Methods and circuits for measuring the conductivity of solutions

DETAILED DESCRIPTION It is therefore an object of the invention to provide a circuit and methods for measuring the conductivity of solutions that minimizes the effects of series and parallel capacitance inherent in cells containing samples of such solutions.
The above object of the invention and others which will appear as the discussion below proceeds are satisfied by the present invention, which comprises circuits and methods for accurately measuring the conductivity of solutions of widely varying conductivities.
More particularly, it has been realized by the present inventor that entirely different techniques are desirably employed for compensating for the parallel and series capacitances C.
sub.
P and C.
sub.
X, due principally to their relatively large disparity in values; C.
sub.
P is normally orders of magnitude smaller than C.
sub.
X.
It is an object of the invention to provide such different techniques.
As noted, it is desired to measure the conductivity of the solution by measuring current flowing through the solution responsive to an applied square-wave drive signal.
The effect of the presence of C.
sub.
P and C.
sub.
X is to distort the square-wave current signal due to charge being stored over time on the capacitances 18 represented by C.
sub.
P and C.
sub.
X.
It has been realized by the present inventor that if the parallel capacitance represented by C.
sub.
P is fully charged prior to sampling the conductivity, the presence of C.
sub.
P and the error produced by C.
sub.
P will be fully compensated.
The rate of charge of C.
sub.
P is a function of the total equivalent series resistance through which the drive signal applied to the cell passes.
This series resistance is normally the drive source resistance in parallel with the solution resistance R.
sub.
X.
However, it has been realized by the present inventor that under some circumstances, the input resistance of the current-to-voltage converter typically used to measure the resistance of the cell can contribute to the series resistance.
More particularly, when the feedback resistance R