The Great Tetraborate Experiment!

[chem geek]

waterbear (Evan),

I ran through some numbers on my spreadsheet assuming certain levels in your salt pool with borates and low TA. If I put in a pH of 7.5, TA of 70, CYA of 75 (for your SWG), CH of 300, TDS of 3200 (or salt of about 3000), 50 ppm Borates (Boron), and a temp of 85F, then this gives me a carbonate alkalinity of only 39 and a calcite saturation index of -0.47 (pool-store Langelier is -0.39). Now I know we don't give much credence to these indices and problems aren't normally seen until around +/-0.7 or even +/-1.0, but even so, the exceptionally low carbonate alkalinity in your pool plus the high salt level make the water more corrosive.

I just wanted you to know this and to be on the lookout for any signs of such corrosion, namely any dissolving or pitting of plaster/gunite/grout. If you notice your calcium hardness increasing (or not decreasing as much as it normally does, if it decreases from backwashing), then that would be a sure sign of trouble.

Assuming your CYA is high for the SWG (at 75 ppm), then a TA of 100 (carbonate alkalinity 69 and relative outgas rate 6.0) shouldn't outgas that much carbon dioxide, though it would be at about double the rate of a TA of 70 (carbonate alkalinity 39 and relative outgas rate 2.9). A TA of 100 would bring the "index" to -0.22 while also increasing the CH to 500 would bring the index to -0.01.

Richard

[waterbear]

Richard,
I run a adjusted TA of about 70 ppm (which means I run a TA of about 90), pH of 7.6, salt about 3400 ppm, Cal around 220-250 ppm, and temp around 82 when the heater is on. See what those give. BTW, I have seen no corrosive action to the water. Try plugging those numbers in and see what you get.
Evan

[chem geek]

With the higher pH and TA, this increases the saturation index. The higher salt level and lower CH (235) decrease it somewhat. The net result I now get is -0.35 which is better than the -0.47 I was seeing before. The actual carbonate alkalinity is 57.0. If I remove the Borates and keep the measured TA the same, then I get a carbonate alkalinity of 64.4 so though the Borates do not have a large effect, they do have some effect on alkalinity and therefore any "adjustment" calculation.

I'm glad to hear that you resolved your rising pH issue by lowering the TA only to 90 instead of 70, especially with the high CYA levels needed for the SWG. That makes this "lower TA" recommendation more reasonable for people combating rising pH without having to adjust too much of the other parameters.

If you increased your CH to 300, the saturation index would be -0.21; increasing it to 400 would make the index -0.09. Of course, raising the pH by 0.1 raises the saturation index by the same amount and is another alternative. Since you're not seeing any problems, you could of course just keep things as they are. Raising the CH to 300 or 400 wouldn't be hard now that you've stabilized your pool with all its parameters tweaked the way you want them (i.e. borates, salt, CYA). Just FYI.

Richard

[waterbear]

Just a quick update to anyone interested.
I have not needed ANY acid in my pool for the past 3 months. pH has been steady at 7.6 and ALK at 80-90 ppm. (We has some rains so that might account for a need for less acid and I have had to top off the pool and my fill water has a TA of between 80-110 when I have tested it on various times so this might account for some of the stability but not all of it!)

[salinda]

Can we get an update on this? My acid demand this year has been atrocious. I go through 4 gallons every 2-3 weeks.

[mas985]

I just added 50 ppm to my pool a few weeks ago but I have not had a change in acid usage yet. I have four strikes against me when it comes to PH drift:

1) Plaster pool
2) SWG
3) High PH fill water > 8
4) High TA fill water > 220 ppm

I am not sure if the Borates are going to help my situation much but it is probably too soon to tell. I still have to get my TA down since it sits at about 110 ppm. After I do that I will report back if I see any changes.

EDIT: I actually have 4 issues

[waterbear]

My pool has been holding fairly steady at pH 7.6-7.7. My acid use is very low and I am running my SWG at about 8% output compared to the 15-20% I was running it at before the borates. I keep my TA at about 80-90 ppm before stabilizer adjustment.