PUMP DEVICE AND METHOD FOR DETERMINING THE CONCENTRATION OF A SUBSTANCE INSIDE A LIQUID

Abstract

A pump device has at least one chamber (22) or conduit containing or provided for containing a liquid, a concentration sensor (24) arranged in the chamber (22) or conduit for detecting a concentration of a substance in the liquid and an evaluation unit (28) connected to the sensor (24). The sensor (24) and the evaluation unit (28) are configured for an electrical impedance measurement. The evaluation unit (28) is configured such that a measurement for detecting the concentration is carried out by use of an electrical signal applied to the sensor (24) having at least one frequency corresponding to or above an upper cut-off frequency (f.sub.2) of a frequency range showing a constant electrical impedance (R.sub.m). A method is provided for determining the concentration of a substance inside a liquid.

Claims

1. A pump device comprising: at least one chamber or conduit containing or provided for containing a liquid; a concentration sensor arranged in said chamber or conduit for detecting a concentration of a substance in said liquid; and an evaluation unit connected to said sensor, wherein said sensor and said evaluation unit are configured for an electrical impedance measurement, and said evaluation unit is configured such that a measurement for detecting said concentration is carried out by use of an electrical signal applied to said sensor having at least one frequency corresponding to or above an upper cut-off frequency of a frequency range showing a constant electrical impedance.

2. A pump device according to claim 1, wherein said chamber or circuit is part of a sealing system.

3. A pump device according to claim 2, wherein said circuit is a closed circuit.

4. A pump device according to claim 1, wherein said conduit is part of a flow path for a liquid to be pumped.

5. A pump device according to claim 1, wherein said sensor and said evaluation unit are configured for measurement of a concentration of glycol in said liquid.

6. A pump device according to claim 1, wherein said sensor comprises two electrodes which are distanced from one another.

7. A pump device according to claim 6, wherein one of said two electrodes forms an inner electrode which is surrounded by the other electrode forming an outer electrode.

8. A pump device according to claim 1, wherein the sensor comprises two electrodes being in contact with said liquid.

9. A pump device according to claim 1, wherein said evaluation unit is configured such that the at least one frequency is greater than 250 kHz.

10. A pump device according to claim 1, wherein said evaluation unit comprises a frequency generator generating an electrical signal of variable frequency or a signal comprising a range of frequencies or a white noise signal.

11. A pump device according to claim 1, wherein said evaluation unit is configured to measure an impedance of the liquid between two sensor electrodes of the sensor by use of an electrical signal applied to the electrodes having at least one frequency below said upper cut-off frequency.

12. A pump device according to claim 10, wherein said evaluation unit is configured such that in a first measurement step said impedance value is detected and in a second measurement step said detection of said concentration is carried out using the impedance value detected before.

13. A pump device according to claim 1, wherein said evaluation unit is configured such that for detection of the concentration said cut-off frequency is detected and a capacitance of the liquid is calculated on basis of said cut-off frequency detected, wherein preferably the capacitance is determined by following equation: $C_m=\frac{1}{2\pi R_m{f}_2}$ wherein C.sub.m is the capacitance of the liquid, R.sub.m is the resistance of the liquid and f.sub.2 is the upper cut-off frequency.

14. A pump device according to claim 1, wherein said evaluation unit is configured such that a resistance or impedance value and said upper cut-off frequency are detected in a single measurement step, by using an electrical signal having a multi-frequency spectrum and a spectral analysis of the measured impedance frequency response.

15. A pump device according to claim 1, wherein the evaluation unit is configured to determine said concentration on basis of the detected capacitance of the liquid, by use of a predefined relation of capacitance and concentration stored in a memory of the evaluation unit.

16. A pump device according to claim 1, wherein the evaluation unit and the sensor form a resonator and the evaluation unit is configured to determine a resonance frequency and to determine the capacity of the liquid on basis of the detected resonance frequency.

17. A method for determining the concentration of a substance in a liquid contained in a chamber or conduit, the method comprising determining the concentration on basis of an impedance measurement; and measuring for detecting said concentration by use of an electrical signal having at least one frequency corresponding to or above an upper cut-off frequency of a frequency range showing a constant impedance.

18. A method according to claim 17, wherein said impedance measurement is carried out by use of a sensor having two electrodes in contact with said liquid, wherein said electrical signal is applied to said sensor electrodes.

19. A method according to claim 17, wherein the capacitance of the liquid is detected and said concentration is determined on basis of said capacitance, on basis of a relation of capacitance and concentration determined before.

20. A method according to claim 17, wherein the capacitance of the liquid is detected and said concentration is determined on basis of said capacitance, wherein said capacitance is determined by detecting the resistance of said liquid and said upper cut-off frequency and/or detecting a resonance frequency of a resonator influenced by the capacity of said liquid.

21. A sensor unit comprising: a sensor provided for arrangement inside a conduit or space containing a liquid; and an evaluation unit connected to said sensor, wherein said sensor and said evaluation unit are configured for detecting a concentration of a substance in a liquid by an electrical impedance measurement, wherein said evaluation unit is configured such that a measurement for detecting said concentration is carried out by use of an electrical signal applied to said sensor having at least one frequency corresponding to or above an upper cut-off frequency of a frequency range showing a constant electrical impedance.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In the drawings:

[0037] FIG. 1 is a perspective view showing a pump device according to the invention;

[0038] FIG. 2 is a partial cross-sectional view of a sealing chamber of the pump device according to FIG. 1;

[0039] FIG. 3 is a schematical cross-section of an impedance sensor as shown in FIG. 2,

[0040] FIG. 4 is a front view of the sensor according to FIG. 3;

[0041] FIG. 5 is a circuit model for the metallic electrode of the sensor according to FIGS. 3 and 4;

