A Theory About Rising pH in SWG Pools

[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.

[nater]

Great thread!!

My experience closely matched bbb's with regards to stable pH in a vinyl pool with a SWC. At least until I added a....drumroll....FOUNTAIN!!!!

The fountain was added back in early June to combat rising pool temps. It's done a great job of that. Last year by this time my water temp was in the mid 90's, this year I've kept it between 82 and 86.

I added my SWC in ealry March, before the swim season started. I then found this site (thank God) and balanced my water within a week or two. I've reduced my CYA down from 120+ to a current level of 50-60. Prior to June, I had added nothing but salt when needed.

Since installing the fountain I've been using about 1 gallon of MA and 4# of Baking Soda every 2 weeks to maintain a pH of roughly 7.5 and Alk around 120-140. Why? To keep my pool "on target".

Since reading the latest posts on lowering ALK to keep pH rise under control, I'm fully ready to let things go for a couple weeks and see where they settle out naturally.

Chem Geek/others, any suggestions to make my experiment more controlled and useful to the larger group?

Current numbers as of last night (I had to drain about 7,000 gallons Sunday to repair a crack in my steps and rebalanced after the re-fill).

Temp: 82
FC: 3.5
TC: 3.5
pH: 7.2
Alk: 140
CH: 100
CYA: 55
Salinity: 3300
Borates: 50-60 ppm (strips)
Phosphates: less than 1000 ppb (Hach test)

[chem geek]

nater,

Thank you for your very detailed set of numbers. Now that I have borates added to my spreadsheet, I can give you an idea of where I think you are at and where you can go from here.

Your CO2 outgassing rate number (at pH 7.2) is 27.8 which is very, very high so it's no wonder you are having to add so much acid and baking soda to your pool (at a pH of 7.5 it's a much more reasonable 12.8 which would probably be stable were it not for the fountain -- which is exactly what you have been seeing). In general, whenever you are not only adding acid, but also have to add baking soda to maintain TA, then you are seriously outgassing CO2 and should consider lowering TA and/or raising pH (unless you like spending money and time on acid and baking soda!). Even before the fountain, I would say you were somewhat lucky in not having to add so much acid and baking soda, but now with the fountain adding to the aeration, you can really see how "overly carbonated" your pool is.

If I assume you are at a baseline pH of 7.5 most of the time, then your acid and baking soda additions mean that you outgas 11.5% of your carbonate in your pool every 2 weeks and you have an additional source of base (pH rise) in your system since your additions would normally lower the pH to 7.36 and the TA to 130.6 over a 2 week period. It is as if you have an additional source of rising pH equivalent to 20.5 ounces of lye (sodium hydroxide) so perhaps this is from your chlorine generator. In fact, if your SWCG were to produce 5 ppm of chlorine over the 2 weeks that got outgassed as chlorine gas (i.e. didn't dissolve into the water to produce hypochlorite), then that would be consistent with the numbers that you gave.

So here's a plan that can be done in two steps to isolate the two sources of rising pH that you have -- CO2 outgassing and SWCG chlorine outgassing (assuming that's what's going on). First, follow Ben's Lowering Your Alkalinity Procedure to get your TA down to 80. Then we can see what happens. My guess is that you will still need to add acid (though less of it -- probably a little more than 6 cups every 2 weeks), but that you may not need to add baking soda anymore. I assume you will try to keep your pH at 7.5 (not 7.2).

Assuming the lower TA works for you (and you may need to get it lower than 80, but let's see where you are at there first), then you can see if you can lower the power level of your SWCG and have it run for a longer period of time (to make up for the lower power level) to see if this helps reduce the amount of wasted generated chlorine that may be escaping as chlorine gas and not getting dissolved into your pool as hypochlorite. If this works, then this will further lower how much acid you need to add to your pool.

Sound like a plan?

Richard