Corrosion and Scaling with SWG

[Steve456]

As a soon to be new SWG user I am trying to understand and balance my water before I start my SWG. After I plug the recommended values from the "Water Balance for SWGs" into the Pool Calculator I find a Calcite Saturation Index of -.58 to -.2. The advice is to reduce the pH to 7.5 when the pH reaches 7.8. This one change brings the Calcite Saturation Index to -.44.

While I realize that with a SWG the pool owner must avoid scaling on the SWG plates the recommended ranges appear to be borderline corrosive to plaster. Also, the Calcite Saturation Index appears to indicate more corrosion than the Langelier Saturation index does. Which index is the better predictor of corrosion and scaling?

 

[JasonLion]

CSI is a better predictor than LSI. LSI takes more numerical shortcuts so that it is easier to calculate by hand. However, neither of them are really great predictors of corrosion and scaling, more like suggestive guidelines.

There are a couple of reasons why the recommended levels give you a fairly low CSI. First, the CSI inside the SWG cell will be significantly higher than it is in the pool. There are regions of high PH inside the cell. Second, it is far more common for a SWG pool to have the PH go out of range high, while it is quite rare for a SWG pool to have the PH go out of range low.

 

[Guest]

The LSI has never been a predictor of 'corrosive' (actually a misnomer) water but is a predictor of the scaling propensity. There are other indicies that better predict the aggressiveness of water against plaster. Scaling conditions are much easier to model and predict and the majority of these saturation index models were originally designed to model scaling in boiler and water systems, not pools per se.