Lowered TA to 50, let's see if pH stabilizes...

[aztodd]

After dealing with chronic high pH and SWCG cell scaling issues, I've taken the advice here and lowered my TA from 100 to 50 over the past month using aeration and regularly adding muratic acid to bring the pH back down.

One observation, is that it is surprising to me how long this process takes. I have a waterfall and then also an aerator that I pointed down so that it's creating a lot of bubbles. I ran these features 24/7 (not at full pump speed but still) and it still took about a month to finally get to where I'm at.

Anyway, I'm hoping this will help keep my pH more stable, I'll monitor results and report back here for anyone else who finds themselves in the same boat. If this doesn't work my next step will be adding boric acid.

 

[ManiacalMama]

I never could get my TA down that low (highish TA fill water) so I ended up adding boric acid. Scaling stopped almost immediately.

 

[Bperry]

After dealing with chronic high pH and SWCG cell scaling issues, I've taken the advice here and lowered my TA from 100 to 50 over the past month using aeration and regularly adding muratic acid to bring the pH back down.

One observation, is that it is surprising to me how long this process takes. I have a waterfall and then also an aerator that I pointed down so that it's creating a lot of bubbles. I ran these features 24/7 (not at full pump speed but still) and it still took about a month to finally get to where I'm at.

Anyway, I'm hoping this will help keep my pH more stable, I'll monitor results and report back here for anyone else who finds themselves in the same boat. If this doesn't work my next step will be adding boric acid.

The lower the TA gets the longer it takes the pH to rise, which incidentally is the goal.

 

[Sunnydaze]

Unfortunately your ph isn't going to stabilize completely because your (our) fill water has high TA and you'll be fighting that. You will certainly see a slowing in the ph rise though.

 

[aztodd]

Unfortunately your ph isn't going to stabilize completely because your (our) fill water has high TA and you'll be fighting that. You will certainly see a slowing in the ph rise though.

Yeah I figured I would still need to stay on top of it, no pool is truly "set it and forget it". You remind me I need to test my fill water. I have a water softener, curious if that has any effect on the TA of the fill water.

 

[PoolStored]

I have no problem lowering my TA to 50 very quickly. I can lower TA by 10ppm, and have the same pH, in 12 hours in a 30K gallon pool.

Two tricks:
1) Make LOTS of TINY bubbles. Below is a picture of what I built and put it on top of a sump pump. Works great.
2) CO2 outgasses at the highest rate when pH is 7.0. I lowered pH to 7, let it rise to 7.4, rinse and repeat.

Doing these two things will speed the process.

 

[mknauss]

I have a water softener, curious if that has any effect on the TA of the fill water.

It does not.

 

[Gary Davis]

The OP mentioned "chronic high pH and SWCG cell scaling" where the TA was subsequently lowered to combat the pH rise, but what about CH (and the other scaling/etching factors) to account for the scaling?

What is the OP's carbonate saturation index reading?

 

[SoDel]

The OP mentioned "chronic high pH and SWCG cell scaling" where the TA was subsequently lowered to combat the pH rise, but what about CH (and the other scaling/etching factors) to account for the scaling?

What is the OP's carbonate saturation index reading?

I had the same thought. It’s not so much just getting TA down to a point where pH rise slows, but maintaining a neutral or slightly negative overall saturation index, which is one of the reasons why keeping pH under control matters. The logs show CH at 650 which can make it tough to keep a balanced index as pH heads upward and my guess is results in either very frequent acid additions, or excursions between acid additions that contribute to the scaling problem.

My understanding is that using softened water as fill water can help keep the CH in check, which over the long haul may help reduce CH, which will help make it easier to keep saturation index balanced, which, etc. To try to show what I’m trying to say, the logs show within range negative CSI at pH 7.6 but positive CSI at pH 8.0. Keeping pH at around 7.6 can be a chore (it would be for me, even with TA around 50). Borates might be an answer but seems that another way is get the CH low enough so CSI is slightly negative even at pH 7.9 to 8.0. Maintaining pH at around 7.9 to 8.0 is easier than 7.6. There’s always going to be work involved (acid additions) to keep TA down with high TA fill water, but maybe at least the scaling can be resolved as a benefit of that work.

I went looking for an article that explains the relationships between the factors that make up the saturation index, but no luck in a quick search.

 

[Sunnydaze]

I had the same thought. It’s not so much just getting TA down to a point where pH rise slows, but maintaining a neutral or slightly negative overall saturation index, which is one of the reasons why keeping pH under control matters. The logs show CH at 650 which can make it tough to keep a balanced index as pH heads upward and my guess is results in either very frequent acid additions, or excursions between acid additions that contribute to the scaling problem.

