A Theory About Rising pH in SWG Pools

[KurtV]

Richard,
Updated data. I think the acid demad might be letting up a bit.

Date pH Alk. Acid addition
7/26 7.8 100 24 oz.
7/27 7.4 100
7/28 7.8 100 16 oz.
7/29 7.7 100 16 oz.
7/30 7.2 90 32 oz. (before testing)
7/31 Vacation
8/1 Vacation 16 oz.
8/2 Vacation
8/3 Vacation 16 oz.
8/4 7.8 90 28 oz.
8/5 7.6 80 16 oz.
8/6 7.6 80 16 oz.
8/7 7.4 80 8 oz.
8/8 7.6 70 16 oz.
8/9 7.6 70 16 oz.
8/10 7.5 70 16 oz.
8/11 7.3 70 8 oz.
8/12 7.4 70 16 oz.
8/13 7.6 70 16 oz.
8/14 Work trip
8/15 7.6 70 16 oz.
8/16 7.6 70 24 oz.
8/17 7.4 [IMG]file:///C:/DOCUME%7E1/CLV/LOCALS%7E1/Temp/moz-screenshot-2.jpg[/IMG][IMG]file:///C:/DOCUME%7E1/CLV/LOCALS%7E1/Temp/moz-screenshot-3.jpg[/IMG]65 16 oz.
8/18 7.4 60 8 oz.
9/19 7.5 60 20 oz.
8/20 7.2 60 8 oz.
8/21 7.4 60 8 oz.

[chem geek]

Kurt,

I'll bet that when you hit 50, if you let the pH rise to 7.5 then the continued rise from there will be slow (i.e. you could add acid when it hits 7.6 or 7.7). You could therefore make your pH "target range" 7.5-7.7 and not use too much acid. In fact, the general advice for people with low TA would be to have higher pH for better water balance though this technically is only necessary for grout/plaster/gunite pools. Anyway, thank you so much for keeping such good track of your numbers. It really helps in trying to figure these things out.

By the way, if the TA is somewhat stuck on 60 and getting lowered too slowly, you can always just add some extra acid to get to a pH of 7.2 or below in order to accelerate the outgassing (i.e. Ben's alkalinity lowering technique), but if you have the time and just want to see how things slow down as TA gets lower, then measure on...

Richard

[Davenj]

Great read and interesting research. I was surprized on the acid demand of a gunite pool. I have a vinyl pool, 16oz a week. Didn't mean to muck up your thread. Keep up the work, look forward to future posts.

[South_Texas_Sun]

We just finished (sort of) our 19,000 gallon IG pool here in south central Texas and I've been wondering why I've had to add acid about every 2 or 3 days....lol

We have 4 waterfalls, of which just the spa spillway waterfall runs 8 hours/day, but it's a healthy stream of water, dropping about 5 feet though the air, and there's 2 Jandy Sheer Descents and a 12 foot rock slab "grotto" waterfall.

As part of the pool equipment, we got a cheapo "test kit" for chlorine and PH ONLY, but I ordered the poolforum test kit (with salt option) about 3 or 4 weeks ago, and haven't received ANYTHING......not even an EMAIL when threatening to cancel the order! (so I can't verify any numbers except that my Hayward SWG is working and the PH is rising like crazy.)

Another variable.....we have underwater limestone flagstone on 2 separate larg-ish entry and spillway beaches (each 8'X5'). Would underwater limestone rock increase the alkalinity? (it doesn't seem to be reacting to the salt water, but there is substantial initial flaking which is normal, according to all reports) Would it help to place a less alkaline stone below the spa spillway?

Can anyone recommend a more readily available test kit that I can buy online, so that I can get some numbers on my pool chemistry?

Very interesting topic. Thanks for all the great discussion!

STS

[PatL34]

STS,

I suspect that the limestone will have some effect vis-a-vis your rising pH. Chem_Geek hopefully will chip in here. The muriatic acid will do some work on the limestone, so some adjustments will have to be made.

If you can get access to a Taylor 2006 test kit, it will be a great help in testing your water while waiting for Ben's PS234 kit. The numbers will then help us to help you.

Kurt, the numbers I took this morning were:
FC 5 ppm
pH 7.2
TA 90 ppm
CH 270+ ppm (ran out of R-0012 reagent. Duh!)
CYA 60 ppm

Will get another CH reading later today.

Pat

[chem geek]

Limestone is primarily calcite which is calcium carbonate, the same major component that is used in the production of cement that is in plaster/gunite/grout so the same rules for water balance apply. I don't believe the issue of corrosion vs. scaling will be any different for limestone than for the plaster/gunite/grout except perhaps that the limestone may be more sensitive to this chemical balance since it is much more "pure" calcium carbonate (i.e. doesn't have its calcium carbonate converted to calcium oxide bound to silicon oxide in cement though the aggregate in concrete is often limestone).

