RU2302626C2 - Arrangement for measuring electrical conductivity of liquids - Google Patents

Abstract

FIELD: the invention is designed for measuring electrical conductivity of water and other electrolytes and may be used at physics-chemical investigations of liquids and also at automatic control of technological processes.

SUBSTANCE: the arrangement has a comparison block, a bipolar stabilizer of voltage, a source of current, a condenser, a resistor, three repeaters of voltage, four-electrode conductance-measuring cell. Besides three model resistors, three switchboards, a converter of the period into a code and a computing-controlling device are introduced.

EFFECT: increases accuracy of measuring.

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Description

The invention relates to conductometry, is intended for measuring the electrical conductivity of water and other electrolytes and can be used in physico-chemical studies of liquids, as well as in automated control of technological processes.

The purpose of the invention is to improve the accuracy of measurements of the electrical conductivity of the liquid.

A device for measuring the electrical conductivity of a liquid is known, in which a transformer sensor is used (USSR Author's Certificate No. 949464, class G01N 27/02, published 07.08.82., Bulletin No. 29).

In order to increase the accuracy of measurements in this device, in addition to the main measurement, two additional measurements are performed: when connecting an exemplary resistor using an additional winding and a key and when switching the terminals of the transformer sensor windings. The results of these three measurements are used to calculate the value of the measured electrical conductivity of the liquid. Improving the accuracy of measurements is achieved by eliminating the influence on the measurement result of a number of parameters of the conversion function of the measuring device.

The specified device has the following disadvantages:

1. Due to the use of a transformer sensor, the measuring device has a low level of the output parameter and, accordingly, large errors when measuring low conductivities (almost such devices are used to measure specific conductivities of more than 0.1 S / m).

2. A linear mathematical model of the conversion function of the measuring device is used, so the nonlinearity error is not corrected, which also reduces the accuracy of the measurements.

The closest technical solution to the proposed one is a device for remote measurement of the electrical conductivity of a liquid, which uses a four-electrode conductometric cell (USSR Author's Certificate No. 1635103, class G01N 27/02, published March 15, 91, bulletin No. 10).

The device is a self-generating transducer of electrical conductivity of a liquid during the repetition of rectangular pulses. The use of a contact conductivity cell makes it possible to use it in a wide range of electrical conductivities and, in particular, in the range of low fluid conductivities.

However, this device does not provide high measurement accuracy for the following reasons:

1. The parameters of the function of converting the conductivity of water during the pulse repetition period change in time, mainly due to the influence of changes in external destabilizing factors (primarily temperature).

2. The nominal function of converting conductivity into a period is linear, while the actual calibration characteristic is non-linear, which is mainly due to the non-ideal current source and the non-linear function of converting the voltage on the capacitor to the supply current of the conductivity cell.

To a device comprising a comparison unit, a bipolar voltage stabilizer, a current source, a capacitor, a resistor, three voltage followers, a conductivity cell with two current and two potential electrodes, in which the output of the comparison unit is connected to the first input of the bipolar stabilizer, the output of which is connected to the first input the comparison unit and to the input of the current source, the second input of the bipolar stabilizer is connected to the output of the first voltage follower, the output of the current source is connected to the input of the second repeat voltage reflector and one capacitor plate, the other lining of which is connected to the output of the third voltage follower, the output of the second voltage follower is connected to one output of the current-setting resistor, the other output of which is connected to the input of the third voltage follower, three reference resistors, three switches, a period to code converter are introduced and a computing and control device, wherein all four inputs of the first switch are connected to the input of the third voltage follower, the first output of the first commut the torus is connected to the first current electrode of the conductivity cell, the second, third and fourth outputs of the first switch are connected respectively to the second, third and fourth inputs of the second switch, the first input of which is connected to the first potential electrode of the conductivity cell, all four outputs of the second switch are connected to the second input of the unit for comparison, one terminal of the first model resistor is connected to the second output of the first switch, one terminal of the second model resistor is connected to the third output the house of the first switch, one terminal of the third model resistor is connected to the fourth output of the first switch, the other terminals of the three model resistors and the second current electrode of the conductivity cell are connected to the device common bus, the first input of the third switch is connected to the second potential electrode of the conductivity cell, the second, third and fourth the inputs of the third switch are connected to the common bus of the device, and all four outputs of the third switch are connected to the input of the first voltage follower, the course of the period to code converter is connected to the output of the bipolar voltage stabilizer, the output of the period to code converter is connected to the input of the computing-control device, one output of which is connected to the control input of the period to code converter, the other output is connected to the control inputs of the three switches, and the third output is measuring device output.

