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Patent US5334940A successfully refreshed from Google Patents.
US5334940A
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Methods and circuits for measuring the conductivity of solutions
Assignee: Hach Co
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Filing: 1993-04-16
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Granted: 1994-08-02
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Expires: 2013-04-16
Abstract
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. In a further embodiment, charge proportional to the current through the cell is integrated across the op-amp, and the rate of charge is measured by measuring the time required to integrate the current to a predetermined level, and used to determine the resistivity of the solution. In this embodiment, the parallel capacitance is fully charged before each integration period begins. The voltage developed across the series capacitance is constant and can be compensated mathematically.
Inventors
- 👤 Frederick K. Blades
Bibliographic Details
Application Number
—
Jurisdiction
US
Data Source
google_patents
Last Synchronized
2026-11-08 11:58
CPC Classifications
G; G01; G01R; G01R27/00; G01R27/02; G01R27/22
Claims Analysis
Total Claims: 34
Claim 1
Independent Claim
140 words
A method for measuring the resistivity of a solution confined between two electrodes in a cell such that a series resistivity R x and a series capacitance C x are exhibited between the electrodes, comprising the steps of: applying a bipolar square wave drive signal V drive to one electrode of said cell; connecting the other electrode of said cell to an inverting input of an op-amp; grounding a non-inverting input of the op-amp; connecting an integrating capacitor C i between the inverting input and the output of the op-amp, such that C i is charged at a rate proportional to the conductivity of the solution in said cell; measuring the rate of charge of C i ; and determining the resistivity R x of the solution in said cell responsive tot he rate of charge of C i .
Claim 2
Independent Claim
140 words
A method for measuring the resistivity of a solution confined between two electrodes in a cell such that a series resistivity R x and a series capacitance C x are exhibited between the electrodes, comprising the steps of: applying a bipolar square wave drive signal V drive to one electrode of said cell; connecting the other electrode of said cell to an inverting input of an op-amp; grounding a non-inverting input of the op-amp; connecting an integrating capacitor C i between the inverting input and the output of the op-amp, such that C i is charged at a rate proportional to the conductivity of the solution in said cell; measuring the rate of charge of C i ; and determining the resistivity R x of the solution in said cell responsive tot he rate of charge of C i .
Claim 3
Dependent Claim
69 words
The method of claim 1, comprising the further steps of comparing the output voltage V sig of the op-amp with the amplitude of the square wave drive signal V drive , controlling V drive to reverse polarity when V sig reaches a predetermined fraction of V drive , and determining the rate of charge of C i by measuring the time between reversals of polarity of V drive .
Claim 4
Dependent Claim
23 words
The method of claim 2, comprising the further step of discharging C i upon each reversal of the polarity of V drive .
Claim 5
Dependent Claim
52 words
The method of claim 3, comprising the further step of maintaining C i discharged for a period of time t d after each reversal of the polarity of V drive , during which time t d a parallel capacitance C p exhibited across the electrodes of the cell is substantially fully charged.
Claim 6
Dependent Claim
23 words
The method of claim 4, wherein said period t d is timed by a one-shot triggered upon V sig equaling V drive .
Claim 7
Dependent Claim
23 words
The method of claim 5, wherein said one-shot is triggered by the output of comparator means comparing V drive to V sig .
Claim 8
Dependent Claim
27 words
The method of claim 6, wherein said comparator means comprises two comparators each comparing V drive to V sig , and connected thereto in respectively reversed polarity.
Claim 9
Dependent Claim
41 words
The method of claim 7, wherein the outputs of said two comparators are connected as input signals to a flip-flop, wherein the output signal from said flip-flop controls reversal of the polarity of said square wave drive signal V drive .
Claim 10
Dependent Claim
39 words
The method of claim 8, comprising the further step of generating said bipolar drive signal V drive by inverting a supply voltage with respect to ground, said inverting step being performed by a switching circuit controlled by said flip-flop.
Claim 11
Dependent Claim
31 words
The method of claim 9, comprising the further step of employing a flying capacitor in said switching circuit to provide stored charge for supply during one-half of the bipolar drive signal.
Claim 12
Dependent Claim
92 words
Apparatus for measuring the conductivity of a solution, comprising: a sample cell including two electrodes for receiving a sample of said solution; means for applying a square-wave bipolar drive signal to one of said electrodes, the other of said electrodes being connected to a summing node; a feedback loop for maintaining said summing node at a predetermined potential; means for measuring the current in said feedback loop necessary to maintain said summing node at said predetermined potential; and means for determining the conductivity of the solution responsive to said means for measuring.
Claim 13
Dependent Claim
92 words
Apparatus for measuring the conductivity of a solution, comprising: a sample cell including two electrodes for receiving a sample of said solution; means for applying a square-wave bipolar drive signal to one of said electrodes, the other of said electrodes being connected to a summing node; a feedback loop for maintaining said summing node at a predetermined potential; means for measuring the current in said feedback loop necessary to maintain said summing node at said predetermined potential; and means for determining the conductivity of the solution responsive to said means for measuring.
