Abstract: A system for assessing the effectiveness of a water softener or comparable liquid-processing apparatus whose effectiveness is related to the difference between the electrical conductivity of the liquid supplied to the apparatus and the electrical conductivity of the liquid that has passed through the apparatus includes two inductance loop conductivity sensors excited by a common signal generator. The outputs of the sensors, which typically include unwanted quadrature components, are applied directly to the inputs of a differential amplifier. The resulting difference signal is then applied to a detector that is gated by the common excitation signal, so as to produce a detected signal that is substantially unaffected by the unwanted quadrature components. In a preferred embodiment, each of the inductance loop conductivity sensors includes two electrodes connected by a conductor that links two toroids.