WO2001075459A1 - Circuit de mesure de conductivite electrique - Google Patents

Circuit de mesure de conductivite electrique Download PDF

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Publication number
WO2001075459A1
WO2001075459A1 PCT/AU2001/000364 AU0100364W WO0175459A1 WO 2001075459 A1 WO2001075459 A1 WO 2001075459A1 AU 0100364 W AU0100364 W AU 0100364W WO 0175459 A1 WO0175459 A1 WO 0175459A1
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WO
WIPO (PCT)
Prior art keywords
circuit
substance
conductivity
frequency
voltage
Prior art date
Application number
PCT/AU2001/000364
Other languages
English (en)
Inventor
Clive Barron Chamberlain
Original Assignee
Accent Hydroponics Pty Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Accent Hydroponics Pty Ltd filed Critical Accent Hydroponics Pty Ltd
Priority to AU43947/01A priority Critical patent/AU4394701A/en
Publication of WO2001075459A1 publication Critical patent/WO2001075459A1/fr

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
    • G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
    • G01N27/06—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a liquid
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
    • G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
    • G01N27/045—Circuits
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
    • G01R27/02—Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
    • G01R27/22—Measuring resistance of fluids

Definitions

  • the present invention relates generally to the measurement of the electrical conductivity of substances, in particular the conductivity of liquids.
  • a common practice for determining the conductivity of a liquid is to apply an alternating voltage of low amplitude (typically less than lOOmV RMS, referred to as ELV or Extra Low Voltage drive) and a frequency between 100Hz to over 50kHz.
  • the voltage is applied to the liquid via a Wheatstone bridge arrangement in which the liquid forms an arm of the bridge.
  • the response of the Wheatstone bridge can be measured in a traditional fashion to give a measure directly proportional to the conductivity of the liquid.
  • An alternating current is used to avoid polarisation effects within the liquid, something which is important in hydroponics systems, for example.
  • the conductivity is determined by measuring and precision rectifying the current in the Wheatstone bridge, or may be quantified in any other suitable way. The rectified value is then converted to a frequency which is used to drive a conventional meter display within an appropriate range.
  • an object of the present invention to substantially overcome, or at least ameliorate, one or more disadvantages of existing arrangements.
  • an electrical conductivity circuit arrangement wherein a resistance-dependent variable- frequency oscillator is used to drive a substance whose conductivity is to be measured, an output of said circuit being the frequency of said oscillator, said circuit arrangement characterised in that said substance is driven with a symmetric, amplitude-controlled signal.
  • a circuit for measuring the electrical conductivity of a substance comprising: a supply splitter for providing an analog signal ground, said analog signal ground being a voltage substantially equal to half a supply voltage of said circuit as measured with respect to a supply ground of said circuit; oscillator means for creating a output signal having a frequency proportional to the conductivity of said substance; driver means input with said output signal for applying an amplitude-controlled version of said output signal to said substance, said amplitude-controlled version being symmetric about said analog signal ground; and output means for interpreting said frequency to form a measure of said electrical conductivity.
  • a method for measuring the electrical conductivity of a substance comprising the steps of: generating a drive voltage to apply to said substance, said drive voltage being symmetrical about an analog signal ground and having a controlled amplitude; inducing an oscillating voltage within a measurement circuit, the frequency of said oscillating voltage being proportional to the conductivity of said substance; measuring said frequency, and presenting and/or logging a value linearly related to said frequency.
  • Fig. 1 shows a schematic block diagram of a system for measuring conductivity
