Calcium deposits?

[halefmly]

I just went to the Pool $tore for a replacement bag for my Pentair Pool cleaner, and decided to get a water test. Since the post above, I have kept my Alkalinity down between 80 and 90 for the past two months. (quite a feat with the highly Alkaline water supply in North Texas). Well, the Pool $tore said that I needed to get my Alk up to 100 to prevent "etching the plaster."

Is this real? Should I worry about it? What is the worst thing etched plaster can do?

The Pool $tore numbers are:

FC: 9.4
CC 0.6
TC 10
pH 7.1
Alk 88
TDS 4700
Cya 80
Temp 74
Salt 4000

My numbers with the PS234s:
FC: 11
CC 0
TC 11
pH 7.6
Alk 80
Cal 500
Cya 60
Temp 65
Salt 3600
Do you have any wisdom to share on the Alkalinity and on the variance in the numbers?

[chem geek]

The numbers from your test kit are perfect. The higher calcium hardness compensates for the lower total alkalinity so your water is in perfect balance (and remember that it can get quite a way out of balance before there are problems).

The variation with the pool store numbers that is most distrubing is the pH. The store didn't measure calcium (or at least you didn't show that) and with their measured pH of 7.1 that would indeed indicate corrosive water that could etch or dissolve plaster, though it's still only starting to get out of balance (saturation index of -0.46 though if the pool store assumed a calcium of 300, then the index would be -0.67 which is more worrisome and probably why they made their comment).

Basically, the pH affects the corrosive vs. perfect vs. scaling tendency of your pool water more than any other factor so if your pH is correct (7.6) then you are fine but if the pool store pH measurement is correct (7.1) then you would need to get the pH up. I would trust your numbers over the pool store, but perhaps you could get a cheap separate test for pH just to be sure (perhaps some test strips). If your test kit and cheap test match, then you could let the pool store know about that and see if they can do a similar test with their own water to see if there's something wrong with their pH measurement (might be a meter that uses a pH measurement cell that needs calibration).

Richard

[halefmly]

Richard,
Thanks for the reply. I took your advice and retested my pH with a test strip and again with my drop test. The strip indicated 7.8 and the drop test 7.6.

The store printout did not give me a value for calcium, just Total Dissolved Solids. I think my calcium is so high because I was shocking with CalHypo before I discovered this forum and switched to the simpler method of testing weekly and adjusting only as needed.