Scale and the CSI vs LSI


Active member
Apr 18, 2010
I lowered my water line by an inch and now I see a new scale line.

I have recently been enlightened, having learned about TFP, water balance, CSI, etc. Before my enlightenment, I had allowed a stark ring of scale to form. Most likely by allowing my CSI to drift too high in the positive range. At the beginning of the summer I removed this scale with elbow grease by chipping and scrubbing with pumice stones.

I am now a little discouraged because the scale is beginning to return. I have tested my water almost daily throughout the summer and have kept my CSI in the negative range -0.1 to -0.3.

The CSI is an overall indicator of corrosive versus scale-prone water. Water with a CSI level that is slightly negative should remove just a tiny bit of scale - not deposit it.

For example:

FC 4.6
CC 0.2
pH 7.5
TA 50
CH 550 (fill water 110)
CYA 40
Salt 3800-4500 (depending on who does the test)
Temp 88
CSI -0.27
LSI 0.13 (from the Taylor Watergram wheel)

My CH is high so I have to be careful to keep the CSI out of the positive range. However, the corresponding LSI is slightly positive. I hate to say this but does this mean that the LSI is more accurate? If it were more accurate it would explain the deposit of the scale. Am I correct in assuming that slightly corrosive water should not deposit scale?

I may need to do a partial drain to get the CH down so that I don't have to fight the scale as hard.

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
Scale can deposit on the water line even with a somewhat negative saturation index due to evaporation of the water which concentrates calcium carbonate (you can get salt deposits this way as well). That is, as the water evaporates it can deposit solids on the surface. If the saturation index were too high, you would be getting scale in submerged areas as well. You want the saturation index set to protect your overall pool surfaces. Weekly brushing will usually be enough to prevent a scale line -- when I brush my pool I include the tile line during the brushing. If that isn't enough, then someone else may suggest a product that might help.