Help! Pool math leads me to CSI of -0.65?

xyz

Gold Supporter
Sep 8, 2016
641
Escondido/CA
So I'm getting ready to put in new water into my SWG pool, and am looking for my targets.

So so I ran pool math, shooting for the middle of the range on all measures, with a starting salt level of 3000.

PH - 7.6
TA - 70
CH - 170 (I tested my tap water)
CYA-75

ok, good, so I'm right in the middle of every recommendation.

But this yields a CSI of -0.65!

So can I get some help/thoughts/guidelines for better targets? I don't want to add CH because that will just age my water mode quickly (or maybe I don't need to care?

Maybe I'm not doing it right?
 

bdavis466

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Aug 4, 2014
5,529
San Clemente, CA
You would target a slightly higher TA and pH to compensate for the lower CH.

A pH of 7.8 and a TA of 100 should help for the short while your calcium is low. With all that said, that CSI value you got really isn't all that bad for the short term. You definitely do not want to add any calcium.

I recall you mentioning borates as well. Keep in mind that 50ppm will further drop the CSI by 0.1.
 

xyz

Gold Supporter
Sep 8, 2016
641
Escondido/CA
Thanks Brian. I think I'm going to spend more time looking into the chemistry of TA. I have a chemist that I work with that may be able to help me understand better also.

- - - Updated - - -

Ah yes, let me add the borate level also.
 

bdavis466

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Aug 4, 2014
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San Clemente, CA
CSI takes every tested water parameter into account to produce a numerical value that indicates how over/under saturated the water is with calcium carbonate (the primary ingredient of plaster and scale). Some calcium carbonate is needed in the water to protect plaster surfaces, if it is low the water will be aggressive and begin to dissolve and etch the plaster. This is not an overnight thing either, it takes many weeks and months at very low CSI values to produce noticeable effects.

TA is a measurement of bicarbonate but actually gets calculated in ppm of CaCO3 (calcium carbonate). Not to get too in-depth but the bicarbonate in the water combines with the calcium hydroxide in the plaster and forms calcium carbonate at the plaster surface which provides the necessary saturation of the water to not become too aggressive to the plaster but is also not a permanent addition to the water that can't be easily removed (add acid and aerate to lower TA as the CH rises from evaporation and refill).
 

bdavis466

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Aug 4, 2014
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San Clemente, CA
People that have trouble with scale always have high TAs in addition to high pH levels. pH and TA go hand in hand and it is relatively difficult to have a high pH with a low TA without actively monitoring the water chemistry and making efforts to keep that way.

The biggest powerhouse of CSI is pH. The next greatest influence comes from TA. CH really doesn't have as big of an impact on CSI numbers as you would think until the CH becomes very, very high or is absurdly low. Water temperature also greatly influences CSI. Higher water temperatures drastically raise the CSI as it becomes easier for the calcium carbonate to precipitate out of the water and form scale (often time the first place scale is seen is in spas and heaters).

Plug in some different numbers in Pool Math and it's easy to see how each change effects the CSI outcome.
 

xyz

Gold Supporter
Sep 8, 2016
641
Escondido/CA
Yep. borates makes it more extreme. I read up on TA, and it seems that a high T would bring my CSI back up to a reasonable level, and if nothing bad happens with high TA, no ahem no foul. So I'll start there and see how it goes.

Thanks again Brian!
 

jblizzle

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May 19, 2010
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I would slightly disagree. With a SWG, the higher TA will result in your pH climbing faster. I would try to leave the TA and just keep the pH higher.
Ideally the CH needs to be higher, but I have done what you are doing and know that my CH rises about 100ppm every year, so I did not want to add CH intentionally either.
 

bdavis466

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San Clemente, CA
The TA has to be higher since a pH of 7.8 still yields a CSI of -0.61 with 68 degree water. Colder weather is coming as well so with colder water temperatures, the CSI is further decreased. Luckily the pH rise will be slower while the water is cold since the SWG won't be producing chlorine and the pool will not likely see much use so aeration should also be minimized.

A TA of 100 really isn't that high and not difficult to control pH rise with frequent testing.

Same numbers above but with a TA of 100 gives a CSI of -0.34 which is a nice compromise and allows some wiggle room for cooler water temperatures.
 

jblizzle

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Good point about the cooler water. My pH still rises faster than I like with a TA of 70. Next year I am starting over, due to high CH, and adding borates, so we shall see how that goes :D
 

ping

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Jun 24, 2011
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Long Beach, CA
I'd run the pH at 7.6 to 7.8 and bump up the TA between 90 to 100 for now and skip the borates until spring. There's not much time left for swimming as it is and if you use the spa the hot temp will drive the CSI lower.
 

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bdavis466

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That is what I meant to say:hammer:

I knew what you meant...I'm just making up for my own insecurities


I would slightly disagree. With a SWG, the higher TA will result in your pH climbing faster. I would try to leave the TA and just keep the pH higher.
Ideally the CH needs to be higher, but I have done what you are doing and know that my CH rises about 100ppm every year, so I did not want to add CH intentionally either.

Your CH really only rises by 100 ppm per year? You must be pretty religious with a cover?

Mine rises by 100 ppm per month!
 

jblizzle

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Maybe I have a leak to keep the CH low and CYA disappearing ;) except my salt level does not drop add the same rate. :D
 

bdavis466

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Aug 4, 2014
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San Clemente, CA
Is this because of the Ca of 170 in the fill water and with evaporation calcium would concentrate in the pool and eventually get too high? If the fill water had a Ca of near zero, would you manage the CSI by adding Ca?

Exactly correct. We get very little rainfall here and have high evaporation rates. Calcium levels rise quickly enough on their own that intentionally adding calcium will only ensure that a water replacement will be needed that much sooner. What is important with this is to manage the water in a way that the low calcium levels won't be an issue for the plaster (as discussed earlier in this thread).

If you have the opposite issue and find your CH is falling due to high rainfall, you would target the very high end of the recommended levels and add more calcium as it falls.
 

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