Rising FC Problem

pool512 · Sep 2, 2023

After a couple years getting my TFP/PoolMath methodology in good shape, I'd gotten to the point where I would test weekly, and usually need a small amount of muriatic acid weekly to lower pH. All of the sudden recently things seem to have gone sideways, and I've not been able to figure out what's going on.

The issue started with high pH (8.0+), so I added my muriatic acid (MA) per PoolMath, and checked the next day. It was back to 8.0+, so I added MA again and checked the next day. It was 7.8, so I added MA again (shooting for 7.2). The next day it was 7.6. I added MA to get to 7.2 again, and it was 7.6 again the next day. This went on again for a couple more days, and I began to notice FC rising from 10 to 11.

I bumped the SWG from 30% (where it's been all Summer) to 20%, and checked this morning to find it had gone up to 12. I bumped it down to 5%, and checked just now (about 7 hours later) and it's up to 13.

In addition to the FC issue, my CYA had fallen to 50 yesterday, so I added stabilizer to get it back to 70, but it was only at 60 this morning, so I added more stabilizer again and it's finally at 70 again. Again, all is per PoolMath.

And finally, my TA had dropped to 50 as of yesterday, so I added baking soda (per PoolMath) to get it back up to 70, but as of this morning it was only up to 60.

In sum, all of the sudden and with no changes to my pool maintenance/weather/etc. my FC is rising, pH keeps rising, and CYA/TA have dropped. I brushed, vacuumed, etc. over the week, but I haven't found anything that would cause these issues. Any ideas? Thanks!

Latest PoolMath numbers:
FC 13.0
pH 7.2
TA 60
CH 250
CYA 70
SALT 3600
TEMP 84
CSI -0.90

Final question... is it OK to swim with FC of 13.0 and CYA of 70? I know they say don't swim above FC 10, but also that they aren't considering CYA levels in that recommendation. Thanks again!

Mdragger88 · Sep 2, 2023

Not sure who “they” is but -
It is safe to swim with fc anywhere between minimum & slam level for your cya so long as you can see the bottom of the pool for swimmer safety. So up to 28ppm for cya of 70.
FC/CYA Levels

It takes 24-48 hours for cya to fully register when testing so you may end up higher than you just tested.

Ta of 60 is fine. Leave it alone.
The ph test is invalid at fc levels above 10ppm & often reads falsely high. This leads to overdosing with acid.
Don’t test / adjust ph until fc falls to 10ppm or less.

ajw22 · Sep 2, 2023

Why are you pushing your pH down to 7.2?

You don’t want your CSI below -0.6 and preferably between +0.3 to -0.3. Your low pH is causing low CSI which will etch and dissolve your plaster.

It is safe to swim up to SLAM FC level . For CYA 70 that is 28.

TA of 50 is ok. Leave your TA alone. Raising your TA with baking soda causes your pH to rise and then you chase your TA down lowering your pH to 7.2.

Run your pH at 7.6 to 7.8.

pool512 · Sep 2, 2023

Mdragger88 said:
Not sure who “they” is but -
It is safe to swim with fc anywhere between minimum & slam level for your cya so long as you can see the bottom of the pool for swimmer safety. So up to 28ppm for cya of 70.
FC/CYA Levels

It takes 24-48 hours for cya to fully register when testing so you may end up higher than you just tested.

Ta of 60 is fine. Leave it alone.
The ph test is invalid at fc levels above 10ppm & often reads falsely high. This leads to overdosing with acid.
Don’t test / adjust ph until fc falls to 10ppm or less.

Thank you! Will do.

Any idea why the FC would just start creeping up?

pool512 · Sep 2, 2023

ajw22 said:
Why are you pushing your pH down to 7.2?

You don’t want your CSI below -0.6 and preferably between +0.3 to -0.3. Your low pH is causing low CSI which will etch and dissolve your plaster.

It is safe to swim up to SLAM FC level . For CYA 70 that is 28.

TA of 50 is ok. Leave your TA alone. Raising your TA with baking soda causes your pH to rise and then you chase your TA down lowering your pH to 7.2.

Run your pH at 7.6 to 7.8.

Thank you!

I was pushing pH to 7.2 because it kept rising daily, so I figured that shooting for the bottom of the PoolMath range would be best, assuming it keeps creeping up, though as Mdragger88 pointed out (which I did not know) the pH test is not valid above FC of 10.

Mdragger88 · Sep 2, 2023

pool512 said:
Thank you! Will do.

Any idea why the FC would just start creeping up?

Lower uv day most likely. Or Maybe you checked at a different time of day than you normally do.

PoolStored · Sep 2, 2023

FC Demand.

I had a thread last month with an unexplained FC loss...like low salt caused the cell to stop working. Added salt and I was in business. Now I can't lose FC to save my soul. 10-15% was working great. This week, still sunny and consistent weather, but FC was rising with cell at 5%. Tested this...

www.troublefreepool.com

ajw22 · Sep 2, 2023

pool512 said:
Thank you!

I was pushing pH to 7.2 because it kept rising daily, so I figured that shooting for the bottom of the PoolMath range would be best, assuming it keeps creeping up, though as Mdragger88 pointed out (which I did not know) the pH test is not valid above FC of 10.

Even with a valid pH test the lower you push the pH the faster it will bounce back.

PH TA Relationship - Further Reading

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PH - Further Reading

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mgtfp · Sep 2, 2023

You have a couple of factors coming together here.

One is seasonal, as discussed in the thread that @PoolStored linked to.

Then you increased CYA, that would also reduce chlorine demand.

Then you decreased pH from above 8 to 7.2. This will have reduced the Hypochlorite Ion proportion of FC, which is more vulnerable to UV than Hypochlorous Acid, therefore reducing the FC demand. This effect is normally not that large to be noticeable while maintaining target FC and pH within smallish ranges, but it becomes more pronounced at higher FC levels and for larger pH changes.

This is actually one of the reasons why the SLAM process asks for pH reduction before increasing FC to SLAM level, as this will reduce chlorine decay while at high FC levels. As pH will be elevated after addition of a large quantity of liquid chlorine while maintaining that higher FC level, it should be reduced prior to the liquid chlorine addition (note that pH will come down again once FC is back down to normal levels after the SLAM, the complete chlorination cycle with liquid chlorine is pH neutral).

For normal FC maintenance there is not much point in making deliberate use of that effect as pH just bounces back very quickly from the low to the high sevens, and the extra efforts and amounts of MA (and likely also baking soda) required to maintain lowish pH are just not worth it for the sake of a smallish reduction in FC demand. But it's big enough for making sure that pH doesn't hover well above 8 during a SLAM (which you will be blind for with the pH drop test while above FC 10, hence the need to adjust pH beforehand).

Here is a good summary of that effect by Chem Geek with examples:

Chlorine and pH relationships. Chem experts knowledge?

OK, so I am a newbie. But, I’m ignorant of pH within Chlorine relationships. I can find very much data here on Chlorine/CYA, but what about pH? Basically without getting too technical……haha, (I’ve read Chem Geeks articles) I am trying to figure out the effectiveness of Chlorine at different pH...

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