Why are the values for SI between the dial in my Taylor K-2006C and as calculated in the pool math app so different?

[TazDoug]

Here are my test results from last night:
FC: 8.0
PH: 7.2
TA: 80
CH: 1000
CYA: 37
Temp: 70F (estimated)

SI (using the dial in my test kit): 0.11
SI (Pool match calculated): -0.05

Why are the above values for SI so different?

 

[Donldson]

The wheel with the k2006 calculates LSI (Langelier Saturation Index) but PoolMath and TFP calculates and uses CSI (Calcium Saturation Index).

If you want to get in to the weeds about the difference you can read up on it here: Langelier and Calcite Saturation Indices (LSI and CSI)

The TL;DR of it is that LSI is somewhat simplified and TFP considers the CSI to be more accurate of the scale potential of water.

 

[TazDoug]

Thank you, Donldson, I appreciate your prompt answer!