Can't stabilize spa pH and alkalinity

[RobLane]

The spa is a 5' x 10' with a Diamondbrite interior. Therefore the calcium at 200 ppm minimum. The pH of 7.4 was chosen to match the pH of eyeballs. I was trying to stick the pH in place with the high TA.
If we still aim for a pH of 7.4 and lower the TA to 80 what do you think will happen?
What is your defination of boric acid? I have used sou
dium tetraborate penta hydrate in the past. First from John Girvan and lately from Haviland. Are there other good replacements? Do you think that the borates will help us avoid pH bounce with a low pH?
It is nice to have time sweat these small details.
I don't know how to account for borates with the LSI.

[chem geek]

Thanks for letting me know about the Diamondbrite. Most spas are acrylic so don't need to saturate the water with calcium carbonate. Your spa does need that. As for the pH of the eye, this paper and this paper say 7.5 average while this paper says 7.0, but there's a lot of variability in all these papers. You won't get irritation anywhere from 7 to 9 as noted in this paper (though we've seen some reports of irritation at the low pH range, but not at the higher end near 8) and it will be a lot easier to maintain the pH if you don't try to keep it lower. This chart shows how over-carbonated water is compared to air at various pH and TA levels.

You can use sodium tetraborate pentahydrate, but it raises the pH so you have to add acid. You can get something very similar (sodium tetraborate decahydrate) in 20 Mule Team Borax at lower cost. You can get boric acid from DudaDiesel, The Chemistry Store or AAA Chemicals (the latter source only in 50 pound bags so more suitable for pools). The advantage of boric acid is that you need not alternate adding Borax and acid -- the boric acid is only slightly acidic so you can add it all at once. If you prefer Borax and acid that's fine -- just split up the dose into thirds or fourths and alternate adding them so as not to make the pH swing too much.

The Borates are best at buffering against a rise in pH, but that's what you are seeing. The carbonates will be what buffers more against a drop in pH -- you will still have them even at a lower TA level. You can use The Pool Calculator to calculate dosages and to calculate the Calcite Saturation Index (CSI). Just keep in mind that with a spa you can get scaling pretty easily so target slightly negative or try and keep your pH from rising too quickly.