Compensated conductivity determination

Abstract

The present invention relates to a method for determining and / or monitoring the conductivity of a medium (M) by means of a measuring probe (1) having at least one electrode (4), comprising the following method steps of: Applying an excitation signal (A) to the measuring probe (1), receiving a reception signal (E) from the measuring probe (1), determining an ohmic component (Eo) of the reception signal (E), and determining the conductivity of the medium (3) based on the ohmic component (Eo) of the reception signal (E). The present invention also relates to a device which is designed for carrying out the claimed method.

Description

[0001] Compensated conductivity determination

[0002] The invention relates to a method for determining the conductivity of a medium using a measuring probe having at least a first electrode. The medium is located in a container, which can be, for example, a tank or a pipeline. The invention further relates to a device designed to carry out the method according to the invention.

[0003] The electrical conductivity of a medium can be determined, for example, by means of a conductivity measuring cell, as described in EP0990894A2, or by means of an inductive sensor.

[0004] However, field devices based on the capacitive and / or conductive measuring principle, which are often also used to determine and / or monitor a fill level or limit level, are also suitable for determining the conductivity of a medium. Such capacitive or conductive measuring devices typically have a substantially cylindrical measuring probe with at least one electrode, which is at least partially inserted into a container. On the one hand, rod-shaped measuring probes that extend vertically into the container are widespread, particularly for continuous fill level measurement. However, for detecting a limit level, measuring probes that can be inserted into the side wall of a respective container, and in particular those that are essentially flush with the front of the container wall, have also become known.

[0005] During conventional measurement operation, the measuring probe, in particular a transmitting electrode of the measuring probe, is subjected to an excitation signal, usually in the form of an alternating current signal. The respective process variable can then be determined from the response signal received by the measuring probe. According to the capacitive measuring principle, the dependence of the response signal on the capacitance of a first electrode of the measuring probe and the container wall or a capacitor formed by another electrode of the measuring probe is used to determine the respective process variable. Depending on the conductivity of the medium, either the medium itself or insulation of the measuring probe forms the dielectric of this capacitor.

[0006] To evaluate the response signal received from the measuring probe, an apparent current measurement or an admittance measurement is often performed. In an apparent current measurement, the amount of apparent current flowing through the sensor unit is measured. However, the apparent current has an active and a reactive component. Therefore, in the case of an admittance measurement, in addition to the apparent current, the phase angle between the apparent current and the voltage applied to the measuring probe is measured. The additional determination of the phase angle also allows for statements about possible buildup, as disclosed, for example, in DE102004008125A1.

[0007] In contrast, the conductive measuring principle detects whether there is electrical contact between one of the electrodes and the wall of a conductive container or the second electrode via the conductive medium used when applying the conductive measuring principle.

[0008] Field devices in the form of multisensors that can operate in both capacitive and conductive modes are known, for example, from documents DE102011004807A1, DE102013102055A1, and DE102014107927A1. Such multisensors are also suitable for determining various media-specific properties, such as electrical conductivity, or dielectric properties of the medium, such as the dielectric constant, as described, for example, in DE102013104781A1.

[0009] One of the problems encountered when determining conductivity in process measurement technology is the formation of insulating layers around the electrodes used. Other deposits on the electrode surfaces can also cause similar problems.

[0010] A reduction in the measurement error resulting from such coatings can be achieved, for example, by using alternating signals, increasing the frequency of the excitation signal, and / or reducing the measuring current. Alternatively or additionally, it has also become known to manufacture the electrodes at least partially from graphite or platinum, or to use platinum-coated electrodes.

[0011] However, such measures often limit signal evaluation or make corresponding sensors comparatively expensive.

[0012] Therefore, the object of the present invention is to provide a method for determining and / or monitoring electrical conductivity that is as simple and accurate as possible.

[0013] This object is achieved by the method according to claim 1 and by the device according to claim 8.

[0014] With regard to the method, the object underlying the invention is achieved by a method for determining and / or monitoring the conductivity of a medium by means of a measuring probe which has at least one electrode, comprising the following method steps:

[0015] Applying an excitation signal to the measuring probe, receiving a received signal from the measuring probe, determining an ohmic component of the received signal, and determining the conductivity of the medium based on the ohmic component of the received signal.

