CSI versus TPF Chemical Recommendations

[Naplespool]

My SWG pool chemistry is quite stable and the water is sparkling. I wondering though about my target chemistry levels. My SWG is set to produce a little under 2ppm/day of FC and typical of SWG pools there is constant upward pressure on the pH level, which I've been running about 7.7-7.9.

If I use the TFP recommendations for pool chemistry, summarized below I get a CSI range of -0.62 to -0.16.

TFP Recommendations
FC ~9
CC 0
pH 7.2 to 7.8
TA 70 (TFP range 60 to 80 ppm)
CH 400 (TFP range 350 to 450 ppm)
CYA 80 (TPF range 70-80)
Borate 40 ppm (waiting for Blue/Mannitol parts to confirm)
Salt 3750
Temp 84F
CSI Range based on the above: -0.62 to -0.16

What I've been running though is the following:
FC ~10
CC 0
pH 7.7 to 7.9
TA ~90
CH 400
CYA 80
Borate ~40-50
Salt 3750
Temp 84F
CSI Range: -0.04 to +0.12

Does the TFP recommend targeting a ZERO CSI, or would I be better with the lower pH but an offset CSI average of about -0.3?

Thoughts and comments are appreciated.
RB

 

[mknauss]

With a SWCG it is best to keep your CSI on the negative side of zero. Anywhere between -0.6 and 0.0.

 

[Naplespool]

Thanks, and can you explain why that is?

 

[JoyfulNoise]

You have to realize that the water chemistry inside your cell is very different than the bulk pool water. For a pool with ZERO borates, you most definitely want to have a negative CSI in the bulk pool water because the pH inside the cell when it is running can easily go past 10. With borates in the water, the pH rise inside the cell is cut in half and so you can operate closer to zero CSI without scaling issues.

 

[Naplespool]

Thanks for that input. I'm guessing that the pH rise inside the cell is due to the outgassing occurring as chlorine is manufactured? I'm going to capture some water returning and see what the pH is like.

Just read this: Saltwater Chlorine Generator (SWG) Recommended Levels.

EDIT: Is pH rise within the SWG an argument for a higher rather than lower pump speed? (At 2000 rpm now.)

Just now: Pool Average pH about 7.85, pool return pH (salt cell on) is about 7.91

 

[JoyfulNoise]

The water out of the return is significantly diluted. What matters is what is happen right at the generation plates inside the cell. Two moles of hydroxide (OH-) is formed at the cathode along with hydrogen gas (the bubbles you see emanating from the returns) and so the pH right at the plate surface is very high. This is why calcium can scale at the plate. Borates will absorb and buffer some of that pH rise which helps to minimize scaling.