Method for Determining a Conductivity, Operating Method of a Magnetic-Inductive Flowmeter and Magnetic-Inductive Flowmeter

Abstract

A method for determining conductivity of a medium includes: generating a signal with first and second frequencies; feeding the signal to first and second electrodes; determining a first voltage between the first and second electrodes, a first current through the medium for the first frequency, and a first impedance from the first voltage and first current; determining a second voltage between the first and second electrodes, a second current through the medium for the second frequency, and a second impedance from the second voltage and second current; determining and comparing phases of the first and second impedances; determining a medium resistance with a first formula when the first impedance phase is smaller than the second impedance phase; determining a medium resistance with a second formula when the first impedance phase is equal to or greater than the second impedance phase; and determining conductivity of the medium using the medium resistance.

Claims

1. A method for determining a conductivity (σ) of a medium with a controller and a measuring cell, wherein the measuring cell has a cell constant (k), a first electrode and a second electrode, and wherein the first electrode and the second electrode are in direct contact with the medium, the method comprising the following steps carried out by the controller: generating a signal (S) with a first frequency (f.sub.1) and a second frequency (f.sub.2) and feeding the signal (S) to the first electrode and the second electrode; determining a first voltage (U.sub.1) between the first electrode and the second electrode and a first current (I.sub.1) through the medium for the first frequency (f.sub.1) of the signal (S) and a first impedance (Z.sub.1) from the first voltage (U.sub.1) and the first current (I.sub.1); determining a second voltage (U.sub.2) between the first electrode and the second electrode and a second current (I.sub.2) through the medium for the second frequency (f.sub.2) of the signal (S) and a second impedance (Z.sub.2) from the second voltage (U.sub.2) and the second current (U.sub.2); determining and comparing a phase (φ.sub.1) of the first impedance (Z.sub.1) with a phase (φ.sub.2) of the second impedance (Z.sub.2); determining, when the phase (φ.sub.1) of the first impedance (Z.sub.1) is smaller than the phase (φ.sub.2) of the second impedance (Z.sub.2), a resistance (R) of the medium according to a first formula $R=\mathrm{Re}\left(Z_2\right)-\mathrm{Im}\left(Z_2\right).Math.\frac{\mathrm{Re}\left(Z_2\right)-\mathrm{Re}\left(Z_1\right)}{\mathrm{Im}\left(Z_2\right)-\mathrm{Im}\left(Z_1\right)}.$ determining, when the phase (φ.sub.1) of the first impedance (Z.sub.1) is equal to or greater than the phase (φ.sub.2) of the second impedance (Z.sub.2), the resistance (R) according to a second formula $R=\frac{\left|Z_1\right|}{\cos \left({\phi }_1\right)};$ and determining a conductivity (σ) of the medium using the resistance (R) and the cell constant (k).

2. The method according to claim 1, wherein the controller includes leads; wherein the leads connect the controller to the first electrode and the second electrode; and wherein, if the phase (φ.sub.1) of the first impedance (Z.sub.1) is smaller than the phase (φ.sub.2) of the second impedance (Z.sub.2), a lead resistance (R.sub.L) of the leads is subtracted from the resistance (R).

3. The method according to claim 1, wherein the signal (S) includes a first partial signal with the first frequency (f.sub.1) and a second partial signal with the second frequency (f.sub.2); and wherein the first partial signal and the second partial signal are superimposed on each other in the time domain.

4. The method according to claim 1, wherein a quotient of the second frequency (f.sub.2) divided by the first frequency (f.sub.1) is in a range between 1.8 and 2.2.

5. The method according to claim 4, wherein the first frequency (f.sub.1) is in a range between 450 Hz and 550 Hz.

6. The method according to claim 1, wherein the signal (S) includes a signal voltage (U) and a signal current (I), and wherein the controller carries out the following steps: impressing the signal current into the medium; determining the first voltage (U.sub.1) by generating a first square wave signal (T.sub.1, T.sub.1′) with the first frequency (f.sub.1) and multiplying the first square wave signal (T.sub.1, T.sub.1′) by the signal voltage (U); and determining the second voltage (U.sub.2) by generating a second square-wave signal (T.sub.2, T.sub.2′) with the second frequency (f.sub.2) and multiplying the second square-wave signal (T.sub.2, T.sub.2′) by the signal voltage (U).