Functional diagram of a device for measuring the electrical conductivity of a liquid is shown in the drawing.

The device contains a conductivity cell 1 with two current 2, 3 and two potential 4, 5 electrodes, a comparison unit 6, a bipolar voltage regulator 7, a current source 8, a capacitor 9, a current-setting resistor 10, three voltage followers 11, 12, 13, three reference a resistor 14, 15, 16, a switch torch 17, 18, 19, a period to code converter 20, and a computing and control device 21.

The output of the comparator 6 is connected to the input of the bipolar voltage stabilizer 7, the output of the voltage follower 11 is connected to the second input of the output. The output of the bipolar voltage stabilizer 7 is connected to the inputs of the comparator 6, the current source 8, and the period to code 20 converter. The output of the current source 8 is connected to the input of the voltage follower 12 and with one lining of the capacitor 9, the other lining of which is connected to the output of the voltage follower 13. The output of the voltage follower 12 is connected to one output of the current-setting resistor 10, the other output of which is connected to the input of the voltage follower 13 and to all four inputs of the switch 17. The first output of the switch 17 is connected to the first current electrode 2 of the conductivity cell 1. The second, third, and fourth outputs of the switch 17 are connected to the second, third, and fourth inputs, respectively switch 18, the first input of which is connected to the first potential electrode 2 of the conductivity cell 1. All outputs of the switch 18 are connected to the second input of the comparison unit 6. The findings of the model resistors 14, 15, 16 are connected us respectively with the second, third and fourth outputs of the switch 17. Other conclusions exemplary resistors 14, 15, 16 and a second current electrode 3 conductimetric cell 1 are connected to a common bus device. The first input of the switch 19 is connected to the second potential electrode 5 of the conductivity cell 1. The second, third and fourth inputs of the switch 19 are connected to the common bus of the device, and all outputs of the switch 19 are connected to the input of the voltage follower 11. The output of the period-to-code converter 20 is connected to the computational input -control device 21, one output of which is connected to the control input of the period converter into code 20, another output is connected to the control inputs of switches 17, 18, 19, and the third output is the measurement output tion device.

The device operates as follows.

At the command of the computing and control device 21, the switches 17, 18, 19 close the circuit "a" (the remaining circuits of the switches are open). In this case, the current electrode 2 of the conductivity cell 1 is connected to the current-sensing resistor 10, the potential electrode 4 is connected to the input of the comparison unit 6, and the potential electrode 5 is connected to the input of the voltage follower 11. Depending on the voltage levels at the inputs of the comparison unit 6, a voltage is established at its output either positive or negative polarity.

The voltage at the output of the bipolar stabilizer 7 is equal to

- напряжение на выходе двухполярного стабилизатора;Where

- voltage at the output of a bipolar stabilizer;

U _article - voltage stabilization bipolar stabilizer;

U ₅₀ - voltage at the output of the voltage follower 11, equal to the potential difference between the potential electrode 5 and the common bus device.

The current source 8 produces a direct current, the direction of which is determined by the sign of the output voltage of the bipolar stabilizer 7. This current charges the capacitor 9. With a constant charge current, the voltage across the capacitor 9 varies linearly and at zero initial voltage is

where U _c (t) is the voltage across the capacitor 9;

I is the charge current;

t is the time from the beginning of the charge;

C is the capacitance of the capacitor 9.

The voltage followers 12 and 13 ensure equal voltages across the capacitor 9 and the current-setting resistor 10. As a result, a current equal to 10 flows through the resistor

where I _R (t) is the current through the resistor 10;

R ₀ is the resistance of the resistor 10.

The same current flows through the product meter cell 1.

The potentials of the electrodes 4 and 5 relative to the common bus device are:

where U ₄₀ (t) is the potential of the electrode 4 relative to the common bus;

U ₅₀ (t) is the potential of the electrode 5 relative to the common bus;

R ₄₅ is the resistance of the portion of the test fluid between the electrodes 4 and 5;

R ₅₀ is the resistance between the electrode 5 and the common bus.

