Michael, you are correct. Ca is needed for the saturation index. Water temp, PH, alkalinity, calcium hardness and total dissolved solids are all used in determining the saturation index. If the SI is too low, any metal in the pool will start to dissolve. Heaters are especially vulnerable due to the metal content in the heat exchanger.

Tracy,

If you can post the PH, Alk, Temp, Ca and salt level, I can give you the SI. To learn about it yourself, Google: "Langelier saturation index" for more information.

Also, here is an online calculator: http://www.advancedh2o.com/technical.../langlier.html