So I wouldn't expect a change in pH from the limestone unless your water was out of balance. If your water was too low in TA, pH and/or CH, then you could corrode the calcium carbonate from the limestone and that would slightly increase your CH and TA and would increase your pH.

Richard

[bbb]

Very interesting thread. This is my first year with a SWG (installed in June '06), and I live in the Northeast. The pool is a vinyl liner, 20x40, around 20 years old. CYA around 45. No special features. Over the past 2 years the only thing I have used to raise pH is borax. My pH has been between 7.2 and 7.4 all summer. Total acid added - zero! Contributing factors may include lots of acid rain, and my use of a solar cover when the pool is not in use. I have added virtually no fill water all year.

[chem geek]

Richard,
Updated data. I think the acid demad might be letting up a bit.

Date pH Alk. Acid addition
7/26 7.8 100 24 oz.
7/27 7.4 100
7/28 7.8 100 16 oz.
7/29 7.7 100 16 oz.
7/30 7.2 90 32 oz. (before testing)
7/31 Vacation
8/1 Vacation 16 oz.
8/2 Vacation
8/3 Vacation 16 oz.
8/4 7.8 90 28 oz.
8/5 7.6 80 16 oz.
8/6 7.6 80 16 oz.
8/7 7.4 80 8 oz.
8/8 7.6 70 16 oz.
8/9 7.6 70 16 oz.
8/10 7.5 70 16 oz.
8/11 7.3 70 8 oz.
8/12 7.4 70 16 oz.
8/13 7.6 70 16 oz.
8/14 Work trip
8/15 7.6 70 16 oz.
8/16 7.6 70 24 oz.
8/17 7.4 [IMG]file:///C:/DOCUME%7E1/CLV/LOCALS%7E1/Temp/moz-screenshot-2.jpg[/IMG][IMG]file:///C:/DOCUME%7E1/CLV/LOCALS%7E1/Temp/moz-screenshot-3.jpg[/IMG]65 16 oz.
8/18 7.4 60 8 oz.
9/19 7.5 60 20 oz.
8/20 7.2 60 8 oz.
8/21 7.4 60 8 oz.

On another pool forum (poolcenter.com) in this thread via an unfortunately not-so-nice discussion, I calculated whether the SWG produces enough hydrogen gas bubble volume to have the small amount of carbon dioxide that would be allowed into it (via Henry's Law for equilibrium) to make the pH rise. It isn't enough and that's under ideal conditions with full equilibrium reached (TxPool says the kinetics don't allow for any effective transfer, but I'm not sure about that -- in any event, the volume just isn't enough).

So, I'm back to leaning towards the other explanation I originally started with in this post in this thread where I thought it might be undissolved chlorine gas bubbles. If this is the case, then one way to improve the situation could be to point the returns downward to give the bubbles a greater chance of dissolving more completely into the water. The lower TA effect that KurtV saw wasn't huge and could just be regular carbon dioxide outgassing without much change from the SWG itself -- that is, the SWG effect may mostly be chlorine gas escaping.

KurtV (or anyone else with an SWG that would like to experiment), if you are still experiencing a rising pH in your SWG pool, can you try pointing the returns downward and seeing if that makes any difference? Can you see the bubbles at night to see if they linger longer when the returns point downwards or if you notice any of them getting smaller in the water (dissolving)? Do you ever smell any fresh chlorine smell over the return where the bubbles come up compared to other parts of the pool where there are no bubbles?

If the chlorine outgassing is a primary cause for the incremental pH rise in SWG pools, then I would expect this effect to be worst in pools with short distances between the SWG and the pool returns and for 2-speed or variable speed pumps when the pump is on high speed but not on low speed. If pointing the returns downwards helps, then one has the trade off of rising pH vs. potentially worse cleaning of the pool water surface into the skimmer (which I think is better when the returns are not pointed downwards).

Richard

[Sentient]

My pool has three returns, the third does not bubble at all, I will leave it pointing up and point the other two down. I have a cover, which I keep on most of the time when the pool is not in use and need to add 1Qt of acid a week. It will be interesting to see if this makes any difference. I guess it is finally time to start a log so I can figure out what really is going on versus my gut feel.

Mark

[chem geek]

Mark,

That's REALLY interesting that one of your returns doesn't bubble at all. I assume all three are coming from the output of your SWG, right? You don't have any sort of bypass of flow with some going around your SWG to one return, right?

If the SWG output is going to all three returns, then is the one return with no visible bubbles at a farther distance away from your SWG? If so, then the theory of the chlorine bubbles needing more time to dissolve may very well be on the right track.

I would think that a pool cover would keep the chlorine in contact with the water longer so that it would eventually dissolve and not escape, but I suppose we'll find out.

I'm looking forward to the results of your experiment. Thanks!

Richard