Method for operating an inductive conductivity meter and respective conductivity meter

Abstract

A method for operating an inductive conductivity meter having a transmitting coil, a receiving coil and a terminating impedance device, the transmitting coil having a transmitting coil terminal, the receiving coil having a receiving coil terminal and the terminating impedance device having a terminating impedance, wherein the receiving coil is terminated with the terminating impedance device and wherein the transmitting coil and the receiving coil are inductively coupled with one another by an electrically conductive medium. To provide an improved accuracy of a determination of a conductivity of a medium a setpoint input impedance is specified, an input impedance is determined at the transmitting coil terminal, the terminating impedance is set such that the input impedance is matched to the setpoint input impedance, and a conductivity of the medium is determined using the adjusted input impedance and the set termination impedance.

Claims

1. A method for operating an inductive conductivity meter having a transmitting coil, a receiving coil and a terminating impedance device, wherein the transmitting coil has a transmitting coil terminal, the receiving coil has a receiving coil terminal and the terminating impedance device has a terminating impedance, wherein the receiving coil is terminated with the terminating impedance device and wherein the transmitting coil and the receiving coil are inductively coupled with one another by an electrically conductive medium, the method comprising the following steps: specifying a setpoint input impedance, determining an input impedance at the transmitting coil terminal, setting the terminating impedance such that the input impedance is matched to the setpoint input impedance, and determining a conductivity of the medium using the matched input impedance and the set termination impedance.

2. The method according to claim 1, wherein the matching of the input impedance to the setpoint input impedance is performed in a manner such that a real part of the input impedance corresponds to a real part of the setpoint input impedance.

3. The method according to claim 1, wherein the input impedance is adjusted to the setpoint input impedance such that the input impedance corresponds to the setpoint input impedance.

4. An inductive conductivity meter, comprising: a transmitting coil, a receiving coil, a terminating impedance device, and a control device, wherein the transmitting coil has a transmitting coil terminal, the receiving coil has a receiving coil terminal and the terminating impedance device has a terminating impedance, wherein the receiving coil is terminated with the terminating impedance device and wherein, during operation, the transmitting coil and the receiving coil are inductively coupled with one another by an electrically conductive medium, wherein a setpoint input impedance is stored in the control device, wherein the terminating impedance of the terminating impedance device is adjustable, and wherein the control device has means for determining an input impedance at the transmitting coil terminal, for setting the terminating impedance such that the input impedance is matched to the setpoint input impedance, and for determining a conductivity of the medium using the adjusted input impedance and the set terminating impedance.

5. The inductive conductivity meter according to claim 1, wherein the terminating impedance device is adapted to adjust the input impedance to the setpoint input impedance such that a real part of the input impedance corresponds to a real part of the setpoint input impedance.

6. The inductive conductivity meter according to claim 4, wherein the terminating impedance device has a current sensor for measuring a receiving coil current through the receiving coil terminal, and a controlled voltage source for setting a receiving coil voltage across the receiving coil terminal, and wherein the control device has means for determining the receiving coil current with the current sensor and to set the receiving coil voltage such that the terminating impedance is set.

7. The inductive conductivity meter according to claim 6, wherein the current sensor is a shunt resistor and the controlled voltage source has an operational amplifier for providing the receiving coil voltage.

8. The inductive conductivity meter according to claim 7, wherein the controlled voltage source has a signal generator for controlling the operational amplifier.

9. The inductive conductivity meter according to claim 8, wherein the signal generator has means for generating a continuous-value signal or a pulse width modulated signal for controlling the operational amplifier.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an embodiment of an inductive conductivity meter,

(2) FIG. 2 is a partial electrical equivalent circuit diagram of the inductive conductivity meter from FIG. 1,

(3) FIG. 3 is a partial electrical equivalent circuit diagram of the terminating impedance device from FIG. 2,

(4) FIG. 4 is a partial electrical equivalent circuit diagram of the controlled voltage source from FIG. 3, and

(5) FIG. 5 is a flowchart of an example of a method for operating the inductive conductivity meter from FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

(6) FIG. 1 shows the inductive conductivity meter 1. The inductive conductivity meter 1 has the hollow-cylindrical carrier 2, on which the transmitting coil 3 and the receiving coil 4 are arranged. The transmitting coil 3 and the receiving coil 4 are arranged on the hollow cylindrical carrier 2 by being wound around the hollow cylindrical carrier 2, wherein the transmitting coil 3 has the number N.sub.1 turns and the receiving coil 4 has the number N.sub.4 turns. Furthermore, the transmitting coil 3 has the electrical transmitting coil terminal 5 and the receiving coil 4 has the electrical receiving coil terminal 6. The inductive conductivity meter 1 also has the control device 7, which is designed to control the transmitting coil 3 and the receiving coil 4, which is why the control device 7 is also electrically connected to the transmitting coil terminal 5 of the transmitting coil 3 and to the receiving coil terminal 6 of the receiving coil. In addition, a setpoint input impedance is stored in the control device 7.

