Higher Calcium Numbers Seem to Control PH better

It seems higher calcium numbers seem to control pH better (more stable), less acid is required. Why is this?

I have tried Calcium at 475 and at 575-600 and the latter seems to use quite a bit less acid. For information my fill water alkakinity is 80 and the pool is covered at night to retain the temperature so the heat pump does not need to be run most of the time, at least from May through October. During the other months I am willing to swim in temperatures as low as 78 F to save on energy costs from the heat pump.

FC is relatively constant throughout the day barely fluctuating from 5 or so.

I would have thought that lowering the Calcium to 475 and raising the pH to 7.5 would have used less in a SWG pool but the opposite seems true. As I am not a chemist I do not understand why.

ORP measuresment starts at 11:30 am at about 690, 670 at 1:00 pm, rising to about 685 at 2 :15 pm, and close to either side of 700 at 8pm, ORP is set at 700. The 3/4 hp pump runs 3 1/2 hours from 11:30 am to 2:30 pm, just over 1 turn at 55 gpm, pool is crystal clear. SWG reverses once being set at 2 hours for the reversal. Pool is used daily.

Additionally, the walls seem to get less, or no, iron staining; a problem here in Hawaii. I have read here on the forum that keeping the pH at 7.5 or below helps with this however 7.4 seems better in controlling iron stains. This could of course be helped by the higher calcium, any thoughts?

I initially wished to keep the pH at 7.5 as this was closer to that of the human eye, which I also read somewhere on the forum; although I am not sure this slightly lower pH makes much difference to one's eye comfort while swimming, any thoughts here?

Does anyone see any issues with these numbers, especially in regards to the "Calcite Saturation Index (CSI)" of -0.092, is this negative enough to keep the salt cell and heat pump relatively clear of calcium build-up?

Finally after all the questions I have asked recently I believe and hope that I am done for at least a few months and thank all of those that have helped.

Here are the numbers (using chem geek's "Pool Equations" spreadsheet), initial 1st & revised 2nd:

Measured pH 7.5 7.4
Total Alkalinity (ppm CaCO3) 80 80
Free Chlorine (ppm Cl2) 5 5
Cyanuric Acid (ppm CYA) 30 30
Calcium Hardness (ppm CaCO3) 475 600
Total Dissolved Solids (ppm) 3430 3427
Total Borate (ppm Boron) 50 50
U.S. Gallons 10000 10000
Temperature (F) 86 86

Total Chloride (ppm NaCl) 3000 3000
Carbonate Alkalinity (ppm CaCO3) 63.25 64.82
Langelier Saturation Index (LSI) 0.03 0.04
% HOCl (vs. Total Free Chlorine) 0.015 0.016
OCl- (as ppm Cl2) 0.098 0.081
HOCl (as ppm Cl2) 0.077 0.08
Calcite Saturation Level (CSL) 0.796 0.809
Calcite Saturation Index (CSI) -0.099 -0.092

Hard-sided pool or vinyl? If it's concrete or shotcrete or gunite or tile or plaster it makes sense. You keep calcium in your water to keep it from leaching out of the walls. When it leaches out, it increases the calcium in the water and that is alkaline. So if your water has calcium in it, it cannot leach out and your pH stays where you put it.

CarlD,

Thank you for your quick reply.

It is concrete formed in place with plain white plaster.

Would it be correct to state, if I'm understanding you correctly, that if there were not enough calcium in the pool then the calcium measurement level would slowly rise; as it would be leached out of the wall, and/or the pH would rise to compensate?

Aloha

Quote:

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Originally Posted by smallpooldad

CarlD,

Thank you for your quick reply.

It is concrete formed in place with plain white plaster.

Would it be correct to state, if I'm understanding you correctly, that if there were not enough calcium in the pool then the calcium measurement level would slowly rise; as it would be leached out of the wall, and/or the pH would rise to compensate?


Aloha

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Almost, but not quite. The pH isn't rising to compensate, it's rising because calcium added to the water is alkaline and that increases the pH.

If you find you are having troubles with your FC dropping as well, for you, a good solution may be using tri-chlor pucks--they add CYA (and in Hawaii, you may want to run higher CYA because of the heat and sun) and they are very acidic, which fights the tendency of the calcium to drive pH up. But be sure to test your water every week and stop using the pucks when CYA hits your target, whereever you set it.

That's why, IMHO, in-line chlorinators are a waste of money. Floaters are $10-$15, and you only use them some of the time, until CYA requires stopping.

I respond to this issue in this thread in The China Shop since it's a technical response.

Richard

Thank you both for your responses, they are very helpful.

I do not at the present time have any issues with FC dropping, it stays remarkably static at around 5 even with a cYA of 30-35.

I use to use chlorine from WalMart and adjust the acid manually. Now with the SWG I do not have to do that. I really enjoy the fact that the in-line chlorinator does that for me as I cannot always attend to the pool when I travel for a day or few days a week. So for me it was a good purchase.

My main issue was whether or not high Calcium (575-600) numbers and a lower pH of 7.4 was a good idea. From what chem geek states in the"China Shop" post it was not such a good idea. So I will adjust to Calcium 375 and a pH of 7.5.