Methods and circuits for measuring the conductivity of solutions
First Claim
1. A method for measuring the conductivity of a solution disposed between two electrodes, such that the error in said measurement produced by the conductance of a parallel capacitance CP across said electrodes is minimized, comprising the steps of:
- applying a bipolar square wave drive signal having a period T across said electrodes through a series resistance RS ;
changing CP during a first portion tc of each half cycle T/2 of the bipolar drive signal, while maintaining RS at a sufficiently small value during tc to allow CP to charge substantially to its maximum value during tc ;
measuring the current conducted between said electrodes during a latter sampling time portion ts of each half cycle T/2 of the bipolar drive signal T; and
determining the conductivity of the solution responsive to the current measured in said measuring step.
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Accused Products
Abstract
Improved circuits for measuring the conductivity of a solution confined between two electrodes in a cell compensate for series capacitance and parallel capacitance between the electrodes. A bipolar square-wave signal is applied to the cell. In one embodiment, the current through the cell is measured by an op-amp in current-to-voltage converter configuration. A feedback resistance employed with the op-amp in a feedback loop is controlled to a low value to ensure that the parallel capacitance is fully charged during an initial portion of each half-cycle of the drive signal. The feedback resistance is then selected so that the gain of the feedback loop is responsive to the range of the resistivity of the solution, and the measurement is made. The period of the bipolar signal is selected responsive to the selected loop gain, to ensure that a filter capacitor across the op-amp is fully charged, and to limit distortion caused by the series capacitance. More particularly, the rate of charge of the series capacitance is proportional to the solution resistance; by varying the period of the drive signal in accordance with the solution resistance, the distortion introduced by the series capacitance remains negligible.
72 Citations
33 Claims
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1. A method for measuring the conductivity of a solution disposed between two electrodes, such that the error in said measurement produced by the conductance of a parallel capacitance CP across said electrodes is minimized, comprising the steps of:
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applying a bipolar square wave drive signal having a period T across said electrodes through a series resistance RS ; changing CP during a first portion tc of each half cycle T/2 of the bipolar drive signal, while maintaining RS at a sufficiently small value during tc to allow CP to charge substantially to its maximum value during tc ; measuring the current conducted between said electrodes during a latter sampling time portion ts of each half cycle T/2 of the bipolar drive signal T; and determining the conductivity of the solution responsive to the current measured in said measuring step. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
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12. A method of measuring the conductivity of a solution disposed between two electrodes in a cell, such that a cell resistance RX and a cell capacitance CX are exhibited between said electrodes, comprising the steps of:
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applying a bipolar square wave drive signal of controllable period T to said electrodes, whereby the rate dCX /dt of charging of CX during each half cycle T/2 of the bipolar drive signal is proportional RX ; controlling the frequency of said bipolar drive signal in accordance with the cell resistance RX such that the total charge accumulated by CX during T/2 is small; sampling the current conducted through the cell during each half cycle T/2 of the bipolar drive signal; and determining the conductivity RX-1 of the solution in the cell responsive to said sampling of the current through the cell. - View Dependent Claims (13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23)
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24. A method for measuring the conductivity of a solution disposed between two electrodes in a cell, that the error resulting from the presence of a series capacitance CX between each electrode and the solution is minimized, comprising the steps of:
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applying a bipolar square wave drive signal with a controllable period T to said electrodes, wherein the rate of charge of CX during each half cycle T/2 of the drive signal is substantially proportional to the resistivity RX of the cell; varying the period T responsive to the approximate value of RX, such that the voltage drop across CX during T/2 is maintained below a desired error limit; sampling the current conducted through the cell during each half cycle T/2 of the bipolar drive signal; and determining the conductivity of the solution in the cell responsive to said sampling of the current through the cell. - View Dependent Claims (25, 26, 27)
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28. A method for measuring the conductivity of solutions disposed between two electrodes, wherein said solutions exhibit a widely variable series resistance RX, a series capacitance CX in series with RX, and a parallel capacitance CP across said electrodes, comprising the steps of:
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applying a controllable-frequency bipolar square-wave drive signal to one electrode; connecting the second electrode to the inverting input of an op-amp; grounding the noninverting input of said op-amp; connecting a variable feedback resistance Rf between the inverting input and an output of said op-amp; determining the approximate value of RX ; controlling the period T of said bipolar drive signal in accordance with the approximate value of RX, such that when RX is high, T is long, and vice versa; controlling Rf to be low at least during an initial period tc of each half-cycle T/2 of the bipolar drive signal, such that CP is charged to substantially its full value during tc ; controlling Rf during the remainder of T/2 in accordance with the approximate value of RX ; measuring the current Icell output by said op-amp at a sampling time ts during the latter portion of T/2; and determining the conductivity RX-1 response to said measurement of Icell. - View Dependent Claims (29, 30, 31, 32, 33)
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Specification