Conductance measurement circuit with wide dynamic range

• US 5,317,275 A
• Filed: 01/13/1992
• Issued: 05/31/1994
• Est. Priority Date: 01/13/1992
• Status: Expired due to Fees

• Current Assignee: Hamilton Sundstrand Corporation (Rtx Corporation)
• Original Assignee: Orbital Sciences Corporation (Northrop Grumman Corporation)
• Inventors: Von Esch, Myron, Mount, Bruce E.
• Primary Examiner(s): Wieder, Kenneth A.
• Assistant Examiner(s): Regan, Maura K.
• Application Number:
  US07/820,457
• Time in Patent Office: 869 Days
• Field of Search: 324/692, 324/693, 324/439, 128/734, 128/635, 204/406, 328/145, 307/492
• US Class Current: 324/692
• CPC Class Codes: G01R 27/22 Measuring resistance of fluids

Abstract

A conductance measurement circuit to measure conductance of a solution under test with an output voltage proportional to conductance over a 5-decade range, i.e., 0.01 uS to 1000 uS or from 0.1 uS to 10,000 uS. An increase in conductance indicates growth, or multiplication, of the bacteria in the test solution. Two circuits are used each for an alternate half-cycle time periods of an alternate squarewave in order to cause alternate and opposite currents to be applied to the test solution. The output of one of the two circuits may be scaled for a different range optimum switching frequency dependent upon the solution conductance and to enable uninterrupted measurement over the complete 5-decade range. This circuitry provides two overlapping ranges of conductance which can be read simultaneously without discontinuity thereby eliminating range switching within the basic circuitry. A VCO is used to automatically change the operating frequency according to the particular value of the conductance being measured, and comparators indicate which range is valid and also facilitate computer-controlled data acquisition. A multiplexer may be used to monitor any number of solutions under test continuously.

Claims

1. A circuit for measuring the conductance of a solution under test which contains biological growth media where an increase in the conductance of the solution indicates growth, or multiplication, of bacteriological colonies and in which a pair of spaced-apart electrodes are placed for applying current to said solution, comprising,

first means connected to said electrodes for applying current to said electrodes and for maintaining a voltage drop at a predetermined value regardless of any change in conductance of said solution,

second means connected to said first means for adjusting the current necessary to maintain said voltage drop at said predetermined value,

third means connected to said second means responsive to said second means to provide a current output responsive to the change in voltage drop so that the output of said second means remains constant,

said second and third means including means which responds to a small change in voltage drop provides a large current output to produce a wide range of conductance measurement, and

fourth means connected to said third means for producing an output proportional to conductance of said solution.

2. The circuit as claimed in claim 1 wherein said first means comprises,

conductance responsive means having an output connected to one electrode and an input connected to the other electrode so that said conductive responsive means will have the same voltage drop as that of said solution,

wherein said second and third means comprise,

a variable voltage source, and

a variable nonlinear voltage-controlled input resistance means connected to said variable voltage source and to said input to control current at said input according to the magnitude of the voltage applied by said variable voltage source, and

wherein said fourth means comprises,

a second variable nonlinear voltage-controlled input resistance means connected to the same variable voltage source and having the same characteristics as said first variable resistance means.

3. A circuit for measuring the conductance of a solution under test which contains biological growth media where an increase in the conductance of the solution indicates growth, or multiplication, of bacteriological colonies and in which a pair of spaced-apart electrodes are placed for applying current to said solution, comprising,

means connected to said electrodes for applying current to provide a voltage drop of a predetermined value across said solution,

conductance responsive means having an output connected to one electrode and an input connected to the other electrode so that said conductive responsive means will have the same voltage drop as that of said solution,

a pair of circuits alternately connectable to said conductance responsive means and to said electrodes,

means for alternately connecting each of said pair of circuits to said conductance responsive means and to said electrodes,

each circuit having,

means for applying an alternating AC current in square wave form to said conductance responsive means and to said electrodes to provide a peak voltage across said solution,

means for maintaining the output voltage of said conductance responsive means at said predetermined value by varying the current level applied to said conductance responsive means and to said electrodes according to the change in voltage across said solution,

means responsive to said variation in said current level for producing a circuit voltage output proportional to the change in voltage determined by said conductance responsive means, and

means connected to both circuits for determining the output range of the two circuits producing a circuit output voltage representative of the change of conductance in said solution.

