Perfect Range 0 TO 2,000 PPM

Temperature + Total Alkalinity + pH + Calcium Hardness + Stabilizer Adjustment + TDS = Saturation Index

Total Dissolved Solids is a measure of the gradual build up of chemical by products.  The continual "dumping" of chemicals in the pool water and constant chemical reactions taking place eventually leads to a build up of unwanted solids in solution.  However, most outdoor pools lose a significant amount of water due to splashing as well as from partial draining due to rain and filter backwashing.  Spa's are completely drained and refilled on a periodic basis for general maintenance.

Most pool and spa water is below 2,000 PPM and has little effect on water balance.  It is only mentioned here because it is a factor that is plugged into the Saturation Index formula.