Yes I'm impressed -- clear graphs beat equations for understanding. And Evan knows a lot of chemistry and probably remembers some things better than I and has interesting and relevant real-world experience. And sodium chloride (salt) when dissolved in water is really separate sodium and chloride ions so the sodium from sodium hydroxide doesn't really do anything significant at all. If you increase your ion levels A LOT as with saltwater pools, including SWG pools, then the higher concentration of ions affects the ionic strength and that affects all chemical equations that have ions in them (especially ions on one side and not the other, or higher charges on one side compared to the other). This is why the TDS is part of the saturation index -- it's for taking into account this ionic strength effect.

Richard