In accordance with formulas (1) and (5), the output voltage of the bipolar stabilizer 7 is

на его неинвертирующем входе, напряжение на выходе блока сравнения 6 сменит знак и соответственно изменится направление тока I на выходе источника тока 8. Далее процесс повторяется при других полярностях напряжений и направлениях токов.At the time when the voltage U ₄₀ at the inverting input of the comparison unit 6 becomes equal to the voltage

at its non-inverting input, the voltage at the output of the comparison unit 6 will change sign and, accordingly, the direction of the current I at the output of the current source 8. will change. Next, the process is repeated for other voltage polarities and current directions.

а следовательно,At the moment of operation of the block comparison 6

and consequently,

до

и обратно. Учитывая формулы (3) и (7), получимThe considered time interval until the response of the comparison unit 6 is equal to a quarter of the period of relaxation oscillations, since the full period is determined by the change in voltage U ₄₀ from

before

and back. Given formulas (3) and (7), we obtain

where T _x is the period of oscillation.

To go to the electrical conductivity of the liquid, the geometric constant of the cell is used:

where K is the geometric constant of the conductivity cell;

σ is the electrical conductivity of the liquid.

The oscillation period T _x is associated with the measured specific electric conductivity of the liquid formula

where A = 4 · R _{0 ·} C · U _st / I.

Thus, in the described device, the electrical conductivity of the liquid is converted during the repetition of rectangular pulses at the output of a bipolar voltage stabilizer.

On command computationally-control device 21 in the inverter 20 into the code period is performed transformation of period T _x in digital code N _x:

where B is the coefficient of the period to code conversion.

The resulting code N _{x is} stored in the memory of the computing and control device 21.

Formula (11) reflects the nature of the relationship between the values of N _x and σ _x , however, its use as a conversion function of the measuring device does not allow for high measurement accuracy, since this formula does not take into account the bias of the characteristic due to the presence of time delays in the links of the device and non-linearity of the characteristic caused by the imperfection of the current source 8 and the non-linearity of the conversion of the voltage across the capacitor 9 into the power supply current of the conductivity cell 1 in the presence of significant polarization of the current electrode in. In addition, the coefficient A depends on many parameters of the device (R ₀ , C, U _st , I), the instability of which causes measurement errors.

In this regard, to increase the accuracy of measurements, a nonlinear mathematical model of the function is used, the inverse calibration characteristic of the device:

where σ _x is the electrical conductivity of the liquid;

N _x - code output period code converter 20;

a ₀ , a ₁ , and ₂ are coefficients depending on the parameters of the elements and nodes of the device.

After storing the code N _{x, the} computing and control device 21 generates a command by which circuits "b" are closed in the switches 17, 18, 19 (the remaining circuits are open). In this case, the conductivity cell 1 is turned off, and instead of it, a model resistor 14 is connected to the converter. The corresponding pulse repetition period is set in the device, which, upon the command of the computing and control device 21, is converted into a digital code N ₁ stored in this device.

In accordance with the mathematical model used (12), the formula connecting the code N ₁ with the conductivity g _{1 of the} model resistor 14 can be represented as

where K is the geometric constant of the conductivity cell.

Then, in the same way, according to the instructions of the computing and controlling device 21, instead of the conductivity cell 1, exemplary resistors 15 and 16 are connected in turn. The corresponding codes N ₂ and N _{3 are} stored in the computing and controlling device 21.

By analogy with formula (13), we have

where g ₂ is the conductivity of the exemplary resistor 15;

g ₃ - conductivity of the reference resistor 16.

The conductivities g ₁ , g ₂ , g _{3 of the} reference resistors are selected so that the values of K · g ₁ , K · g ₂ , K · g ₃ correspond to the range of measured specific electrical conductivities of the liquid, and g ₁ ≠ g ₂ ≠ g ₃ .

Formulas (12) - (15) can be considered as a system of equations with unknown σ, and ₀ , a ₁ , a ₂ . Solving this system of equations for σ by eliminating the unknowns a ₀ , a ₁ , and ₂ , we obtain

where: C ₀ = R · g _1;

C ₁ = K · (g ₃ -g ₁ );

C ₂ = K · (g ₁ -g ₂ );

Z ₁ = [(N _x -N ₁ ) · (N _x -N ₂ )] / [(N ₃ -N ₂ ) · (N ₃ -N ₁ )];

Z ₂ = [(N _x -N ₁ ) · (N _x -N ₃ )] / [(N ₃ -N ₂ ) · (N ₂ -N ₁ )].

