Conductivity measuring circuit

A conductivity measuring circuit for providing a compact portable instrument for measuring the conductivity of a solution, comprising:

• a pair of electrode input terminals for connecting to electrodes forming a conductivity cell;
• voltage input terminals for connection to a source of known dc voltage;
• a known resistance;
• a pair of current input terminals;
• first circuit means connecting said dc voltage input terminals in series with said current input terminals and said known resistance so as to produce a known dc current source across said current terminals;
• an analog-to-digital converter of the dual slope type having an input signal integration phase and a reference voltage integration phase, said converter being in the form of an integrated package and having associated circuitry which includes
• a clock for timing the conversions of said converter,
• a reference capacitor,
• a pair of differential converter signal input terminals for receiving a voltage drop as the input signal for one of said integration phases of said converter, and
• a pair of differential converter reference input terminals for receiving another voltage drop as the input signal for the other of said integration phases of said converter;
• first circuit means connecting said dc voltage input terminals in series with said current input terminals and said known resistance and for connecting said known resistance across said reference input terminals so as to produce a known dc current source across said current terminals;
• switching means for connecting said current input terminals alternately to said electrode input terminals in one sense and then the other at a rate synchronous with the operation of said converter and sufficient to prevent polarization of the electrodes;
• and
• second circuit means for connecting said electrodes to said converter signal input terminals for conversion of the voltage drop across said electrodes to a digital output indicative of the magnitude of the conductivity of said solution in said cell.

In a ratiometric conductivity measuring circuit for measuring the conductivity of a solution in a conductivity cell wherein the circuit has an analog-to-digital converter of the dual slope type which includes an integrating circuit, a clock circuit for timing the periods of integration carried out by the integrating circuit in sequential conversion cycles, first and second pairs of voltage input terminals, first switch means for connecting said integrating circuit to receive a first input from said first pair of voltage input terminals to integrate the voltage received at those terminals for a fixed period of each sequential cycle as timed by said clock circuit to produce a voltage output from said integrating circuit ramping from a reference value for said fixed period as a first part of the conversion cycle, and for subsequently switching said integrating circuit to integrate a second voltage sampled over a short period from the voltage appearing at second terminals as a second part of said conversion cycle to produce a voltage output from said integrating circuit which ramps back toward said reference value for a variable period sufficient to bring the output of the integrating circuit back to said reference value so that said converter produces a digital output proportional to the second voltage, the combination comprising:

• a pair of current input terminals;
• cell terminals for connection to a conductivity cell immersed in the solution whose conductivity is to be measured;
• a known standard resistance;
• circuit means forming a series circuit including said current input terminals and said standard resistance in series with a voltage source and for connecting said cell input terminals to said second voltage input terminals and said standard resistance across said first voltage input terminals;
• and
• second switch means for connecting said current input terminals alternately to said cell terminals in one sense and then the other at a rate determined by said clock circuit and sufficient to prevent polarization of said cell, said switch means being driven in synchronism with said clock, whereby said cell receives an ac voltage across its terminals and said second voltage input terminals of said converter receive a voltage drop for conversion to a measurement of the conductivity of said solution in digital form.

A circuit for measuring the conductivity of a solution, comprising:

• electrode terminal means for receiving a pair of electrodes immersed in said solution;
• means connecting a source of known ac current across said electrode terminal means;
• a dual slope analog-to-digital converter of the integrated circuit type having a conversion cycle including a signal input integration phase and a reference integration phase, said converter including signal input terminals for receiving the signal to be converted, a clock for controlling said phases of converter operation, and a capacitor for sampling the signal input in synchronism with said clock;
• means for connecting said signal input terminals to receive as the input for conversion the voltage drop across said electrode terminal means due to said ac current;
• and
• means for synchronizing said ac current to said clock.