Understanding CSI (Calcite Saturation Index)

[steveg_nh]

I'm helping a friend open up his pool and make sure his tests and water are in good shape. Things are looking very good, but CH is a bit high. But from what I remember last year (when we took this pool from green to clean), CSI must also be looked at before you determine if you really need to replace any water to lower it.

First some background.

24,000 gallon gunite pool, about 15 years old I'd say. Manually dosed (no SWG).

Test Results:

pH: 7.6
TA: 90
CYA: 0 (adding CYA to get to 40, so doing that way)
FC: 5
CC: 0
CH: 475-500

For his pool, we see CH as ideal around 300. But his CSI is 0.25. If I understand poolmath correctly, until you are below -0.6 or above 0.6, he is still ok and doesn't need to change out any water.

If this is true, why not? What does CSI tell you, and why is it not an issue, even though it's way above the range?

Thanks for the (upcoming) chemistry lesson!

 

[JoyfulNoise]

I'm helping a friend open up his pool and make sure his tests and water are in good shape. Things are looking very good, but CH is a bit high. But from what I remember last year (when we took this pool from green to clean), CSI must also be looked at before you determine if you really need to replace any water to lower it.

First some background.

24,000 gallon gunite pool, about 15 years old I'd say. Manually dosed (no SWG).

Test Results:

pH: 7.6
TA: 90
CYA: 0 (adding CYA to get to 40, so doing that way)
FC: 5
CC: 0
CH: 475-500

For his pool, we see CH as ideal around 300. But his CSI is 0.25. If I understand poolmath correctly, until you are below -0.6 or above 0.6, he is still ok and doesn't need to change out any water.

If this is true, why not? What does CSI tell you, and why is it not an issue, even though it's way above the range?

Thanks for the (upcoming) chemistry lesson!

First off, you really want to stay within -0.3to +0.3. 0.6 is the outer limits of what's safe.

Second, you can easily manage water with a CH of 500ppm. My water is 850ppm and I get no scaling. If you adjust (lower) your TA down to 60-80, you'll easily get close to 0 CSI. Also, you should use 40ppm as CYA since that affects CSI too. Test for salt at the pool store unless you have a kit for it, I bet your friends salt level is a lot more than 0. Salt also affects CSI.

Finally, CSI tells you if scaling or etching of the plaster is possible. It does not predict how fast it will happen. If you can get your friends pool close to 0 CSI, it will be perfectly fine. No need to waste water.

 

[Richard320]

Go plug all your numbers into poolmath and look at CSI. Then see what happens when the temperature goes up or down. See what happens when pH creeps up to 7.9. Heck, see what happens if someone slacks off and it gets to 8.

+/- .6 it where it says it will start scaling or etching. In reality, the range is more like +/- .3 because there can be areas with poor circulation where the pH or the CH can be higher or lower.

 

[steveg_nh]

I did enter CYA of 40 in poolmath, just an FYI. But I did not adjust temp...I just adjusted temp to 60, and CSI is now 0.04. Perfect, right? And with these levels, at 82 degrees (summer temps), CSI would be 0.24. So as the water gets warmer, we should just lower TA....cool.

 

[JoyfulNoise]

I did enter CYA of 40 in poolmath, just an FYI. But I did not adjust temp...I just adjusted temp to 60, and CSI is now 0.04. Perfect, right? And with these levels, at 82 degrees (summer temps), CSI would be 0.24. So as the water gets warmer, we should just lower TA....cool.

Pool Math assumes a small amount of salt if you put 0 in for salt. However, most people using chlorinating liquids, powders and muriatic acid will have chloride concentrations in the 1000ppm range if they are not doing fresh water exchanges. Salt modifies the CSI by a little bit so at some point you might want to get a reading on his salt (chloride level) to be more accurate.

Yes, adjusting pH and TA targets will have very significant impact on the CSI value.

 

[steveg_nh]

Cool. Thanks.

I can test his pool for salt, as I have the K1766 kit. So anyway, looks like all is perfectly fine with those CH readings. Thought so, and really cool to understand the relationships of all the various levels.