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Liquid conductivity measuring circuit

  • US 4,656,427 A
  • Filed: 01/24/1986
  • Issued: 04/07/1987
  • Est. Priority Date: 02/05/1985
  • Status: Expired due to Fees
First Claim
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1. A liquid conductivity measuring circuit for use a four electrode conductivity cell having two potential electrodes (P1 and P2) and two current electrodes (C1 and C2) positioned in the cell in spaced relation in the sequence (C1, P1,C2,P2) comprising:

  • (a) a first operational amplifier having its output connected to C1 and its inputs connected via an isolating capacitor to P1 and via a buffer amplifier and a voltage setting resistance to a source of square-wave voltage,(b) a first switch connected across the first operational amplifier between its output and noninverting input,(c) a reference resistance connected at one end to current electrode C2 and having current and potential tapping points,(d) a second operational amplifier having its inputs connected to ground and via an isolating capacitor to P2 and its output connected to the reference resistance such as to control the current through the reference resistor to maintain electrode P2 at a constant potential,(e) a second switch connected across the second operational amplifier between its output and non-inverting input,(f) readout means for measuring the voltage across appropriate points of the reference resistor,(g) said first and second switches when closed defining a quiescent mode of operation in which the electrode voltages and amplifier outputs are maintained at ground potential so that the only loads on the power supply are the quiescent currents of the operational amplifiers and the readout means and when open, an active mode in which the voltage between the potential electrodes of the cell is controlled by the first operational amplifier to bear a fixed relation to a square-wave driving voltage by supplying current to electrode C1,(h) said readout means adapted to measure, on initiation of a single or small number of square-wave voltage cycles in a period short enough that no significant change of polarization of the potential electrodes can occur, the voltage between the positive and the negative half cycles or the change from zero to either half cycle of the voltage across the resistor, said measurement being a function of the current through the reference resistor and conductivity cell and therefore a measure of the conductivity of the liquid in or passing through the cell.

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