Claim 14
Dependent Claim
50 words
The apparatus of claim 11, wherein said feedback loop includes an op-amp, said summing node being connected to the inverting input of said op-amp, and a feedback network connected between the output and inverting input of said op-amp, the noninverting input of said op-amp being connected to said predetermined potential.
Claim 15
Dependent Claim
50 words
The apparatus of claim 12, wherein said feedback network includes an integrating capacitor, whereby said integrating capacitor is charged at a rate responsive to the rate of current flow through said cell, and the resistivity of said solution is determined responsive to the rate of charge of said feedback capacitor.
Claim 16
Dependent Claim
42 words
The apparatus of claim 13, wherein the voltage at the output of the op-amp is compared to a predetermined fraction of the voltage of the bipolar drive signal, and the polarity of said bipolar input signal is reversed responsive to said comparison.
Claim 17
Dependent Claim
26 words
The apparatus of claim 14, wherein the conductivity of the solution is determined responsive to the frequency of reversal of polarity of said bipolar drive signal.
Claim 18
Dependent Claim
50 words
The apparatus of claim 13, further comprising means for discharging said integrating capacitor at the commencement of each half-cycle of the bipolar drive signal, and maintaining said integrating capacitor discharged for a time t d during which any parallel capacitance C p across said electrodes may be substantially fully charged.
Claim 19
Dependent Claim
44 words
The apparatus of claim 12, wherein said feedback network includes a feedback resistance R f , and said apparatus further comprises means for controlling R f to control the rate of charge of a parallel capacitance C p between the electrodes of said cell.
Claim 20
Dependent Claim
17 words
The apparatus of claim 17, wherein R f is provided as a network of selectible discrete resistances.
Claim 21
Dependent Claim
20 words
The apparatus of claim 18, further comprising controller means for selecting ones of said discrete resistances as R f .
Claim 22
Dependent Claim
39 words
The apparatus of claim 19, wherein said controller means is operated such that R f is maintained low during at least an initial charge period t c at the commencement of each half-cycle T/2 of the bipolar drive signal.
Claim 23
Dependent Claim
18 words
The apparatus of claim 17, further comprising full-wave rectifier means for rectifying the output signal from said op-amp.
Claim 24
Dependent Claim
34 words
The apparatus of claim 21, wherein said full-wave rectifier means comprises a pair of flying capacitors arranged to store charge from the output of said op-amp during alternate half-cycles of said bipolar input signal.
Claim 25
Dependent Claim
40 words
The apparatus of claim 22, wherein said full-wave rectifier means further comprises a pair of holding capacitors arranged to store charge transferred from said flying capacitors so as to cancel any dc bias in the output signal from the op-amp.
Claim 26
Dependent Claim
25 words
The apparatus of claim 22, wherein said flying capacitors are switched to rectify said output signal responsive to a control signal provided by controller means.
Claim 27
Dependent Claim
15 words
The apparatus of claim 21, comprising further full-wave rectifier means for rectifying the drive signal.
Claim 28
Dependent Claim
23 words
The apparatus of claim 25, wherein said further full-wave rectifier means comprises a pair of flying capacitors and a pair of holding capacitors.
Claim 29
Dependent Claim
32 words
The apparatus of claim 25, further comprising means for dividing said full-wave rectified output signal by said full-wave rectified drive signal, to provide an output representative of the conductivity of said solution.
Claim 30
Dependent Claim
29 words
The apparatus of claim 11, wherein said bipolar drive signal is generated by alternate inverting and direct connection of a source of potential to said one of said electrodes.
Claim 31
Dependent Claim
24 words
The apparatus of claim 28, wherein said alternate inverting and direct connection of a source of potential is accomplished using a switched flying capacitor.
Claim 32
Dependent Claim
51 words
The apparatus of claim 29, wherein said cell is disposed in the vicinity of a source of high frequency noise driven by a periodic signal, and said switching of said flying capacitor is synchronized with said periodic signal, whereby said noise is canceled from the average signal in said feedback loop.
Claim 33
Dependent Claim
24 words
The apparatus of claim 30, wherein the period of said bipolar drive signal is an integral multiple of the period of said periodic signal.
Claim 34
Dependent Claim
45 words
The apparatus of claim 31, wherein said solution is water containing organic carbon and said source of high frequency noise is an ultraviolet lamp operated in juxtaposition to a window in said cell to oxidize organic carbon compounds in said water to CO 2 .
References Cited (Backward Citations)
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| 7.5 Years (Year 8) | 2001-08-01 | 2002-01-31 | 2002-08-02 | $3760.00 | Window Closed / Lapsed |
| 11.5 Years (Year 12) | 2005-08-01 | 2006-01-31 | 2006-08-02 | $7700.00 | Window Closed / Lapsed |