4. The circuit as claimed in claim 3 further including means for varying the frequencies of said alternating AC current.

5. The circuit as claimed in claim 4 further including a multiplexer for connecting a plurality of solutions under test to the conductance responsive means for measuring the conductance of said plurality of said solutions continuously.

6. The circuit as claimed in claim 4 wherein said conductance responsive means comprises,

an operational amplifier with its negative input connected to one electrode and its output connected to the other electrode.

7. The circuit as claimed in claim 6 wherein said means for maintaining said output voltage at a predetermined value comprises,

a comparator connected to the output of said operational amplifier to compare the voltage of the output of said operational amplifier with a precise voltage source to produce an error signal when the output voltage of said operational amplifier differs from the voltage of said precise voltage source,

the output of said comparator being connected to a second voltage source which applies a current to the input of said operational amplifier to maintain the output of said operational amplifier at its predetermined value.

8. The circuit as claimed in claim 7 including a first transistor connected as a diode to the negative input of said operational amplifier and to the second voltage source and a second transistor also connected to the first transistor and to said second voltage source to reflect the current through said first transistor and produce said circuit voltage output.

9. The circuit as claimed in claim 8 wherein said first and second transistors are a matched pair.

10. The circuit as claimed in claim 9 wherein said means for applying current to the input of said operational amplifier comprises a voltage-controlled resistance connected between said second voltage source and the input of said first transistor.

11. A method for determining growth, or multiplication, of a solution under test containing biological growth media, said solution also containing electrodes so that said solution is subject to a current to measure the conductance of said solution as a first output voltage of a preselected level, comprising the steps of,

applying current to said electrodes and to said solution,

measuring the conductance of said solution wherein a change in conductance is a function of biological change by maintaining the first output voltage at said preselected level,

logarithmically adjusting the current necessary to maintain the first output voltage applied to said electrodes at a selected level due to a change in conductance to provide a wide range of conductance measurements, and

producing a second voltage output as a function of the adjusted current.

12. A circuit for measuring the conductance of a solution under test which contains biological growth media where an increase in the conductance of the solution indicates growth, or multiplication, of bacteriological colonies and in which a pair of spaced-apart electrodes are placed for applying current to said solution, comprising,

an operational amplifier with its output connected to one electrode and its negative input connected to the other electrode so that said operational amplifier responds to conductance and produces an output voltage of a preselected value,

a variable voltage source and a transistor connected as a diode between said voltage source and said negative input for applying current to said negative input for maintaining the output voltage at said preselected level regardless of any change in conductance,

a second transistor connected as a diode to said variable voltage source with characteristics substantially identical to the first transistor,

the output of said second transistor being connected to a second operational amplifier having a fixed resistance feedback to provide a circuit output proportional to change in conductance.

13. A circuit for measuring the conductance of a solution under test which contains biological growth media where an increase in the conductance of the solution indicates growth, or, multiplication, of bacteriological colonies and in which a pair of spaced-apart electrodes are placed for applying current to said solution, comprising,

an operational amplifier with its output connected to one electrode and its negative input connected to the other electrode for applying current to said electrodes and to said solution so that said operational amplifier responds to the conductance of said solution and produces an output voltage of a preselected value, and

a variable voltage source connected to said output of said amplifier and a transistor connected as a diode between said voltage source and said negative input for applying current to said negative input for maintaining the output voltage of said amplifier at said preselected value regardless of any change in conductance of said solution,

said transistor having a logarithmic output response so that a small change in voltage from said voltage source produces a large change in current through said transistor because of the logarithmic response of said transistor thus providing a wide range of current variation through said solution from a small change in the voltage from said voltage source.

