Saturation Index

[chem geek]

Donnie,

I would not test for TDS. It takes a HUGE change in TDS to affect the saturation index so it is almost negligible. Besides, you can readily approximate your TDS by measuring the amount of chloride you have, which is measured using the Taylor K-1766 which is only $21.35. Most of your TDS is likely to be salt so measuring the chloride will give you a reasonable idea of your TDS -- not that it matters much anyway. Remember that the main chemical you are adding to your pool is chlorine and that this mostly gets converted to sodium chloride (if using chlorinating liquid or bleach) or calcium chloride (if using Cal-Hypo).

With normal levels of TA and CH and CYA, the amount of TDS is about 175-185 ppm more than the amount of ppm salt from the chloride test. This ignores organics that may stick around in your pool and assumes no sulfates, but that's usually a reasonable assumption. My spreadsheet will calculate this relationship between measured chloride (as ppm sodium chloride) and TDS assuming that there is nothing in the water except what you list as coming from CH, TA and CYA and sulfates (and FC, but that's very minor).

Richard