7. An operating method of a magnetic-inductive flowmeter including a controller, a measuring tube, a first electrode and a second electrode, wherein the first electrode and the second electrode are arranged on the measuring tube in direct contact with a medium in the measuring tube, wherein the controller is arranged to determine a flow rate of the medium through the measuring tube using the first electrode and the second electrode wherein the first electrode, the second electrode and the measuring tube form a measuring cell having a cell constant (k) for determining a conductivity (σ) of the medium, the method comprising the following steps carried out by the controller: generating a signal (S) with a first frequency (f.sub.1) and a second frequency (f.sub.2) and feeding the signal (S) to the first electrode and the second electrode; determining a first voltage (U.sub.1) between the first electrode and the second electrode and a first current (I.sub.1) through the medium for the first frequency (f.sub.1) of the signal (S) and a first impedance (Z.sub.1) from the first voltage (U.sub.1) and the first current (I.sub.1); determining a second voltage (U.sub.2) between the first electrode and the second electrode and a second current (I.sub.2) through the medium for the second frequency (f.sub.2) of the signal (S) and a second impedance (Z.sub.2) from the second voltage (U.sub.2) and the second current (I.sub.2); determining and comparing a phase (φ.sub.1) of the first impedance (Z.sub.1) with a phase (φ.sub.2) of the second impedance (Z.sub.2); determining, when the phase (φ.sub.1) of the first impedance (Z.sub.1) is smaller than the phase (φ.sub.2) of the second impedance (Z.sub.2), a resistance (R) of the medium according to a first formula $R=\mathrm{Re}\left(Z_2\right)-\mathrm{Im}\left(Z_2\right).Math.\frac{\mathrm{Re}\left(Z_2\right)-\mathrm{Re}\left(Z_1\right)}{\mathrm{Im}\left(Z_2\right)-\mathrm{Im}\left(Z_1\right)};$ determining, when the phase (φ.sub.1) of the first impedance (Z.sub.1) is equal to or greater than the phase (φ.sub.2) of the second impedance (Z.sub.2), the resistance (R) according to a second formula $R=\frac{\left|Z_1\right|}{\cos \left({\phi }_1\right)};$ and determining a conductivity (σ) of the medium using the resistance (R) and the cell constant (k).

8. The operating method according to claim 7, wherein a flow rate of the medium through the measuring tube is determined by the controller by carrying out a magnetic-inductive flow measurement method using the first electrode and the second electrode.

9. The operating method according to claim 8, wherein, in the magnetic-inductive flow measuring method, an alternating magnetic field for determining the flow is generated by the controller; and wherein, in a transient region of the alternating magnetic field, the signal (S) is fed into the first electrode and the second electrode by the controller.

10. The operating method according to claim 7, wherein the controller includes leads: wherein the leads connect the controller to the first electrode and the second electrode; and wherein, if the phase (φ.sub.1) of the first impedance (Z.sub.1) is smaller than the phase (φ.sub.2) of the second impedance (Z.sub.2), a lead resistance (R.sub.L) of the leads is subtracted from the resistance (R).

11. A magnetic-inductive flowmeter comprising: a controller; a measuring tube; a first electrode; and a second electrode; wherein the first electrode and the second electrode are arranged on the measuring tube in such a way that the first electrode and the second electrode are in direct contact with a medium in the measuring tube during operation of the magnetic-inductive measuring device; wherein the controller is adapted to determine a flow rate of the medium through the measuring tube using the first electrode and the second electrode; wherein the first electrode, the second electrode and the measuring tube form a measuring cell having a cell constant (k) for determining a conductivity (σ) of the medium; and wherein the controller is designed to carries out the following steps: generating a signal (S) with a first frequency (f.sub.1) and a second frequency (f.sub.2) and feeding the signal (S) into the first electrode and the second electrode; determining a first voltage (U.sub.1) between the first electrode and the second electrode and a first current (I.sub.1) through the medium for the first frequency (f.sub.1) of the signal (S) and a first impedance (Z.sub.1) from the first voltage (U.sub.1) and the first current (I.sub.1); determining a second voltage (U.sub.2) between the first electrode and the second electrode and a second current (I.sub.2) through the medium for the second frequency (f.sub.2) of the signal (S) and a second impedance (Z.sub.2) from the second voltage (U.sub.2) and the second current (I.sub.2); determining and comparing a phase (φ.sub.1) of the first impedance (Z.sub.1) with a phase (φ.sub.2) of the second impedance (Z.sub.2); determining, when the phase (φ.sub.1) of the first impedance (Z.sub.1) is smaller than the phase (φ.sub.2) of the second impedance (Z.sub.2), a resistance (R) of the medium according to a first formula $R=\mathrm{Re}\left(z_2\right)-\mathrm{Im}\left(Z_2\right).Math.\frac{\mathrm{Re}\left(Z_2\right)-\mathrm{Re}\left(Z_1\right)}{\mathrm{Im}\left(Z_2\right)-\mathrm{Im}\left(Z_1\right)};$ determining, when the phase (φ.sub.1) of the first impedance (Z.sub.1) is equal to or greater than the phase (φ.sub.2) of the second impedance (Z.sub.2), the resistance (R) according to a second formula $R=\frac{\left|Z_1\right|}{\cos \left({\phi }_1\right)};$ and determining a conductivity (σ) of the medium using the resistance (R) and the cell constant (k).