[0016] The method according to the invention can be applied to all types of measuring probes suitable for capacitive and / or conductive measurement methods. The measuring probe can have one or more electrodes with different functions, such as a transmitting electrode, a receiving electrode, a ground electrode, and / or a guard electrode. The same electrode can also have different functions.

[0017] According to the invention, the at least one electrode serves as a transmitting electrode, and a container wall or an optional second electrode serves as a ground electrode. The second electrode can also serve, at least temporarily, as a receiving electrode.

[0018] By determining the ohmic component of the received signal, the influence of insulation and / or other layers or deposits on the at least one electrode on the received signal can be eliminated or reduced. The method according to the invention accordingly makes it possible to use a simple sensor, for example, with a stainless steel electrode or similar, without any loss of measurement accuracy due to deposits, in particular insulating deposits or insulating deposits, and / or polarization layers. Furthermore, the range of applications can be significantly expanded, since no special requirements for the excitation signal, such as a high frequency, need be placed on it.

[0019] In one embodiment, the measuring probe is operated in a capacitive and / or a conductive mode. In this case, the measuring probe preferably has at least two electrodes. In both operating modes, additional process variables of the medium can advantageously be determined.

[0020] For example, in this case, it may also be a multi-sensor suitable for both capacitive and conductive process variable determination. Both operating modes can be carried out alternately, simultaneously, or at predefined times. In this context, it is possible, for example, to combine the excitation signal from two sub-signals. A first sub-signal can be selected for use with the capacitive measuring mode and a second sub-signal for use with the conductive measuring mode. In this context, particular reference is also made to DE102014107927A1, which is incorporated by reference in this application and which describes the use of an excitation signal composed of two sub-signals to implement a capacitive and conductive multi-sensor.It is also advantageous if the fill level or limit level of the medium in the respective container is also determined. In addition, dielectric properties of the medium, such as a dielectric constant, can also be determined.

[0021] A further embodiment of the method according to the invention involves recording the received signal as a function of time. Recording as a function of time can significantly improve the evaluation of the received signal with regard to conductivity.

[0022] The excitation signal can be, for example, a sine signal, a triangular signal, or a trapezoidal signal.

[0023] However, it is particularly advantageous if a square-wave signal is used for the excitation signal. Using a square-wave signal as the excitation signal offers particular advantages with regard to determining the resistive component of the received signal, as will be explained below.

[0024] With regard to the use of a square-wave signal for the excitation signal, it is further advantageous to determine a first point in time at which the received signal exhibits a step from a first, in particular a maximum or minimum value, to a second value, and to determine the difference between the first and second values. The received signal, as a function of time, thus exhibits a jump or step, which is detected or determined. The height of the step can then be used for further signal analysis.

[0025] It is also advantageous to determine the resistive component of the received signal based on the difference between the first and second values, i.e., the height of the step. This resistive component of the received signal is free from effects due to insulation layers or other deposits on the respective electrode surface.

[0026] The object underlying the invention is further achieved by a device for determining and / or monitoring the conductivity of a medium with a measuring probe which has at least one electrode, which device is designed to carry out the method according to the invention according to at least one of the described embodiments.

[0027] The device can be configured, in particular, analogously to the device described in DE102014107927A1, particularly with regard to the electronics. It should be noted that the configurations described in connection with the method according to the invention can also be applied, mutatis mutandis, to the device according to the invention.

[0028] The invention will now be described in more detail with reference to the following figures, Fig. 1 to Fig. 4. It shows:

[0029] Fig. 1 : exemplary schematic representations of a measuring probe for use with the method according to the invention;

[0030] Fig. 2: a block diagram for an electronic system according to the prior art;

[0031] Fig. 3: a block diagram illustrating the method according to the invention; and

[0032] Fig. 4: (a) a rectangular excitation signal and (b) a corresponding

[0033] Received signal as a function of time.

[0034] Fig. 1a shows an exemplary embodiment of a measuring probe 1 of a field device V, by means of which a process variable can be monitored using a capacitive or conductive measuring method. The measuring probe 1 is arranged in a container 2, which is at least partially filled with a medium 3. In this case, it protrudes from the top of the container into the container. However, it goes without saying that in other embodiments, the measuring probe 1 can also be designed so that it is flush with the wall of the container 3.

