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Abstract

A device for measuring liquid conductivity or resistivity also quantifies series capacitance and compensates for parallel capacitance to achieve accuracy over a range of several orders of magnitude. A first signal of known frequency is applied to a conductivity cell and reference resistors, and the voltage drop across the cell is sampled. This is repeated at a second frequency. The sampled voltages are proportional to the cell's impedance, and the difference in impedance at the two frequencies allows calculation of the capacitive and resistive components of the impedance. Subtraction of the capacitive components yields resistivity measurements that are highly linear. When the cell contains a reference solution, the capacitive component can be correlated to monitor electrode surface area and thus electrode corrosion. With an unknown liquid, its contribution to capacitance can be used for species identification such as pH or concentration of components in multi-component liquids.

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First Claim