12. The magnetic-inductive flowmeter according to claim 11, wherein the controller includes leads; wherein the leads connect the controller to the first electrode and the second electrode; and wherein, if the phase (φ.sub.1) of the first impedance (Z.sub.1) is smaller than the phase (φ.sub.2) of the second impedance (Z.sub.2), a lead resistance (R.sub.L) of the leads is subtracted from the resistance (R).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In detail, a multitude of possibilities are given for designing and further developing the method for determining a conductivity of a medium, the method for operating a magnetic-inductive flowmeter and the magnetic-inductive flowmeter. For this purpose, reference is made to the following description of a preferred embodiment in connection with the drawings.

[0056] FIG. 1 illustrates an embodiment of a magnetic-inductive flowmeter.

[0057] FIG. 2 illustrates a plot of signal over time of the magnetic-inductive flowmeter.

[0058] FIG. 3 illustrates an electrical equivalent circuit of a medium.

[0059] FIG. 4 illustrates a plot of phases of impedances at two different frequencies of a signal over a conductivity of a medium.

DETAILED DESCRIPTION

[0060] FIG. 1 shows an embodiment of a magnetic-inductive flowmeter 1 during operation while carrying out an operating method. The magnetic-inductive flowmeter 1 has a controller 2, a measuring tube 3, a first electrode 4, a second electrode 5 and an electromagnet 6. A medium 7, which has an electrical conductivity, flows through the measuring tube 3. The first electrode 4 and the second electrode 5 are arranged on the measuring tube 3 such that the first electrode 4 and the second electrode 5 are in direct contact with the medium 7 in the measuring tube 3.

[0061] The electromagnet 6 has a yoke 8 and a coil 9 arranged around the yoke 8. The coil 9 is driven by a driver 10 and the driver 10 is controlled by a microcontroller 11, which is indicated by a double arrow in FIG. 1. The driver 10 and the microcontroller 11 are arranged in the controller 2. The electromagnet 6 is set up in such a way that a magnetic field 12 generated by it, which is indicated in FIG. 1 by a plurality of parallel arrows and has a magnetic flux density B, partially penetrates the medium 7 located in the measuring tube 3, and the flow of the medium 7 in the measuring tube 3 induces a voltage in the medium 7 which can be tapped between the first electrode 4 and the second electrode 5. The induced voltage is proportional to a flow rate of the medium 7 in the measuring tube 3.

[0062] The controller 2 has a first lead 13 and a second lead 14, wherein the first lead 13 connects the microcontroller 11 to the first electrode 4 and the second lead 14 connects the microcontroller 11 to the second electrode 5 so that the induced voltage is applied to the microcontroller 11. The microcontroller 11 is designed to evaluate the induced voltage. The first lead 13 and the second lead 14 together have a lead resistance R.sub.L. In this sense, the controller 2 is connected to the first lead 13 and to the second lead 14.

[0063] The controller 2 is designed to determine a flow rate of the medium 7 through the measuring tube 3 using the first electrode 4 and the second electrode 5 during the operating method. In the present case, the operating method is designed such that a flow rate of the medium 7 through the measuring tube 3 is determined by the controller 2 by carrying out an electromagnetic flow measurement method using the first electrode 4 and the second electrode 5. In the electromagnetic flow measurement method, a rectangular alternating magnetic field having the magnetic flux density B is generated by the controller 2 for determining the flow rate. The microcontroller 11 controls the driver 10 in such a way that the square-wave magnetic field is generated. The plot of the magnetic flux density B of the magnetic square-wave field over time t is shown in the first line of the diagram in FIG. 2. Successive constant ranges of magnetic flux density B are connected by transient ranges. From the induced voltage, the microcontroller 11 first determines a flow velocity of the medium 7 in the measuring tube 3, and then from the flow velocity, using an inner cross-sectional area of the measuring tube 3, determines a volumetric flow rate of the medium 7 through the measuring tube 3.