The calculation of the measured specific electrical conductivity of the liquid according to the formula (16) is performed in the computing and control device 21.

An analysis of formula (16) shows that the estimate of the measured conductivity of a liquid calculated by this formula does not depend on the coefficients a ₀ , a ₁ , a _{2 of the} mathematical model (12), and therefore, on a number of unstable parameters of the elements and components of the device: stabilization voltage U _article , the charge current of the capacitor 9 and the capacitance of this capacitor, the resistance of the current-setting resistor 10, the zero offsets of the comparison unit, voltage followers and other units of the device. Since the mathematical model used (12) is a second-order polynomial, the error in the nonlinearity of the measuring device is also corrected.

The advantage of the model used (12) is also that the estimate of the measured specific conductivity of the liquid can be calculated by formula (16) both in the case of a nonlinear transformation function (a ₂ ≠ 0) and in the case of a linear transformation function (a ₂ = 0) measuring device.

Thus, in this device, when the measurement algorithm described above is implemented and the value of the measured specific conductivity of the liquid is calculated using formula (16), the additive and multiplicative components of the error, as well as the nonlinearity error, are automatically corrected. In this case, the achievable measurement accuracy is determined mainly by the stability of the resistances of the reference resistors 14, 15, 16 and the stability of the geometric constant of the conductivity cell.

MEGGITT HOLSWORTNY model resistors with a temperature coefficient of resistance of 10 · 10 ^-6 1 / ° С were used in the manufactured device. To ensure the stability of the parameters of the conductivity cell, it is made of quartz glass and has platinum electrodes. The accuracy and stability of the period to code converter is provided by a quartz resonator. The remaining elements and components of the device are not precision. The digital part of the device is based on a microprocessor.

Experimental studies have shown that the total measurement error in the range of specific electrical conductivity (0.05 ... 6.5) S / m and temperature range (0 ... 50) ° C does not exceed 0.05%.

The proposed device is intended for high-precision measurements of the electrical conductivity of a liquid and can be used both in studies of various water bodies and in automatic control of technological processes.

Claims (1)

A device for measuring the electrical conductivity of a liquid, comprising a comparison unit, a bipolar voltage stabilizer, a current source, a capacitor, a resistor, three voltage followers, a conductivity cell with two current and two potential electrodes, in which the output of the comparison unit is connected to the first input of the bipolar voltage stabilizer, output which is connected to the first input of the comparison unit and to the input of the current source, the second input of the bipolar stabilizer is connected to the output of the first repeater voltage, the output of the current source is connected to the input of the second voltage follower and to one capacitor plate, the other lining of which is connected to the output of the third voltage follower, and the output of the second voltage follower is connected to one output of the current-setting resistor, the other output of which is connected to the input of the third voltage follower, different the fact that three reference resistors, three switches, a period-to-code converter and a computing-control device are introduced into it, all four inputs of the first to the mutator are connected to the input of the third voltage follower, the first output of the first switch is connected to the first current electrode of the conductivity cell, the second, third and fourth outputs of the first switch are connected respectively to the second, third and fourth inputs of the second switch, the first input of which is connected to the first potential electrode of the conductivity cell , all four outputs of the second switch are connected to the second input of the comparison unit, one output of the first model resistor is connected to the second the output of the first switch, one terminal of the second model resistor is connected to the third output of the first switch, one terminal of the third model resistor is connected to the fourth output of the first switch, the other terminals of the three model resistors and the second current electrode of the conductivity cell are connected to the device common bus, the first input of the third switch is connected with the second potential electrode of the conductivity cell, the second, third and fourth inputs of the third switch are connected to a common bus of the device, and all four outputs of the third switch are connected to the input of the first voltage follower, the input of the period converter in code is connected to the output of the bipolar voltage stabilizer, the output of the period converter in code is connected to the input of the computing and control device, one output of which is connected to the control input of the period converter in code, the other the output is connected to the control inputs of the three switches, and the third output is the output of the measuring device.