Abstract: Detection of exhaustion of sodium form water softeners is accomplished by subjecting sample of the outflow from a water softener to reverse osmosis in order to separate monovalent (sodium) and divalent (hardness or magnesium and calcium) ions differentially from the outflow and measuring conductivity before and after the reverse osmosis. The two conductivities are compared in a ratiometer or differential device. After the softener becomes exhausted the divalent ion concentration in the outflow increases. Consequently the conductivity ratio changes to indicate that the water softener has become exhausted.