14. The circuit as claimed in claim 13 further including a second transistor connected as a diode to said variable voltage source with characteristics substantially identical to the first transistor to mirror the current through said first transistor and produce an identical output current, and means connected to the output of said second transistor to transform said identical output current to a linear voltage proportional to the current applied to said solution which is directly proportional to the conductance changes in said solution over a wide range, typically five decades.

15. The circuit as claimed in claim 13 further including a second transistor connected as a diode to said variable voltage source with characteristics substantially identical to the first transistor and having its output connected to a second operational amplifier having a fixed resistance feedback to provide a wide range circuit output proportional to a wide range in conductance change.

16. A measuring circuit for measuring the conductance of a solution under test which contains biological growth media where an increase in the conductance of the solution indicates growth, or multiplication, of bacteriological colonies and in which a pair of space-apart electrodes are placed for applying current to said solution, comprising,

conductance responsive means having an output connected to one electrode and an input connected to the other electrode for applying current to said electrodes to provide a voltage drop of a first polarity and of a predetermined value across said solution, said conductance responsive means having the same voltage drop as that of said solution,

a first circuit means for maintaining the voltage drop across said solution at a constant value by maintaining the output voltage of said conductance responsive means at a constant value by varying the current level applied to said conductance responsive means and to said electrodes according to any change in voltage across said solution, comprising,

a first variable voltage source having a source output voltage of a first polarity responsive to any change in the output voltage in said conductive responsive means, and

a first variable input resistance having logarithmic characteristics connected between said variable voltage source and said conductive responsive means so that a small change in voltage from said voltage source produces a large change in current flow in one direction through said first variable input resistance because of the logarithmic response of said first variable resistance thus providing a wide range of current variation through said solution from a small change in voltage from said voltage source.

17. The circuit as claimed in claim 16 further including a second circuit means connectable alternately to this same conductive responsive means when said first circuit means is disconnected from this same conductive responsive means for maintaining the voltage drop across said solution at a second constant value by maintaining the output voltage of this same conductance responsive means at a second constant value by varying the current level applied to said conductance responsive means and to said electrodes according to any change in voltage across said solution, comprising,

a second variable voltage source having a second source output voltage of a second polarity responsive to any change in the output voltage in said conductive responsive means, and

a second variable input resistance having logarithmic characteristics connected between said second variable voltage source and said conductive responsive means so that a small change in voltage from said second voltage source produces a large change in current flow in a direction opposite from the current flow in said first circuit means through said first variable input resistance of the logarithmic response of said second variable input resistance thus providing a wide range of current variation through said solution from a small change in the voltage from said second voltage source,

means for alternately connecting said first and second circuit means to said conductive responsive means and to said electrodes at a predetermined rate depending upon the conductance characteristics of the solution under test, and

means for determining which of said circuit means is connected to said conductance responsive means at the time said conductance responsive means is responding to changes in said solution.

18. The circuit as claimed in claim 16 further including a third variable input resistance having the same characteristics as said first variable input resistance to mirror the current of the first polarity through said first variable input resistance and produce an identical output current and means connected to the output of said third variable input resistance to transform the current from said third variable input resistance into a linear voltage proportional to the current of the first polarity applied to said solution which is directly proportional to conductance changes in said solution over a wide range.

19. The circuit as claimed in claim 17 further including a fourth variable input resistance having the same characteristics as said second variable input resistance to mirror the current of the second polarity through said second variable input resistance and produce an identical output current and means connected to the output of said fourth variable input resistance to transform the current from said fourth variable input resistance into a linear voltage proportional to the current of the second polarity applied to said solution which is directly proportional to conductance changes in said solution over a wide range.