(7) The hollow cylindrical carrier 2 with the transmitting coil 3 and the receiving coil 4 is immersed in the medium 8 and the inductive conductivity meter 1 is in operation. The medium 8 surrounds the hollow cylindrical carrier 2 and is also present in its interior. The medium 8 is electrically conductive and thereby couples the transmitting coil 3 and the receiving coil 4 inductively with one another. Since it is an abstracted schematic representation of the inductive conductivity meter 1, a housing, which is usually present and prevents direct contact of the transmitting coil 3 and the receiving coil 4 with the medium 8, is not shown here. By avoiding contact of the transmitting coil 3 and the receiving coil 4 with the medium 8, it is possible to use the inductive conductivity meter 1, in contrast to conductive conductivity meters, in aggressive and corrosive media such as industrial wastewater, seawater and acid solutions, without the functionality of the inductive conductivity meter 1 being impaired. The housing also makes it suitable for hygienic applications in processes in the fields of food, beverage and pharmaceuticals.

(8) FIG. 2 shows a partial electrical equivalent circuit diagram, on the one hand, of the inductive conductivity meter 1 and, on the other hand, of the electrically conductive medium 8.

(9) The equivalent circuit diagram of the inductive conductivity meter 1 has the transmitting coil 3, the transmitting coil terminal 5 and the transmitting alternating voltage source 9, wherein the transmitting alternating voltage source 9 is connected to the transmitting coil terminal 5 such that the transmitting alternating voltage source 9 and the transmitting coil 3 are electrically connected in parallel. Furthermore, the equivalent circuit of the inductive conductivity meter 1 has the receiving coil 4, the receiving coil terminal 6 and the terminating impedance device 10, which is part of the control device 7 in this embodiment. The terminating impedance device 10 is connected to the receiving coil terminal 6 such that the terminating impedance device 10 and the receiving coil 4 are electrically connected in parallel.

(10) The equivalent circuit diagram of the electrically conductive medium 8 has the medium resistor 11 with the medium resistance R.sub.W, which represents the electrical resistance of the medium 8 between the transmitting coil 3 and the receiving coil 4.

(11) The transmitting alternating voltage source 9 is designed to generate a sinusoidal transmitting coil voltage U.sub.1 given by the control device 7, and the control device 7 is designed to measure the transmitting coil current I.sub.1. Thus, the transmitting coil voltage U.sub.1 is a transmitting alternating voltage and the transmitting coil current I.sub.1 is a transmitting alternating current. Due to the parallel connection of the transmitting alternating voltage source 9 and the transmitting coil 3, the transmitting coil voltage U.sub.1 is also applied across the transmitting coil 3 and the transmitting coil current I.sub.1 flows through the transmitting coil 3. Further, the control device 7 is designed to determine the input impedance Z.sub.E=U.sub.1/I.sub.1 at the transmitting coil terminal 5 using the transmitting coil voltage U.sub.1, which is usually measured by the control device 7, and the transmitting coil current I.sub.1.

(12) The inductive coupling of the transmitting coil 3 and the receiving coil 4 with one another by the electrically conductive medium 8 is given in that the transmitting alternating signal fed into the transmitting coil 3, which is characterized by the transmitting coil voltage U.sub.1 and the transmitting coil current I.sub.1, generates eddy currents in the medium 8 and the eddy currents induce a receiving alternating signal in the receiving coil 4, which is characterized by the receiving coil voltage U.sub.4 and the receiving coil current I.sub.4. The receiving coil voltage U.sub.4 is incident across the receiving coil 4 and the receiving coil current I.sub.4 flows through the receiving coil 4. Due to the parallel connection of the receiving coil 4 and terminating impedance device 10, the receiving coil voltage U.sub.4 is also incident across the terminating impedance device 10 and the receiving coil current I.sub.4 also flows through the terminating impedance device 10. The receiving coil voltage U.sub.4 and the receiving coil current I.sub.4 are specified by the terminating impedance Z.sub.A=U.sub.4/I.sub.4. The generation of eddy currents in the medium 8 by the transmitting alternating signal takes place using a transformer coupling between the transmitting coil 3 and the medium 8 according to U.sub.1=N.sub.1.Math.U.sub.2 and the transformer coupling between the medium 8 and the receiving coil 4 takes place according to U.sub.4=U.sub.3.Math.N.sub.4.

