I've noticed a significant difference between the SI (Saturation Index) given by the Taylor Watergram wheel and the CSI (Calcite Saturation Index) given by the TFP Pool Calculator (or the "poolcalculator.com" calculator.) These are supposed to give you the same indication of how "balanced" your water is, so it's troubling that they're far enough apart to indicate both "balanaced" and "unbalanced" water depending on which one I look at.
For example, today my readings are:
FC - 9
pH - 7.6
TA - 70.2 (corrected for 60ppm CYA at pH 7.6)
CH - 825
CYA - 60
Salt - 1800
Borate - 0
Temp - 90
On the Taylor Watergram wheel, this gives me an SI of about +0.54, which is outside of the "balanced" range of -0.5 to +0.5. The Pool Calculator, however, gives me a CSI of +0.26, which is WELL within the "balanced" range of -0.6 to +0.6. My instinct would be to trust the Pool Calculator's CSI number because is utilizes more parameters, but I can't help questioning the numbers given that they don't agree by a significant margin. Note also that this margin can be even larger under the right conditions.
Do I just disregard the Taylor Watergram wheel?
PS. As of this writing, the "poolcalculator.com" site has stopped working. The issue started with none of the "About..." links on the site working, then the whole site stopped. I just keep getting a message that says "This webpage is not available.