[0035] In this example, the measuring probe 1 itself consists of two electrodes: a first electrode 4 and a second electrode 5, which serves to prevent the formation of deposits. The container wall also forms a ground electrode 6. The measuring probe 1 is also connected to an electronic unit 8, which is responsible for signal acquisition, evaluation, and / or power supply. In particular, the electronic unit 8 determines the respective process variable based on the received signals.

[0036] Fig. 1b shows a sectional view of a measuring probe 1 with three electrodes 4, 5, 6. The electrodes 4, 5, 6 are electrically separated from each other by insulation 7a, 7b and surround each other concentrically. Such a design for a measuring probe 1 is particularly advantageous for a measuring probe 1 that is flush with the container wall.

[0037] Within the scope of the present invention, the use of a measuring probe 1 with a single electrode 4 is also possible in principle. In this case, for example, the container wall can form a ground electrode. Fig. 2 shows a block diagram of an electronics unit 8, by means of which the measuring probe 1 can be operated in both capacitive and conductive modes. However, it should be noted that the present invention is by no means limited to such an electronics unit 8, but that the embodiment shown merely represents one possible example of a suitable electronics unit 8. The electronics unit 8 in Fig. 2 corresponds to the electronics unit described in DE102014107927A1.

[0038] The electronics 8 includes a microcontroller 9 and is divided into a

[0039] Area for generating the transmission signal 10a from different partial signals and an area for evaluating the reception signals 10b dependent on the respective partial signals.

[0040] Two voltage dividers 11, 11a are used to generate a transmission signal in the form of a square wave for the conductive operating mode: a low-impedance voltage divider (R1 / R2) 11 for highly conductive media and a high-impedance voltage divider (R3 / R4) 11a for slightly conductive media. These two voltage dividers 11, 11a are clocked via corresponding port outputs 12, 12a of the microcontroller 9. In the example shown here, a transmission signal for the capacitive operating mode—a triangular voltage generated by the integration amplifier (Block A) 13—is selected via another port output of the microcontroller 12b.

[0041] The area for evaluating the received signals 10, which depend on the respective subsignals, comprises blocks B to D, which all include three operational amplifiers. To minimize the influence of parasitic effects and buildup on the measuring probe 1, the guard technique according to DE00102008043412A1 can also be used.

[0042] Block B 14 contains an amplifier that provides the reference signal, in this case the guard voltage, to the analog-to-digital converter (ADC) 15 of the microcontroller 9. B 14 can also be used to shield at least one circuit board.

[0043] Block e 16 also includes an amplifier, which is responsible for supplying the received signal to the ADC 15. A measuring resistor 17 is also provided, with which the difference between the voltages at the transmitting electrode and the guard electrode can be determined.

[0044] To evaluate the received signal obtained from the capacitive measurement, block D 18 is also required. This block contains a differential amplifier to subtract and amplify the two received signals from the transmit and guard electrodes. This is done via the measuring resistor 17. The difference between the two received signals is directly proportional to the capacitance at the measuring probe 1 . With such an electronics 8, a measurement resolution of a few femtofarads is possible. Furthermore, the block diagram in Fig. 2 shows four decoupling capacitors.

[0045] 19, 19a, 19b, and 19c are shown, which filter the DC voltage components from the respective signals. Finally, the ESD protection circuit 20 is shown. Furthermore, offset compensation, not shown here, can also be performed.

[0046] The determination of electrical conductivity according to the invention can be explained in more detail using the block diagram in Fig. 3. A measuring probe 1 with an electrode 4 is subjected to an excitation signal A, and the received signal E is evaluated by the region 10b for evaluating the received signals.

[0047] The block diagram further includes an equivalent circuit diagram for an impedance Zi of the electrode 4, represented here in simplified form by a resistor, as well as an impedance Z2 of any insulating layers and / or other deposits present in the region of the electrode 4, which is given by a parallel connection of a capacitance C and a resistor R. By means of the present invention, the ohmic component of the received signal E is determined in order to determine the conductivity of the medium 3.