  • Fig. 2A shows a schematic block diagram of the conductivity measurement circuit of Fig. 1;
  • Fig. 2B shows an electronic circuit diagram of the conductivity measurement circuit of Fig. 2 A
  • Figs. 3 A to 3F show typical voltage trends at selected points around the circuit of Fig. 2B;
  • Fig. 4 shows an alternative arrangement of the circuit of Fig.2B
  • Fig. 5 shows a circuit diagram of a conductivity measurement circuit that can be switched between different ranges
  • Fig. 6 shows a circuit diagram of a conductivity measurement circuit with autoranging capabilities.
  • an electrical conductivity measuring circuit which applies an alternating voltage of known amplitude to the substance being measured.
  • the frequency of oscillation of the circuit is linearly dependent on the conductivity of the measured substance.
  • problems with electrolysis are addressed, allowing a higher drive voltage to be used than would otherwise be possible.
  • This higher voltage offers a faster measuring speed and higher accuracy than prior art arrangements.
  • electrodes 1 and 2 are positioned in a liquid 3 whose conductivity is to be measured.
  • An alternating voltage of fixed amplitude is generated by a conductivity measurement circuit 4 and applied to the electrodes 1 and 2.
  • the circuit 4 operates such that the frequency of oscillation at points (A) and (E) is proportional to the conductivity of the liquid 3.
  • a voltage at point (E) serves as an input to a microcontroller
  • the microcontroller 5 (or any other device capable of measuring and/or displaying the data) which measures the frequency and sends a calculated value to a display 6, and a memory unit 7 which logs the data for later analysis.
  • An input 8 enables conductivity measurement to be made over different ranges.
  • the range change control input 8 is provided to the microcontroller 5.
  • the range change may also be made directly in the conductivity measurement circuit 4, as shown for example in Fig. 5.
  • Autoranging can also be included in the circuit, as shown in Fig. 6.
  • the conductivity measurement circuit 4 is formed by a resistance-dependent oscillator, in which the frequency-controlling resistance is that presented by the liquid 3.
  • the oscillator produces oscillations of high duty-cycle symmetry having a linear relationship between frequency and resistance values over at least two decades of measurement, with an accuracy in the region of about 1% of the full- scale reading.
  • the circuit 4 is formed essentially of four stages: a supply splitter 10; an integrator 11; a Schmitt trigger 12, and a driver 13.
  • the supply splitter 10 provides an accurate reference voltage to the integrator 11, which integrates a signal output from the driver 13.
  • the output of the integrator 11 provides an input to the Schmitt trigger 12.
  • the level adjustment circuit 14 is connected between the Schmitt trigger 12 and the driver 13.
  • the output voltage of the Schmitt trigger at point (E) is the output of the circuit 4.
  • the output voltage at point (A) of the driver 13 is the voltage applied to the liquid 3.
  • Fig. 2B shows a preferred circuit diagram implementation of the block diagram of Fig. 2 A.
  • the supply splitter 10 is formed by an operational amplifier 2A connected so as to buffer a voltage divider formed by resistors R7 and R8, which divide a supply voltage between Vcc and supply ground by one half.
  • R7 and R8 Accurate matching of R7 and R8 can result in net drive-symmetry errors being less than 1% across the measuring electrodes. In a battery-operated application, such an arrangement accurately maintains symmetry even whilst a voltage output from the battery (e.g. Vcc) declines.
  • the operational amplifier 2A outputs an analog signal ground that is shown in Fig. 3B.
  • the analog signal ground is supplied as a first reference input to the integrator 11, formed by an operational amplifier IB, resistor R5 and capacitor Cl.
  • the second input to the integrator 11 is the square wave formed at point (C), the output of the operational amplifier 1 A.
  • the integrator 11 acts to integrate the square wave at point (C), seen in Fig. 3C, to form a triangular waveform at point (D), seen in Fig. 3D.
  • the triangular waveform is input to the Schmitt trigger 12 which displays hysteresis and is formed by an operational amplifier 2B and resistors R9 and R13.
