Saturation Index

[Charlie]

Hi everyone,
I have a question on the saturation index. The goal is to keep it close to 0 as possible, but how much of an excursion can be tolerated before you start to have issues? I've heard values between -0.5 (corrosive) to +0.5 (scale forming) is an acceptable range.

What do the experts here think? I find it difficult to try to keep it at 0 unless you check the water everyday which I can't do. I can only check it every weekend.

[chem geek]

It partly depends on the quality of the plaster, but nearly everyone says that being within +/- 0.3 is perfectly fine and some say that +/- 0.6 is OK as well. Any problems with being too negative are going to take a while to show up anyway unless the pH is very low. As for scaling with being too positive, most pools don't see that showing up until at least +0.7 or for others +1.0. However, in saltwater chlorine generator cells it shows up a lot more because the pH near the hydrogen gas generation plate is high so such pools can be run at a somewhat negative saturation index.

I'll bet you aren't varying by more than 0.3 anyway. It sounds, however, like your pH is changing more than it should. If your pH tends to rise, then your TA is probably too high.

[Charlie]

Thanks for the feedback. I last checked the pool on 1-27-2011:
FC: 4.0
CC: 0
pH: 7.8
TA: 60
CH: 425
Water Temp: 60°F
CYA: 60
SI = -0.4
If I bring up my TA to 100, it will be 0. I also have a SWG @ ~13,000g. It also has a spa that spills over into the pool. Yeah, I have to adjust my pH ever weekend as it can go from 7.2 to 7.8 within a weeks time.

[chem geek]

Do not raise the TA level (especially not to 100 -- I wouldn't go higher than 70 with an SWG). That will only make the rate of pH rise even faster. You might consider using 50 ppm Borates to provide some additional pH buffering that won't contribute to the carbon dioxide outgassing that causes some of the pH rise. Also, don't try and get the pH lower than 7.5 -- have 7.7 or so as a target pH.

[Charlie]

Do not raise the TA level (especially not to 100 -- I wouldn't go higher than 70 with an SWG). That will only make the rate of pH rise even faster. You might consider using 50 ppm Borates to provide some additional pH buffering that won't contribute to the carbon dioxide outgassing that causes some of the pH rise. Also, don't try and get the pH lower than 7.5 -- have 7.7 or so as a target pH.

OK thanks for the tip. As for Borates, how much (guessing) do you think I would need?
As for the SI again, so if I stay between -0.3 and +0.3 I should be okay right?

[chem geek]

You can use The Pool Calculator to calculate dosing where 50 ppm Borates in 13,000 gallons would be 496 ounces by weight or 534 ounces by volume of boric acid OR is 765 ounces weight (about 10 76 ounce boxes) or 734 ounces volume of 20 Mule Team Borax with 365 ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid).

Borax plus acid is less expensive, but boric acid is more convenient since it only lowers the pH a small amount (roughly from 7.5 to 7.2). If you use Borax plus acid, add one-quarter of the total amount at a time, starting with the acid first adding slowly over a return flow, then add the Borax, wait 10 minutes, add acid, then Borax, etc.

[Charlie]

Where can I buy boric acid?

[chem geek]

The Chemistry Store or AAA Chemicals. Shipping determines which source is least expensive depending on where you live.

[CarlD]

Boric Acid is more commonly used as an eyewash in dilute amounts. However, large amounts may be tougher. However, Borax is everywhere and Muriatic Acid is sold in hardware stores and some Home Depots.