[0048] An exemplary possibility for determining the resistive component of the received signal E in the case of a rectangular excitation signal A is illustrated in Fig. 4. Fig. 4a shows the excitation signal A as a function of time t and Fig. 4b shows three different received signals E1-E3 as a function of time t.

[0049] In the case that there are no insulation layers and / or deposits on the electrode 4, the received signal Ei is also a square wave and from the height of the steps E01 at the predeterminable times t s Starting from a maximum or minimum value, conclusions can be drawn about its ohmic component Eo and thus about the conductivity of the medium 3.

[0050] In the case of the slight presence of insulation layers and / or deposits on the electrode 4, a reception signal E2 is generated, which has rising and falling edges and jumps or steps at the predeterminable times ts The height of these jumps or steps E02 from a maximum or minimum value to a second value at the specified times t s results from the difference ö between the maximum or minimum value and the second value and is again a measure of the ohmic component Eo and thus of the conductivity of the medium 3.

[0051] The height of the jumps or steps in the received signal E or the difference δ, and thus Eo, increases with increasing insulation layer or deposit, as illustrated by the received signals E3 and E03, which relate to the case of significant insulation layers and / or deposits in the area of ​​the electrode 4. The slope of the edges also increases with increasing insulation layer and / or deposit. It is an advantage of the present invention that the conductivity can be determined without the influence of the presence of insulation layers and / or deposits in the area of ​​the electrode 4 or the electrodes 4-6. The conductivity results directly from the ohmic component of the received signal Eo. Using an excitation signal A in the form of a square wave signal also results in particularly simple signal evaluation, which only requires the detection of jumps or steps in the received signal E. The capacitive component of the received signal E is negligible.Thus, in this case, the method according to the invention can be implemented in a particularly simple manner.

[0052] List of reference symbols

[0053] 1 measuring probe

[0054] 2 containers

[0055] 3 Medium

[0056] 4 First electrode

[0057] 5 Second electrode

[0058] 6 Third electrode

[0059] 7 a,b Insulations

[0060] 8 Electronics

[0061] 9 microcontrollers

[0062] 10 a,b Areas for generating the transmitted signal and for evaluating the received signals

[0063] 11 11a Voltage divider

[0064] 12 12a port outputs

[0065] 13 Block A, integration amplifier

[0066] 14 Block B, Amplifier

[0067] 15 analog-to-digital converters (ADC)

[0068] 16 Block C, Amplifier

[0069] 17 Measuring resistor

[0070] 18 Block D, differential amplifier

[0071] 19 19a 19b 19c Decoupling capacitors

[0072] 20 ESD protection circuit

[0073] A excitation signal

[0074] E Ei, E2, E3 receive signals

[0075] Z Zi, Z2 impedances

[0076] E01-E03 Ohmic component of the received signal t s specifiable times ö difference

[0077] V device

Claims

Patent claims 1. Method for determining and / or monitoring the conductivity of a medium (3) using a measuring probe (1) which has at least one electrode (4), comprising the following method steps: Applying an excitation signal (A) to the measuring probe (1), Receiving a received signal (E) from the measuring probe (1), Determining an ohmic component (Eo) of the received signal (E), and Determining the conductivity of the medium (3) based on the ohmic component (Eo) of the received signal (E).

2. Method according to claim 1, wherein the measuring probe (1) is operated in a capacitive and / or a conductive operating mode.

3. Method according to at least one of the preceding claims, wherein a fill level or a limit level is determined.

4. Method according to claim 1 or 2, wherein the received signal (E) is recorded as a function of time (t).

5. Method according to at least one of the preceding claims, wherein a square wave signal is used for the excitation signal (A).

6. The method of claim 4, wherein a first time point (t s ) is determined to which the received signal (E) has a step from a first, in particular a maximum or minimum value, to a second value, and wherein the difference (θ) of the first and second value is determined.

7. Method according to claim 5, wherein the ohmic component (Eo) of the received signal (E) is determined based on the difference (ö).

8. Device (V) for determining and / or monitoring the conductivity of a medium with a measuring probe (1) which has at least one electrode (4), which device (V) is configured to carry out the method according to at least one of the preceding claims.

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