loss. The party may just be a coincidence an the day the FC was going to tank either way.

The OCLT is step 1. If you pass and the FC drops again tomorrow with no party/calcium add, it's the CYA. If you pass and the FC holds tomorrow, it was the party/calcium.
 
The calcium chloride for sure crashed your FC, no doubt, especially adding 8 lbs of it. A common phenomenon. Add liquid chlorine to bring it back up.
 
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revitup said:
The calcium chloride for sure crashed your FC, no doubt, especially adding 8 lbs of it. A common phenomenon. Add liquid chlorine to bring it back up.
Click to expand...
I was reading other posts on here about people who said the same thing.

I added some liquid chlorine and will still do an OCLT but thinking the calcium is the culprit.
 
plnewb said:
You have forgotten more than I know but is there a specific reason why it has nearly doubled?
Click to expand...

Yep - because I deliberately raised it! LOL

It took me a few weeks to re-plumb the pool how I wanted it, so after I lowered water level to install new skimmer and return, I didn’t bump up the CYA and just used liquid chlorine during that time. Once I got SWCG back online, I boosted my CYA as well.

I won‘t mess with it again until spring when water starts to warm up, then I will see where the CYA is and adjust accordingly.
 
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