If you want to test the performance of your softener, you want a hardness test kit. The water would be considered soft if the hardness is less than 1 grain per gallon.
There are no practical digital test kits for hardness. Typical hardness test kits use a drop-count titration method. You add a color indicator to the sample; if the color is red the water contains hardness. You add a second chemical (EDTA) to the water dropwise, with each drop typically representing 1 gpg, until the solution turns blue.
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