How critical is CSI for an old Gunite pool?

bostonpool

Well-known member
May 29, 2017
79
Worcester, MA
Hi folks:

Just wrapping up our second season with the pool in our new home. Its a 40 year old Gunite pool.

Overall, this year has been relatively easy. My FC, PH, CH, and TA have been pretty much in range all season with just a few minor corrections here and there.

I did a full test of the pool today (except for CYA.) And here are my results:
FC: 9.5 (A little high as my pump timer had been mistakenly left on for the past 3 days.)
CC: 0.5
PH: 7.5
CH: 400
TA: 50
Salt: 3200 ppm
Temp: 80 deg

I'm pretty much right down the middle / in the target areas for all of the values except for TA which is a little low.

Imagine my surprise when I punch all of the above into PootMath and it says by CSI is -0.63 which gives a warning that it is likely "corrosive to plaster."

I've never really paid much attention to CSI. How critical is it and why is it I may have a problem if most of the values are in range except FC being a bit high and TA being a bit low?
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
22,396
Laughlin, NV
The low CSI is due to your low TA. Add 20 ppm of TA with baking soda.

What is your CYA reading? That is critical to know what your FC should be.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
22,396
Laughlin, NV
The low CSI is due to your low TA. Add 20 ppm of TA with baking soda.

What is your CYA reading? That is critical to know what your FC should be.
 

JimMarshall

Gold Supporter
TFP Guide
Jun 5, 2017
804
Oil City PA
Corrosive is corrosive, regardless of the age of the pool.

CSI is a measure of the calcium saturation in the water, and a predictor of how likely the calcium is to either a) come out of solution in the form of scale on your water line, swg plates, etc, or b) be so undersaturated that it causes calcium to leach out of the concrete surfaces in the pool in an attempt to balance the water. This will weaken the pool, among other problems. As Marty stated, raise your TA and you’ll find the CSI back well within range.

FYI a negative CSI is corrosive and causes calcium to leach, a positive CSI is a predictor of scale formation.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
9,677
Northern NJ
You should raise your CSI to get it above -.6 You don't say what your CYA is but if I assume a CYA of 70 when I plug your numbers into PoolMath I get a CSI of -.55.

Let your PH rise to 7.8 and increase your CH from 400 to 500 and I calculate your CSI at a nice -.19

2018-08-28_2047.jpg
 

bostonpool

Well-known member
May 29, 2017
79
Worcester, MA
Thanks all. I haven't measure CYA in a while. It was a tad low (60-70) when last I read it in July. All I know is my FC is holding (although also high).

Is there a reason I should raise CYA if my FC is holding? I suppose one reason is I could potentially back down on my SWG which right now is at 50% running for 12 hours.

I'm raising TA today. That should do it.
 

bostonpool

Well-known member
May 29, 2017
79
Worcester, MA
zero that I can see. But I haven't really checked.

Putting the values in today it seems poolmath shows me at -0.41

Still too high but better. Will increase TA tomorrow and that will get it down.
 

mdogan

Bronze Supporter
Nov 20, 2017
96
Frisco,TX
i also advise you to keep you PH around 7.8. TA will go little higher and should be stable around 60.

then summer/winter never had issue with CSI. Only the issue It may take little time to stablize PH at 7.8 but after that you would be very comfortable.
PH 7.8 sometimes hard to figure out on chart but i resolved issue to use acid demand reagent. Always one drop after PH and make sure it should be stable as well :)

Good luck.
 

duraleigh

Admin
Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
In The Industry
Apr 1, 2007
32,729
Sebring, Florida
zero that I can see. But I haven't really checked.
Well, then csi would not make my list of top 50 things I need to check. You don't post the numbers but I bet your fill water does not lend itself to calcium saturation.......ever.

Keep your five basic parameters within the TFP guidelines and let the folks in the desert Southwest worry about csi......you don't need to.

Also it is NEVER a good idea to increase CH unless it is below 150 or so. It will rise somewhat on it's own and, like CYA, tough to deal with when it's too high.