What is the best Calcite Saturation Index (CSI) for a saltwater pool

[smallpooldad]

What is the best Calcite Saturation Index (CSI) for a saltwater pool. I read somewhere on the forum that a slightly negative reading is good for the unit. Would -0.16 to - 0.23 be ok?

[waterbear]

I am going to gently remind you that it is a pool and not a science experiment. What is important in a salt pool is watching your pH and not letting it climb above 7.8. High pH is was causes scaling, period! Watch the ph and don't lose sleep over the CSI!

[chem geek]

You also need to be fairly far away from "zero" before problems are seen. As waterbear says, pH is most important to watch as it directly impacts CSI where a 0.1 change in pH is a 0.1 change in the CSI -- the other parameters have a logarithmic relationship so have to move pretty far and that generally doesn't happen very quickly as it can with pH.

[smallpooldad]

Thank you both for your replies.

The pool is saltwater set at 3000 ppm and is controlled by a Pool Pilot Auto Digital Controller. Setting the ph at 7.5 allows it, under normal circumstances, to never exceed 7.5.

As the pool never gets colder than 78 F in the winter and never hotter than 88 F in the summer it seems the following parameters would be best according to the "Pool Equations" spreadsheet. This way no parameter would need to be changed.

Measured pH 7.5 7.55
Total Alkalinity (ppm CaCO3) 70 70
Free Chlorine (ppm Cl2) 5 5
Cyanuric Acid (ppm CYA) 35 35
Calcium Hardness (ppm CaCO3) 450 450
Total Dissolved Solids (ppm) 4,000 4,000
Total Sulfate (ppm SO42-) 0 0
Total Borate (ppm Boron) 50 50
U.S. Gallons 10,000 10,000
Temperature (F) 78 88
Total Chloride (ppm NaCl) 3585 3588.1
Carbonate Alkalinity (ppm CaCO3) 52.5 50.3
Langelier Saturation Index (LSI) -0.16 -0.03
% HOCl (vs. Total Free Chlorine) 1.30% 1.20%
OCl- (as ppm Cl2) 0.074 0.093
HOCl (as ppm Cl2) 0.064 0.062
Calcite Saturation Level (CSL) 0.51 0.66
Calcite Saturation Index (CSI) -0.29 -0.18

[chem geek]

I wouldn't worry about the winter since the colder water temperature will naturally have the pH move upward, all else equal (so let the pH drift up on its own to 7.6 or so during the winter). As for the summer numbers, the roughly -0.2 saturation index is fine since you don't want scaling in the salt cell where one plate is at high pH. Since you are using borates and keeping a lower TA to avoid the rate of pH rise (I assume), you have a higher than average CH to compensate. This all looks good.

Richard

[smallpooldad]

chem geek,

Thank you for your very quick reply. Yes the 70 setting is so we do not have to compensate for pH too often. The 7.5 pH we try to achieve to offset Hawaii's Volcanic Dust, aka Brown Dirt (Ferric Oxide or Iron Rust), which blows in at 16 mph to 26 mph. At higher pH(s), as you know, it goes out of solution and stains the pool brown.

One quick quetion should I use a CYA adjustment for Alk or is it in your spreadsheet?