My understanding is that using softened water as fill water can help keep the CH in check, which over the long haul may help reduce CH, which will help make it easier to keep saturation index balanced, which, etc. To try to show what I’m trying to say, the logs show within range negative CSI at pH 7.6 but positive CSI at pH 8.0. Keeping pH at around 7.6 can be a chore (it would be for me, even with TA around 50). Borates might be an answer but seems that another way is get the CH low enough so CSI is slightly negative even at pH 7.9 to 8.0. Maintaining pH at around 7.9 to 8.0 is easier than 7.6. There’s always going to be work involved (acid additions) to keep TA down with high TA fill water, but maybe at least the scaling can be resolved as a benefit of that work.

I went looking for an article that explains the relationships between the factors that make up the saturation index, but no luck in a quick search.

Without a water softener, people in AZ would have to drain their pool yearly to keep the CH under 500. Realistically, that's not an option for most people. TA and pH are the things we can manage to keep CSI down without having to do a full water replacement annually.

 

[aztodd]

Without a water softener, people in AZ would have to drain their pool yearly to keep the CH under 500. Realistically, that's not an option for most people. TA and pH are the things we can manage to keep CSI down without having to do a full water replacement annually.

That or we keep dumping pucks and pool store potions into our pools year after year thinking "There's got to be a better way!" - at least that's how I was before TFP enlightened me.

 

[Gary Davis]

I went looking for an article that explains the relationships between the factors that make up the saturation index, but no luck in a quick search.

The math is too deep for me but it's almost certainly described in the link I had provided in my prior post.

As I recall, the major contributor to the saturation index is temperature, where colder is more "hungry" for calcium carbonates, and the next major contributor is pH, where more acidic is more "hungry" for the carbonates.

Carbonate alkalinity and CH would be next on in effect, where lowering either (or both) makes the water more hungry for calcium.

Lastly, CYA and TDS (including Salt) levels make the water hungrier for calcium carbonates when they go up.

I'm pretty sure the equations are described in the respective papers, but that's my gist of how these half dozen factors operate together to cause either definite scaling or the greater chance of etching.

 

[mgtfp]

I went looking for an article that explains the relationships between the factors that make up the saturation index, but no luck in a quick search.

In a nutshell, CSI works like that:

The product of the concentrations of Carbonate and Calcium ions divided by the solubility of Calcium Carbonate is 1 in equilibrium (that's essentially how the solubility is defined).

Take the logarithm and the one turns to zero, and the product and division become a sum of the logarithms of Calcium and Carbonate ion concentrations minus (which is a "negative plus") the logarithm of the solubility. That sum of the logs will be negative when undersaturated and positive when oversaturated. You start recognising our CSI here. It's the product of both, Calcium and Carbonate ion concentrations, that matters. Or the sum of the logarithms.

Now you want to express the carbonate and calcium ion concentrations by parameters you get from pool tests. The concentrations are actually "activities", considering ionic strengths, which is where the TDS comes in, basically to describe "shielding" of carbonate and calcium ions by other ions. High TDS basically means that Calcium and Carbonate ions are more shielded and you need more of them to reach saturation.

The temperature comes in via the Calcium Carbonate solubility. At lower temperatures more Calcium Carbonate can remain dissolved in water, i.e. reducing the CSI (that's why kettles scale up when heating up water in them).

The Carbonate ion concentration can be calculated from Carbonate Alkalinity, which is where pH comes in - at high pH Carbonate Alkalinity is nearly all Carbonate ion, at lower pH is us nearly all Bicarbonate ion, at very low pH there is no Carbonate Alkalinity left, all the Carbonates are essentially dissolved CO2 (which doesn't contribute to Alkalinity) at this point which mostly outgasses while pH is low.

Carbonate Alkalinity can be calculated from Total Alkalinity by subtracting CYA- and Borate Alkalinities, which is where CYA and BOR come in, and another pH dependency.

By the way: This is the only place that "corrected" Alkalinity has, it is just required as an intermediate value to calculate CSI - pool stores reporting this "corrected" Alkalinity to their customers, and sometimes even still call it "total", is utter nonsense that simply creates confusion (including amongst pool store staff). No one wants to do these calculations manually these days, that's what computers are for. Most customers don't want to do the calculation at all, they just want to know if their water is OK. But they end up buying overpriced baking soda because their "corrected Total" Alkalinity is "low". Even though CYA-Alkalinity has to be subtracted to calculate CSI, it still provides buffering and has its place in Total Alkalinity. End of rant, back to CSI.

Calcium ion concentration gets approximated by CH.

That's really it, plus a lot of unit juggling between PPMs and moles. And a couple of approximations and assumptions which other ions are in the water at a given TDS, CH, etc. to calculate the ionic strengths.

If you are interested in the details, then do a forum search for chem geek's Pool Equations spreadsheet, he shows the full derivation of the CSI at the bottom of the sheet.


And regarding scaling in the SWG cell: The best thing to combat this are Borates. They are the best of our available buffers at higher pH, and are quite efficient at limiting the pH rise right next to the cell cathode plates, and therefore keep the local cell CSI in check.