(13) A partial equivalent electrical circuit diagram of the terminating impedance device 10 is shown in FIG. 3. The terminating impedance Z.sub.A of the terminating impedance device 10 is adjustable, and the control device 7 is designed to set the terminating impedance Z.sub.A of the terminating impedance device 10. Thus, the control device 7 controls the terminating impedance device 10. The terminating impedance device 10 has a current sensor 12 for measuring the receiving coil current I.sub.4 through the receiving coil terminal 6 and a controlled voltage source 13 for adjusting the receiving coil voltage U.sub.4 across the receiving coil terminal to set the terminating impedance. The control device 7 is designed to set the terminating impedance Z.sub.A, to determine the receiving coil current I.sub.4 with the current sensor 12 and to set the receiving coil voltage U.sub.4 such that the terminating impedance Z.sub.A is set. Thus, an active terminating impedance Z.sub.A is implemented by the terminating impedance device 10 and the control device 7. In the present embodiment, the current sensor 12 is a shunt resistor and the controlled voltage source 13 has the operational amplifier 14 shown in FIG. 4 for providing the receiving coil voltage U.sub.4 and the signal generator 15 for controlling the operational amplifier 14. In this case, the control device 7 is designed to control the signal generator 15. In the present embodiment, the signal generator 15 is designed to generate a continuous-value signal for controlling the operational amplifier 14.

(14) A transfer function from the transmitting coil 3 to the receiving coil 4 is given by the following formula:

(15) $\frac{I_4}{U_1}=\frac{N_4}{N_1}\frac{1}{Z_A+R_W{N}_4^2\left(1+\frac{Z_A}{j{L}_{44}}\right)}$

(16) The reciprocal of the input impedance Z.sub.E, i.e., the input admittance Y.sub.E, is given by the following formula:

(17) $Y_E=\frac{1}{N_1^2{R}_W+{\left(\frac{N_1}{N_4}\right)}^2(Z_A.Math.j{L}_{44})}+\frac{1}{j{L}_{11}}$

(18) In the above formulas, j is the imaginary unit, is the angular frequency, L.sub.11 is an inductance between transmitting coil 3 and the medium 8, L.sub.44 is an inductance between the receiving coil 4 and the medium 8, and (Z.sub.AjL.sub.44) symbolizes a calculation instruction for calculating the impedance of a parallel connection of Z.sub.A and jL.sub.44.

(19) It can be seen from the above formulas that the terminating impedance Z.sub.A is a parameter of both the transfer function and the input impedance Z.sub.E. Thus, the input impedance Z.sub.E can be matched to the setpoint input impedance by adjusting the terminating impedance Z.sub.A. For this, the control device 7 is designed for carrying out the method illustrated in the flowchart in FIG. 5 with the following method steps.

(20) In the first method step 16, a setpoint input impedance is specified. This is the setpoint input impedance that is stored in the control device 7.

(21) In the second method step 17, the input impedance Z.sub.E is determined at the transmitting coil terminal 5.

(22) In the third method step 18, the terminating impedance Z.sub.A is adjusted such that the input impedance Z.sub.E is matched to the setpoint input impedance. In the fourth method step 19, a conductivity of the medium 8 is then determined using the adjusted input impedance Z.sub.E and the set terminating impedance Z.sub.A. The conductivity is determined by first determining the medium resistance value R.sub.W and then the conductivity of the medium 8 using the medium resistance value R.sub.W and the geometry of the medium 8 between the transmitting coil 3 and the receiving coil 4.

(23) The listed method steps are also carried out several times by the control device 7 as required. This applies, in particular, to the second method step 17 and the third method step 18 when the input impedance Z.sub.E is adapted to the setpoint input impedance by means of an iterative method by varying the terminating impedance Z.sub.A.