[0064] The first electrode 4, the second electrode 5, and a portion of the measuring tube 3 form a conductive measuring cell 15 having a cell constant k for determining a conductivity σ of the medium 7. A conductivity σ of the medium 7 is determined according to the formula

[00008]$\sigma =\frac{\text{?}}{\text{?}}$$\text{?}\text{indicates text missing or illegible when filed}$

[0065] FIG. 2 shows an electrical equivalent circuit 16 for the medium 7 between the first electrode 4 and the second electrode 5. The equivalent circuit 16 is an approximation of the medium 3 for alternating signals. The equivalent circuit 16 has a parallel circuit consisting of a series circuit and a parallel capacitor 17. The series circuit has a medium resistor 18 with resistance R and a series capacitor 19. Using the resistance R and the cell constant k, the conductivity σ is determined according to the above formula. Thus, the resistance R is a resistance of the medium 7 between the first electrode 4 and the second electrode 5.

[0066] The controller 2 has a sinus wave generator 20 and a drive 21 for feeding an electrical signal S to the first electrode 4 and the second electrode 5. The sinus wave generator 20 is controlled by the microcontroller 11, which is indicated by a double arrow in FIG. 1, and is designed to generate a generator signal G having at least a first frequency f.sub.1 and a second frequency f.sub.2. The controller 21 has a first path 22 and a second path 23, into which the generator signal G is fed during operation. The first path 22 opens into the first lead 13 and the second path opens into the second lead 14. The first path 22 has a series circuit with a first amplifier 24, a first resistor 25 and a first capacitor 26. The first amplifier 24 has a gain of +1. The first resistor 25 and the first capacitor 26 form a first high-pass filter. The second path 23 has a second amplifier 27, a second resistor 28, and a second capacitor 29. The second amplifier 27 has a gain of −1. The second resistor 28 and the second capacitor 29 form a second high-pass filter. While the first amplifier 24 only buffers the generator signal G, it is additionally inverted by the second amplifier 27. The signal S has a signal voltage U and a signal current I. The controller 21 generates the signal S, which is a differential signal, from the generator signal G. The signal S is inverted by the second amplifier 27.

[0067] A cut-off frequency of the first high-pass filter and a cut-off frequency of the second high-pass filter are selected such that no substantial attenuation of the signal S occurs. These high-pass filters serve to prevent a DC signal from being fed into the first electrode 4 and the second electrode 5. The values of the first resistor 25 and the second resistor 28 are chosen such that the resistance R is negligible in an approximate determination of the signal current I. Namely, the signal current I is determined by the microcontroller 11, that is, the controller 2, by dividing a voltage of the generator signal G by half the value of the sum of the values of the first resistor 25 and the second resistor 26. The approximate determination of the signal current includes magnitude and phase and is sufficiently accurate for the methods described.

[0068] In addition to the electromagnetic flow measurement method previously described, the controller 2 also carries out the following steps as part of the operating method, which it is also designed to carry out: [0069] In one step, the signal S is generated with a first frequency f.sub.1=500 Hz and a second frequency f.sub.2=1 kHz and is fed into the first electrode 4 and the second electrode 5. The signal S is fed in the present case by impressing the signal current I into the medium 7 via the first electrode 4 and the second electrode 5. A quotient of the second frequency f.sub.2 divided by the first frequency f.sub.1 is 2. For this, the sinus wave generator 20 first generates the generator signal G which has both the first frequency f.sub.1 and the second frequency f.sub.2. The generator signal G is then fed into the first path 22 and the second path 23 of the drive 21. The drive 21 then generates the signal S from the generator signal G, which is fed into the first electrode 4 and the second electrode 5.

[0070] The signal S has a first partial signal with the first frequency f.sub.1 and a second partial signal with the second frequency f.sub.2. The first partial signal and the second partial signal are superimposed in time. The second lead in FIG. 2 shows the signal S with the two temporally superimposed partial signals. The signal S is fed into the first electrode 4 and the second electrode 5 only in transient areas of the alternating magnetic field B. The signal S is not fed into the first electrode 4 and the second electrode 5. This does not impair the determination of the flow rate. Neither the quality suffers nor the duration of the determination of the flow rate increases.

