Methods and circuits for measuring the conductivity of solutions

Abstract

Improved circuits for measuring the conductivity of a solution confined between two electrodes in a cell compensate for series capacitance and parallel capacitance between the electrodes. A bipolar square-wave signal is applied to the cell. In one embodiment, the current through the cell is measured by an op-amp in current-to-voltage converter configuration. A feedback resistance employed with the op-amp in a feedback loop is controlled to a low value to ensure that the parallel capacitance is fully charged during an initial portion of each half-cycle of the drive signal. The feedback resistance is then selected so that the gain of the feedback loop is responsive to the range of the resistivity of the solution, and the measurement is made. In a further embodiment, charge proportional to the current through the cell is integrated across the op-amp, and the rate of charge is measured by measuring the time required to integrate the current to a predetermined level, and used to determine the resistivity of the solution. In this embodiment, the parallel capacitance is fully charged before each integration period begins. The voltage developed across the series capacitance is constant and can be compensated mathematically.

Dependencies are inferred from the claim text.