Conductivity measuring apparatus

Claim 1

Apparatus for measuring the conductivity of a liquid comprising: a. First and second current electrodes adapted to pass a current through the liquid whose conductivity is to be measured, b. First and second potential electrodes positioned adjacent said current electrodes, c. a source of regulated DC voltage comprising a regulated DC current source connected to a first resistor, d. a first reversing isolating chopper means whose input is connected across said first resistor for converting the voltage obtained therefrom to a square wave voltage having positive and negative pulses and whose output is connected across said first and second potential electrodes such that the square wave voltage is applied thereto, e. an operational amplifier having a resistance feedback path and having its input connected across said potential electrodes and providing at its output a square wave current, f. a regulated square wave power supply connected to said operational amplifier comprising a regulated DC voltage source connected to a second reversing isolating chopper means providing a square wave voltage at its output, said second chopper means being ganged to said first chopper means such that switching of the chopper means is in predetermined synchronous relation, g. a reference standard resistance connected to the output of the amplifier and in series with the current electrodes such that the output current of the amplifier passes through the standard resistance and the liquid under measurement via the current electrodes, h. a third reversing isolating chopper means operated in synchronism with the first and second reversing isolating chopper means and havIng its input connected across the reference standard resistance for demodulating the square wave voltage appearing across the reference standard resistance and providing a demodulated voltage at its output, i. a source of regulated reference DC voltage comprising the said regulated DC current source connected to a second resistor, j. means for measuring the difference between the demodulated square wave voltage and the regulated reference DC voltage and providing an output related to said difference.