  • the hysteresis of the Schmitt trigger 12 is set so that switching takes place at each of about one-third of the supply voltage Vcc, and at about two-thirds of Vcc.
  • the triangular waveform at point (D), shown in Fig. 3D varies between one-third and two-thirds of the supply voltage and is centred on the analog signal ground.
  • Preferred devices for the operational amplifiers are LMC662 devices, which have CMOS output stages capable of saturation at supply rail values, i.e. they can supply a rail-to-rail voltage swing, from the supply ground to the supply voltage Vcc.
  • the output generated at point (E) of the Schmitt trigger 12 is the square-wave voltage shown in Fig. 3E. This signal is symmetrical about the analog ground of Fig. 3B, the excursion in each direction being equal to half of the supply voltage.
  • the voltage at point (E) is input to the level adjustment circuit 14, which is formed by resistors R4 and R6 and a thermistor Tl. Resistors R6 and R4 are connected in series between point (E) and the analog signal ground.
  • the thermistor Tl is connected in parallel with R4.
  • the resistors R4 and R6 divide the voltage at point (E) to produce a voltage at point (F) shown in Fig. 3F.
  • the square-wave signal at point (F) has a fixed amplitude which is determined by R4 and R6.
  • the resistors R4 and R6 are preferably chosen such that the peak amplitude of the voltage at point (F) is 250mV above or below the analog ground (B).
  • the thermistor Tl increases the precision of a driving signal to the liquid by providing temperature compensation. This is desirable because the conductivity of many solutions rises as temperature increases. Such solutions are described as having positive temperature coefficients (PTC). The change in conductivity can be of the order of several percent per degree Celcius.
  • the thermistor Tl is chosen to counteract the temperature- related rise in conductivity.
  • the thermistor Tl has a negative temperature coefficient and acts to lower the drive voltage to the liquid 3 as the temperature rises. Judicious scaling and balancing of resistors R4 and R6 and the thermistor Tl provides a useful normalisation of conductivity as the temperature varies.
  • the thermistor Tl is housed in a tubular enclosure exposed at the front of a housing containing the conductivity measurement circuit 4.
  • the tubular enclosure Prior to measuring the conductivity of the liquid 3, the tubular enclosure is immersed in the liquid 3 for a period of about one minute. This procedure ensures that the thermistor Tl compensates for changes in the temperature of the liquid 3.
  • the voltage at point (F) is applied to the driver 13, which is formed by an operational amplifier 1A configured as a modified gain-of-one voltage follower. Operational amplifier 1A operates to adjust the voltage at point (C) so as to equalise the voltage on its positive and negative inputs. To do this, 1A sources a current which flows through resistor R2 and then through a parallel combination of the liquid 3 and a resistor Rl to the analog ground at point (B). The resistor R3 serves as series input protection for the negative input of operational amplifier 1A.
  • the driver 13 operates such that the voltage at point (A), shown in Fig. 3 A, is the same as the voltage at point (F).
  • the voltage at point (A) is thus a constant-amplitude square-wave which is applied to the liquid 3 via electrodes 1 and 2. Because the voltage amplitude at point (A) is set by the driver 13, the current which flows through the liquid 3 is determined by the conductivity of liquid 3. The higher the conductivity, the higher the current and the greater the voltage at point (C), this being shown in Fig. 3C.
  • the voltage at point (C) determines the rate of change of the voltage at point (D) across the integrator components R5 and Cl. The faster the voltage at point (D) changes the more frequently the Schmitt trigger 12 will switch. In this way, the conductivity of the liquid 3 determines the frequency of oscillation of the circuit 4.
  • the voltage at point (E) at the output of the Schmitt trigger 12 is a suitable output from the circuit 4 (this being indicated as F_out in Fig. 2B and Figs. 4-6).
  • F_out in Fig. 2B and Figs. 4-6.
  • the frequency of oscillation tracks the conductivity of
  • the liquid within ⁇ 1% over a range of conductivity values from about O.lmS to about