 

[SoDel]

In a nutshell, CSI works like that:

The product of the concentrations of Carbonate and Calcium ions divided by the solubility of Calcium Carbonate is 1 in equilibrium (that's essentially how the solubility is defined).

Take the logarithm and the one turns to zero, and the product and division become a sum of the logarithms of Calcium and Carbonate ion concentrations minus (which is a "negative plus") the logarithm of the solubility. That sum of the logs will be negative when undersaturated and positive when oversaturated. You start recognising our CSI here. It's the product of both, Calcium and Carbonate ion concentrations, that matters. Or the sum of the logarithms.

Now you want to express the carbonate and calcium ion concentrations by parameters you get from pool tests. The concentrations are actually "activities", considering ionic strengths, which is where the TDS comes in, basically to describe "shielding" of carbonate and calcium ions by other ions. High TDS basically means that Calcium and Carbonate ions are more shielded and you need more of them to reach saturation.

The temperature comes in via the Calcium Carbonate solubility. At lower temperatures more Calcium Carbonate can remain dissolved in water, i.e. reducing the CSI (that's why kettles scale up when heating up water in them).

The Carbonate ion concentration can be calculated from Carbonate Alkalinity, which is where pH comes in - at high pH Carbonate Alkalinity is nearly all Carbonate ion, at lower pH is us nearly all Bicarbonate ion, at very low pH there is no Carbonate Alkalinity left, all the Carbonates are essentially dissolved CO2 (which doesn't contribute to Alkalinity) at this point which mostly outgasses while pH is low.

Carbonate Alkalinity can be calculated from Total Alkalinity by subtracting CYA- and Borate Alkalinities, which is where CYA and BOR come in, and another pH dependency.

By the way: This is the only place that "corrected" Alkalinity has, it is just required as an intermediate value to calculate CSI - pool stores reporting this "corrected" Alkalinity to their customers, and sometimes even still call it "total", is utter nonsense that simply creates confusion (including amongst pool store staff). No one wants to do these calculations manually these days, that's what computers are for. Most customers don't want to do the calculation at all, they just want to know if their water is OK. But they end up buying overpriced baking soda because their "corrected Total" Alkalinity is "low". Even though CYA-Alkalinity has to be subtracted to calculate CSI, it still provides buffering and has its place in Total Alkalinity. End of rant, back to CSI.

Calcium ion concentration gets approximated by CH.

That's really it, plus a lot of unit juggling between PPMs and moles. And a couple of approximations and assumptions which other ions are in the water at a given TDS, CH, etc. to calculate the ionic strengths.

If you are interested in the details, then do a forum search for chem geek's Pool Equations spreadsheet, he shows the full derivation of the CSI at the bottom of the sheet.


And regarding scaling in the SWG cell: The best thing to combat this are Borates. They are the best of our available buffers at higher pH, and are quite efficient at limiting the pH rise right next to the cell cathode plates, and therefore keep the local cell CSI in check.

Thank you but I sort of knew this was coming. I’ve been through everything chem geek has ever posted and have calculated for myself the equilibriums for HOCL, OCL- and the bulk species of chlorinated isocyanurates at as best I could not being a chemist and following along with his work, to ensure I understand why I keep my FC where I keep it (and I don’t quite adhere to the chart). Been through everything I could find from Ben Powell as well. I’d like to think I have a very solid understanding of calcite saturation, certainly enough to deal with a pool as a not-a-chemist What I’m more driving at is all the guides I can find to link to for folks who don’t want to learn chemistry treat the elephant as tail, trunk and body separate. I can’t find a simple guide that explains it’s an elephant so if you pull the tail the trunk follows, if that makes any sense.

Can you think of something that explains it in simple terms but integrates the whole? I see a lot of pieces on TA / pH because of their inter-relation. Some on winterizing that discuss temperature. But not finding much I could give to any very bright and eager neighbor, say, who needs to understand that range chemistry is stone age and there’s not really any difference (within reason) between any of the factors when working as a whole to achieve balanced water under whatever the local constraints may be. Squishing one just means un-squishing another, etc.

 

[mgtfp]

I think in simple terms it's: Use PoolMath.

The highest level of looking at it is that you have to keep both, Calcium ions and Carbonate ions, low enough to not create scale, but high enough to not damage plaster.

Why for example low pH reduces the CSI requires a bit of chemical background (carbonate equilibriums).

I like to to put it in simple terms like that:

Increasing Calcium and Carbonate ion both increase CSI, that's kind of obvious but requires understanding that we are talking about Calcium Carbonate scale here, not other forms of scale like Calcium Phosphate or Calcium Sulfate.

It's the water kettle where you get scale -> Higher Temperature increases CSI

To remove the scale from my kettle, I use acid -> lower pH reduces CSI

CYA only appears because we use TA to estimate the amount of Carbonates, it doesn't have a direct influence. But the same TA with high CYA means that there are less Carbonates involved compared to a situation with the same TA but a lower CYA level.