PH Questions

[Charlie]

My saturation index is -0.8 with these readings this morning:
Water Temp: 60°F
FC: 5.0
CC: 0
pH: 7.4
CH: 425
CYA: 80
Salt: 3200

I can lower the index by increasing the CH, but what are the disadvantages of increasing the CH? Looks like I need to add a little of Arm & Hammer baking soda but it still will not get me close to -0.1 or zero.

[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

[Charlie]

Old age...
TA=60
I did add 2 cups of Arm & Hammer baking soda yesterday. I'm targeting 70-80.

I do not have Borates in the pool. Where or how do I go about getting some?

My pH does rise since I have a small water fall from the spa into the pool. As for targeting 7.5 as the low, yeah I've been trying to figure out how much to add so I have a general idea when its at 7.8 I add "xx" amount in. OK so I'll target a pH of 7.6-7.8 or even 8.

[chem geek]

The addition and use of borates is described in this thread. You can use The Pool Calculator to calculate dosages of 20 Mule Team Borax and Muriatic Acid, or of boric acid if you get that directly (see this post).

The Borates will make the pH rise slower and might lower the total cumulative acid addition needed over time if you are able to lower your SWG on-time due to the algaecidal properties of the borates. As for pH, I wouldn't go so far as 8.0. I think that after the borates a ph of 7.7 is going to be reasonable stable.

[Charlie]

Thanks Rich for that info. Interesting on how much of 20 Mule Team you need to add, which brings me to this question: since you are adding so much in the pool, what does that do to the DE filter pressure? Will I see a huge increase? My pool is ~14,500 gallons.

[chem geek]

You won't see any change in pressure -- I presume you are adding it directly to the bulk pool water. What you add to water is like adding salt -- it fully dissolves. It does have a small effect on Total Alkalinity -- 50 ppm Borates at a pH of 7.5 increases TA by about 5 ppm -- but you can use The Pool Calculator to calculate the saturation index and it will account for the Borates (assuming you put that into the calculator).