  • the conductivity measurement circuit 4 may be calibrated by either a one-point or a two-point procedure.
  • the electrodes 1 and 2 are dried and are held in air. The reading taken in air determines a zero point of the conductivity measurement.
  • the resistor Rl ensures that a current still flows between point (A) and the analog signal ground at point (B) even when there is an open circuit between the electrodes 1 and 2. This makes it possible to distinguish between a reading of OmS and a circuit failure or lack of a supply voltage Vcc.
  • the first measurement point is in air as described above and the second measurement point is obtained by placing the electrodes 1 and 2 in a solution of known conductivity and taking a reading.
  • the integrator 11 and the Schmitt trigger 12 can be reversed in position.
  • the voltage applied to the liquid 3 will be a triangular wave rather than a square wave.
  • Fig. 4 shows a variation of the circuit of Fig. 2B, where like components have like reference numerals and operate in a like manner.
  • a modified adjustment circuit 14A is shown in which the thermistor Tl is connected in series with the resistor R4. The effect is the same as the arrangement shown in Fig. 2B where Tl is in parallel with R4. In both cases the negative temperature coefficient of the thermistor Tl acts to compensate for the positive temperature coefficient of the liquid 3.
  • the thermistor Tl can alternatively be used in parallel or in series with resistors R5, R9 or R13 in order to provide temperature compensation.
  • the thermistor Tl is connected in parallel with the resistor R5.
  • an auxiliary circuit 50 seen in Fig.l, can be used to measure the temperature of the liquid 3.
  • An output of the auxiliary circuit 50 may be a frequency, an analog value or a digital value.
  • the microcontroller 5 processes the output of the auxiliary circuit 50 and uses the information to adjust the uncompensated signal F_out received from the conductivity measurement circuit 4.
  • different compensation factors can be used depending on the solution being measured.
  • the conductivity of the solutions being measured can vary considerably.
  • the conductivity of a saturated salt solution is about one million times that of ultra-pure water. It is consequently desirable to confine the measurement range to a smaller span of interest to the user of the circuit 4, thereby increasing the accuracy and resolution.
  • the user may wish to make measurements in the ranges 0-
  • a further modified level adjustment 14B is shown in which the resistor R4 of Fig. 2B is replaced by resistors RIO and Rl 1.
  • a switch SW01 selects either one or the other of RIO and Rl 1.
  • the resistance of RIO is chosen to be ten times greater than the resistance of Rl l.
  • the switch SW01 may be used to vary the span of the measurement by a factor of 10.
  • Resistors RIO and Rl l are chosen to have highly accurate resistance values.
  • the measurement span may be varied in a similar fashion by changing other components of the circuit 4.
  • the same effect may be achieved by varying resistors R2,
  • R4, R5, R6, R9, R13 or capacitor Cl The components may be changed by means of a mechanical switch as shown in Fig. 5, or by means of a semiconductor switch as will be described below with reference to Fig. 6.
  • the circuit shown in Fig. 6 has a level adjustment circuit 14C which differs from that shown in Fig. 2B in that the resistor R6 has been replaced by the two resistors R20 and R21 and the semiconductor switch 30.
  • the switch 30 connects either R20 or R21 in series with R4.
  • Resistors R20 and R21 are typically selected to have highly accurate values having a ratio of 1 : 10.
  • the semiconductor switch is preferably a CMOS type 4053 or similar.
  • the range switching process can be carried out automatically.
  • the microcontroller 5 can be programmed to detect that the present span of the circuit is inappropriate for the solution being measured. The microprocessor 5 can then send a signal 5 to the semiconductor switch 30 in order to alter the measurement span by switching between components.
  • circuit can be produced using individual components as shown, but is also readily manufacturable as a customised integrated circuit.
  • circuit can be produced using individual components as shown, but is also readily manufacturable as a customised integrated circuit.