[0071] In one step, a first voltage U.sub.1 is determined between the first electrode 4 and the second electrode 5 and a first current I.sub.1 is determined through the medium 7 for the first frequency f.sub.1 of the signal S and a first impedance Z.sub.1 is determined from the first voltage U.sub.1 and the first current I.sub.1. This step includes a substep in which the first voltage U.sub.1 is determined by generating a first square-wave signal T.sub.1 with the first frequency f.sub.1 and multiplying the first square-wave signal T.sub.1 by the signal voltage U. The first square wave signal T.sub.1 also includes a copy T.sub.1′ of the first square wave signal T.sub.1, which has a phase shift with respect to the first square wave signal T1, preferably by 90°. The first square wave signal T.sub.1 is shown in line 3 and its copy T.sub.1′ is shown in line 4 of FIG. 2. Accordingly, both the first square wave signal T.sub.1 and its copy T.sub.1′ are multiplied by the signal voltage U so that the voltage U.sub.1 is determined by magnitude and phase. The first current I.sub.1 is determined according to the approximate determination described above, also by magnitude and phase. The first impedance Z.sub.1 is also determined by magnitude and phase φ.sub.1.

[0072] In one step, a second voltage U.sub.2 between the first electrode 4 and the second electrode 5 and a second current I.sub.2 through the medium 7 are determined for the second frequency f.sub.2 of the signal S and a second impedance Z.sub.2 is determined from the second voltage U.sub.2 and the second current I.sub.2. Also included in this step is a substep in which the second voltage U.sub.2 is determined by generating a second square wave signal T.sub.2 with the second frequency f.sub.2 and multiplying the second square wave signal T.sub.2 by the signal voltage U. The second square wave signal T.sub.2 also includes a copy T.sub.2′ of the second square wave signal T.sub.2, which has a phase shift with respect to the second square wave signal T.sub.2, preferably by 90°. The second square wave signal T.sub.2 is shown in line 5 and its copy T.sub.2 is shown in line 6 of FIG. 2. Accordingly, both the second square wave signal T.sub.2 and its copy T.sub.2′ are multiplied by the signal voltage U so that the voltage U.sub.2 is determined by magnitude and phase. The second current I.sub.2 is determined according to the approximate determination described earlier, also by magnitude and phase. The second impedance Z.sub.2 is also determined by magnitude and phase φ.sub.2.

[0073] In one step, the phase φ.sub.1 of the first impedance Z.sub.1 and the phase φ.sub.2 of the second impedance Z.sub.2 are compared after they have been determined.

[0074] In one step, if the phase φ.sub.1 of the first impedance Z.sub.1 is smaller than the phase φ.sub.2 of the second impedance Z.sub.1, that is, φ.sub.1<φ.sub.2, the resistance R of the medium is determined according to the first formula

[00009]$R=\mathrm{Re}\left(Z_2\right)-\mathrm{Im}\left(Z_2\right).Math.\frac{\mathrm{Re}\left(Z_2\right)-\mathrm{Re}\left(Z_1\right)}{\mathrm{Im}\left(Z_2\right)-\mathrm{Im}\left(Z_1\right)}-R_L.$

[0075] In the first formula, the lead resistance R.sub.L is also subtracted.

[0076] In one step, if the phase φ.sub.1 of the first impedance Z.sub.1 is equal to or greater than the phase φ.sub.2 of the second impedance Z.sub.2, i.e., φ.sub.1≥φ.sub.2, the resistance R is determined according to the second formula

[00010]$R=\frac{\text{?}Z\text{?}}{\text{?}}$$\text{?}\text{indicates text missing or illegible when filed}$

[0077] The two steps described immediately above represent alternative steps to each other, which are carried out depending on what the ratio of the phase (pi of the first impedance Z.sub.1 to the phase φ.sub.2 of the second impedance Z.sub.2 is. FIG. 4 shows the suitability of the phase of the impedance as a selection criterion for the first or second formula. It shows, on the one hand, a plot of the phase φ.sub.1 of the first impedance Z.sub.1 over the conductivity σ and, on the other hand, a plot of the phase φ.sub.2 of the second impedance Z.sub.2 over the conductivity σ.

[0078] In one step, the conductivity σ of the medium 7 is determined using the resistance R and the cell constant k according to the formula

[00011]$\sigma =\frac{\text{?}}{\text{?}}$$\text{?}\text{indicates text missing or illegible when filed}$