PH Questions

[chem geek]

Your Minimum column has too low an FC (2) for the CYA level (60). For an SWG pool, it should be an FC of 3 for a CYA of 60. Basically, shoot for an FC that is a minimum of around 5% of the CYA level. This is lower than in Ben's chart used for manually dosed pools, but seems to work well for SWG pools.

You don't need to worry about the phosphate level. I've got 2000-3000 ppb phosphates in my pool and am able to control algae using chlorine alone (well, this year I also have borates in the pool, but I didn't before this year). You might consider using 50 ppm Borates in your pool since this helps to control pH rise, helps prevent scaling in the salt cell, and also inhibits algae growth.

Your minimum numbers are way too low when taken as a whole. The low pH, TA and CH combine to produce a saturation index of around -0.8 which would tend to dissolve the plaster in your pool. The ideal levels you listed have a saturation index of around -0.3 while the maximum levels have a saturation index of around +0.2. You should use The Pool Calculator to calculate the saturation index and try to target something close to zero or perhaps slightly below (i.e. -0.1 or -0.2 but not much lower) to lessen scaling in your salt cell.

Generally, to minimize the rate of pH rise you want your TA at around 80 ppm and your CYA at 80 ppm. That means your FC should be 4 ppm minimum and for decent water balance with salt at 3000 ppm your pH target should be around 7.7 and your CH at 300-500. If you target your pH at 7.6, then your CH should be 350-600.

Richard

[Charlie]

Thanks Richard for your review. That makes sense - the saturation index is what I should use to target minimum and maximum levels.

[chem geek]

Roughly speaking, one sets the CYA level according to the amount of sunlight protection they need, though usually not too high (with SWG pools, 80 ppm is OK if the FC is set at 4 ppm or higher). Then with the CYA level, this determines your FC target.

You set your TA appropriately for your source of chlorine. For hypochlorite sources, you set your TA on the low side, usually around 80 ppm but sometimes lower if the pH tends to rise. The pH target is usually around 7.5, but for SWG pools where the pH tends to rise it could be 7.7. After all of these are set, then you add enough calcium to have the CH high enough to have the saturation index close to zero. So in an SWG pool, with its higher salt level and a tendency for the pH to rise, one may have a lower TA and a higher CH than in non-SWG pools.

Richard

[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.