PH Questions

[chem geek]

You didn't list the Total Alkalinity (TA), but with the numbers you did list then the TA would be about 55 ppm to get a saturation index of -0.8. Is your TA really that low? I would raise your TA level to 80 ppm using baking soda (or at least 70 ppm, but see below for more info). This may somewhat raise your pH which would also be fine. After adjusting the TA, if the pH is below 7.7, then you can raise it using some 20 Mule Team Borax (one could raise both pH and TA using pH Up, but that gets tricky to do properly).

Also, do you have 50 ppm Borates in your pool?

The adjustment of TA to 80 and pH to 7.7 would bring the saturation index to -0.25 which is on the low side, but not as bad as -0.8. As for whether it makes sense to raise the CH to 500 or 600 ppm, this depends on what normally happens in the summer when the pool water is warmer and you are using the pool. If a pH of 7.5 were fairly stable, then a higher CH would be appropriate, but if the pH tended to rise and was more stable at 7.7, then keeping the CH where it is and targeting a 7.7 pH would be better.

Normally as the water temperature drops, the pH should rise, all else equal. I'm not sure why, with an SWG and your small waterfall from the spa, your pH is as low as 7.4. Have you been adding acid to keep the pH lower? I'm confused because you used to have a problem with the pH rising so you lowered the TA to 70 and I suggested that you not try and lower the pH below 7.5 and keep a higher pH target. For your TA to have dropped, you must have been adding acid over time. I repeat my advice that you should not try and fight to keep a pH of 7.5 in an SWG pool. If you really, really want to have that pH, then you'll need to not only have a low TA and 50 ppm Borates, but have a much higher CH. Most people just target a pH of 7.7 instead and that works out well.

Richard