what happened to my TA

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rd3500

Bronze Supporter
Sep 28, 2021
42
Las Vegas
so... I finally got around to install a SWG (basically same as circupool UL55). It has been running for 2 days and my PH goes up pretty fast since the installation. And by pretty fast I mean .6 in a day. The pool ph has a tendency to go up up but I don't remember ever that fast:

Screen Shot 2022-05-10 at 2.05.58 PM.png

I figured I would test TA, last time I checked (about 4 months ago maybe), it was over 100 but PH was not going up this fast so I decided not to worry about it at the time (also confirmed by people here).

Well, TA is at 50 now and I did nothing to change it. I tested twice because I could not believe it.

Two questions:

1) if TA is at 50, why would PH rise up this much? Is that the SWG?

2) how did it get that low, as far as I understand there is a process that takes time to lower it on purpose... unless I tested it wrong before but I remember I did it a few times.

FC is at about 7 now
CYA is at 60 to 70
Calcium was at 350 last time, I have not checked it again yet
 
Very hard to believe. Your fill water TA is 130 ppm. And you are adding quite a bit of fill water right now.

What test kit?
 
25 lbs of trichlor in a 12,500 gallon pool will:
Raise fc by 219 ppm
Raise CYA by 133 ppm
Raise salt by 179 ppm
Lower TA by 154 ppm.

About 8.3 lbs of trichlor will lower the TA by about 50 ppm.

That's about (19) 7 oz trichlor tabs.
 
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rd3500 said:
1) if TA is at 50, why would PH rise up this much? Is that the SWG?
Click to expand...
Yes SWG increase PH. The SWG produces Sodium Hydroxide (which raises PH) along with the chlorine. I have no idea why your TA crashed like that, but smarter folks then me are already helping you with that one.. have you gotten a lot of rain lately?
 

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Orion7319 said:
Yes SWG increase PH.
Click to expand...
My observation is that SWGs cause zero pH rise.

In my opinion, it's just not something that happens.

pH rise comes from carbon dioxide offgassing.

The only thing that changes is that you lose about 0.25 lb of water for every pound of chlorine gas generated.

This is due to the water being converted into hydrogen gas and oxygen gas.

Anode 4Cl -> 2Cl2.
Cathode 4H2O -> 2H2 + 4OH.
2Cl2 +2H2O -> 3H + HOCl + OCl + 2Cl
HOCl + OCl + uv light -> O2 + H + 2Cl.
Net reaction:
6H2O + 4Cl --> 4H2O + 4Cl + 2H2 + O2.

Following the process, we can see that there are 4H and 4OH created, which nets out to ph neutral.

The chlorine gas generated is very acidic and creates 3 hydrogen ions for every 4 hydroxide ions created.

As the hypochlorous acid is broken down by UV, 1 more hydrogen ion is created for a net neutral result.

Assuming that chlorine gain and loss are equal, there's no pH rise.

For a SWG, you gain and lose the same amount on a continuous basis once you reach equilibrium.
 
mknauss said:
Test your tap water. Should be right at 130 ppm.
Click to expand...

tested it with a 10ml sample, took 5-6 drops times 25, so it is 125 to 150, right there. And I guess it confirms the test is accurate.

Tested again pool water with 25 ml sample, TA is 50.

JamesW said:
Are you sure that no one added tabs to the floater or to the feeder?
Click to expand...

I have never used the feeder although it was installed, I removed it when I put the SWG.

I did put those pucks in the water for 24 hours 3 days ago to try to raise CYA because it was at 60. I literally kept it in the water for 24 at the most.
 
JamesW said:
My observation is that SWGs cause zero pH rise.

In my opinion, it's just not something that happens.

pH rise comes from carbon dioxide offgassing.

The only thing that changes is that you lose about 0.25 lb of water for every pound of chlorine gas generated.

This is due to the water being converted into hydrogen gas and oxygen gas.

Anode 4Cl -> 2Cl2.
Cathode 4H2O -> 2H2 + 4OH.
2Cl2 +2H2O -> 3H + HOCl + OCl + 2Cl
HOCl + OCl + uv light -> O2 + H + 2Cl.
Net reaction:
6H2O + 4Cl --> 4H2O + 4Cl + 2H2 + O2.

Following the process, we can see that there are 4H and 4OH created, which nets out to ph neutral.

The chlorine gas generated is very acidic and creates 3 hydrogen ions for every 4 hydroxide ions created.

As the hypochlorous acid is broken down by UV, 1 more hydrogen ion is created for a net neutral result.

Assuming that chlorine gain and loss are equal, there's no pH rise.

For a SWG, you gain and lose the same amount on a continuous basis once you reach equilibrium.
Click to expand...
Well ok then! I thought that the cell was producing an equal amount of Sodium Hydroxide to chlorine. Google steered me wrong on that one as it seems to do most folks on anything pool related… I keep lowering my PH with MA every week from 7.8 to 7.5 with a TA of 60 just because I’m expecting to see the PH go up due to the salt cell. If that’s the case then I probably don’t really have to add MA and my ph will probably just stay around 7.8. I adjusted my returns to not disturb the surface of the water very much and I don’t have any water features. Also I never thought about the water loss due to the Oxygen and hydrogen conversion…
For the OP, he’s got something very strange going on then…
 
mknauss said:
Is the pool covered?
Click to expand...

the pool has been under a solar blanket for about 2 months until a couple of weeks ago.

JamesW said:
What was the CYA 4 months ago?
Click to expand...

no, tested it these days when I installed the SWG, so let's say 4 days ago.

JamesW said:
How much MA has been added?
Click to expand...

I don't know if you are asking now or over time. To lower the PH that you see on the graph, I put about 1/2 to 3/4 of a gallon of MA each time.

During the winter it has been fairly stable, rising but not by much. I have been managing the pool since october 2021 and if I had to guess, I probably used 6 to 8 gallons total so far.
 
How long is the return line from your SWG back to the closest outlet? A while back Chem Geek explained that another potential source of pH rise with SWGs could be chlorine gas that got created by the SWG outgassing without dissolving in the water. That would create a pH rise together with a rising TA (that couldn't be explained with high TA fill water top ups). But there never seemed to be much evidence that this actually happens, so that theory doesn't seem to be maintained here anymore.

Does the actual chlorine production of your cell match your expectation? For that massive pH rise a lot of chlorine needed to be outgassing (should the chlorine outgassing theory be valid), and that should be noticeable.

How much MA have you added since installing the SWG and how did your TA react to that?

You could turn your SWG off for a while and add the same amount of chlorine that your SWG should be producing per day via liquid chlorine (not Trichlor or Dichlor), and see how pH, TA and FC react to that. Maybe compare both chlorination methods over a period of one or two weeks each to get some data.

I assume that pH tracking comes from an automated system? With automated MA dosing? Maybe do a few Taylor pH tests as well to verify the automated testing. Just to be sure, but even if the pH sensor needed calibration I'd still expect it to show the relative rise reliably.
 
Just had another idea. If you have a fixed pH probe in your plumbing, where exactly is it? Before or after the SWG?

If after the cell, the results might show a locally increased value. Even when the pH reaches the ceiling where outgassing is in equilibrium with atmospheric CO2, outgassing into hydrogen bubbles still continues as there is no CO2 gas in hydrogen bubbles. Once that water mixes with the pool, further pH rise should be compensated by CO2 being absorbed from the atmosphere, and the bulk water pH should stop rising at some point above pH 8.

But just after the cell, you might see weird looking effects.

I would definitely recommend a few manual pH tests over the day from the pool itself and compare with the automated testing.
 
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