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Physics & Mathematics (AREA)
  • Health & Medical Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Biochemistry (AREA)
  • General Health & Medical Sciences (AREA)
  • General Physics & Mathematics (AREA)
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  • Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)

Abstract

La présente invention concerne un circuit de mesure de conductivité (4), qui est constitué d'un oscillateur dépendant de la résistance, dans lequel la résistance de commande de fréquence est celle présentée par un liquide (3) dont la conductivité doit être mesurée. Ce circuit (4) est principalement formé de quatre étages. Un répartiteur d'alimentation (10) fournit une tension de référence précise à un intégrateur (11) qui intègre une sortie de signal issue d'un pilote (13). La sortie de l'intégrateur (11) assure une entrée à une bascule de Schmitt (12). Un circuit de réglage de niveau (14) est connecté entre la bascule de Schmitt (12) et le pilote (13). Le circuit (4) commande le liquide (3) avec un signal symétrique de tension commandée en amplitude.
PCT/AU2001/000364 2000-04-03 2001-04-03 Circuit de mesure de conductivite electrique WO2001075459A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU43947/01A AU4394701A (en) 2000-04-03 2001-04-03 Electrical conductivity measurement circuit

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
AUPQ6636A AUPQ663600A0 (en) 2000-04-03 2000-04-03 Electrical conductivity measurement circuit
AUPQ6636 2000-04-03

Publications (1)

Publication Number Publication Date
WO2001075459A1 true WO2001075459A1 (fr) 2001-10-11

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PCT/AU2001/000364 WO2001075459A1 (fr) 2000-04-03 2001-04-03 Circuit de mesure de conductivite electrique

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AU (1) AUPQ663600A0 (fr)
WO (1) WO2001075459A1 (fr)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2007117748A1 (fr) * 2006-04-07 2007-10-18 Masco Corporation Detecteur de conductivite permettant de determiner le niveau de granules dans une capsule remplie d'eau
EP2163887A1 (fr) * 2008-09-15 2010-03-17 Ulrich Kuipers Procédé, agencement de commutation, capteur destiné à la mesure de grandeurs physiques dans des fluides et leur utilisation
CN106137193A (zh) * 2016-07-29 2016-11-23 普罗朗生物技术（无锡）有限公司 人体生物电导多值模拟器

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4283675A (en) * 1979-03-12 1981-08-11 Bell Telephone Laboratories, Incorporated Impedance/admittance measuring circuit
WO1982002595A1 (fr) * 1981-01-23 1982-08-05 Inc Rosemount Circuit de mesure de la reactance
WO1982004317A1 (fr) * 1981-06-01 1982-12-09 Inc Roemount Circuit de mesure de l'impedance
US4656427A (en) * 1985-02-05 1987-04-07 Dauphinee Thomas M Liquid conductivity measuring circuit
US5334940A (en) * 1992-07-14 1994-08-02 Anatel Corporation Methods and circuits for measuring the conductivity of solutions
US5708363A (en) * 1995-10-16 1998-01-13 Signet Scientific Company Liquid conductivity measurement system using a variable-frequency AC voltage

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4283675A (en) * 1979-03-12 1981-08-11 Bell Telephone Laboratories, Incorporated Impedance/admittance measuring circuit
WO1982002595A1 (fr) * 1981-01-23 1982-08-05 Inc Rosemount Circuit de mesure de la reactance
WO1982004317A1 (fr) * 1981-06-01 1982-12-09 Inc Roemount Circuit de mesure de l'impedance
US4656427A (en) * 1985-02-05 1987-04-07 Dauphinee Thomas M Liquid conductivity measuring circuit
US5334940A (en) * 1992-07-14 1994-08-02 Anatel Corporation Methods and circuits for measuring the conductivity of solutions
US5708363A (en) * 1995-10-16 1998-01-13 Signet Scientific Company Liquid conductivity measurement system using a variable-frequency AC voltage

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2007117748A1 (fr) * 2006-04-07 2007-10-18 Masco Corporation Detecteur de conductivite permettant de determiner le niveau de granules dans une capsule remplie d'eau
EP2163887A1 (fr) * 2008-09-15 2010-03-17 Ulrich Kuipers Procédé, agencement de commutation, capteur destiné à la mesure de grandeurs physiques dans des fluides et leur utilisation
CN106137193A (zh) * 2016-07-29 2016-11-23 普罗朗生物技术（无锡）有限公司 人体生物电导多值模拟器

Also Published As

Publication number Publication date
AUPQ663600A0 (en) 2000-05-04

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