Sk8,
The flow switch assembly does NOT measure the salt level..
It does have a water temp thermistor that tells the cell what the water temp is.. The cell than calculate the salt level based on current flow through the plates.
Replacing the flow switch often fixes the problem when the thermistor is bad and the salt reading is not correct.
There are two kinds of flow switches.. 3-wire and 4-wire... If your cell has a 3-wire cable between the cell and the flow switch, then you MUST use a 3-wire replacement.
Thanks,
Jim R.