[0042] FIG. 6 is a schematic graph view showing y the impedance vs. frequency of the electrode model; and

[0043] FIG. 7 is a circuit model for the electrode including an inductor to form a resonator.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0044] The example of a pump device according to the invention shown in FIG. 1 is a submersible waste-water pump. The pump device has a motor housing 2 attached to a pump housing 4. The pump housing 4 contains an impeller, which is driven by an electric drive motor arranged in the motor housing 2. The pump housing 4 has an inlet 6 on the bottom side and an outlet 8 extending in radial direction from the pump housing 4 and configured to be connected with a further outlet pipe. On top of the motor housing 2, there is arranged an electronic housing 10 containing a control electronics for the drive motor. Connected to the electronic housing 10 is a supply cable for the electric power supply 12. Between the motor housing 2 and the pump housing 4, there is arranged a sealing assembly to separate the interior of the motor housing 2 from the pump housing 4, since the motor housing 2 is dry, whereas the interior of the pump housing 4 is filled with a liquid as water to be pumped. The shaft 14 of the drive motor 14 with its free end 16 is connected to an impeller (not shown in FIG. 2). The shaft 14 extends through two shaft seals 18 and 20, which are distanced from one another. Between the two shaft seals and surrounding the shaft 14 is arranged a closed sealing chamber 22 filled with a lubricant or coolant in form of a glycol-water-mixture. The lubricant serves for lubrication of the shaft sealings 18 and 20 and for cooling. Inside the sealing chamber 22, there are arranged two sensors, an impedance sensor 24 and a temperature sensor 26, which are connected to an evaluation unit 28. The evaluation unit 28 may be a separate unit and may be connected to the control electronics arranged in the electronic housing. Alternatively, it would be possible to also arrange the evaluation unit 28 inside the electronics housing 10. The impedance sensor 24 serves as a concentration sensor for detecting the concentration of glycol in the lubricant contained in the sealing chamber 22. By change of the concentration of glycol, in particular, if the concentration decreases, entering of water into the sealing chamber 22 from the interior of the pump housing 4 can be detected. This is an indication for a wear of the shaft sealing, i.e. the first shaft sealing 18 towards the pump housing 4.

[0045] The impedance sensor 24 comprises two electrodes 30 and 32, as shown in FIGS. 3 and 4. The electrode 30 forms an inner electrode 30, which is surrounded by an outer electrode 32. The outer electrode 32 has a tubular shape with an open front end. The inner electrode 30 extends in the axial direction of the outer electrode 32 and is arranged centered in the middle of the outer electrode 32, which has a circular cross-section. In the outer electrode 32, there are provided openings allowing an exchange of liquid in the inner space of the outer electrode 32. The electrodes 30 and 32 are connected to sensor electronics 36 arranged in a sealed section inside the outer electrode 32. This sealed section or chamber containing the sensor electronics 36 is separated from the fluid containing area inside the outer electrode 32 by a separating wall 38. The impedance sensor 24 may be connected to the evaluation unit 28 via a sensor cable 40.

[0046] The inner electrode 30 and the outer electrode 32 are metallic electrodes, which are in direct contact with the lubricant or coolant inside the sealing chamber 22. This results in the circuit model as shown in FIG. 5. In FIG. 5, the capacitance C.sub.m is the capacitance of the media between the electrodes, i.e. of the lubricant inside the chamber 22. R.sub.m represents the resistance of the media between the electrodes, i.e. the lubricant. C.sub.p represents the capacitance of a polarization layer occurring on the surface of the electrode. Cc represents the capacitance resulting from a native oxide coating of the electrode. C.sub.s is a stray or parasitic capacitance, in particular the stray capacitance of the cable.

[0047] For the evaluation of the capacitance of the media C.sub.m it is assumed that the capacitance of the coating and the polarization layer is much larger than the capacitance of the media, which is to be detected. Furthermore, it can be assumed that the stray capacitance C.sub.s is much smaller than the capacitance of the electrode.

[0048] These capacitances and the resistance of the media define two characteristic cut-off frequencies f.sub.1 and f.sub.2, as shown in FIG. 6. These cut-off frequencies can be calculated according the following equations:

[00004]$f_1=\frac{1}{2\pi R_m\mathrm{Min}\left(C_p,C_c\right)}$$f_2=\frac{1}{2\pi R_m{C}_m}$

[0049] For electrodes, which are larger than a few mm.sup.2, the capacitances C.sub.p and C.sub.c can be assumed as being much larger than C.sub.m. Therefore, the upper cut-off frequency f.sub.2 is much larger than the lower cut-off frequency f.sub.1. The capacitance of the media, which represents a concentration of glycol inside the lubricant, can be found from the impedance above the upper cut-off frequency f.sub.2. This measurement may be difficult due to the low impedance of the electrode. Between the lower cut-off frequency f.sub.1 and the upper cut-off frequency f.sub.2, the impedance corresponds to the resistance R.sub.m between the two electrodes 30 and 32. Furthermore, the impedance Z is substantially constant in this frequency range. Therefore, it is preferred to measure the resistance R.sub.m of the media in this frequency area, i.e. by applying an electrical signal in the frequency range between the lower cut-off frequency f.sub.1 and the upper cut-off frequency f.sub.2 to the sensor electrodes 30 and 32. For generating the respective frequency signal, a frequency generator is integrated into the evaluation unit 28. Such a frequency generator may be configured to provide single predefined frequencies or ranges of frequencies, i.e. a spectrum of frequencies allowing a measurement at several frequencies at the same time.

[0050] The capacitance of the media of such model would be as following:

[00005]$C_m=\frac{2\pi {\mathrm{.Math.}}_0{\mathrm{.Math.}}_{r}L}{\ln \left(\frac{b}{a}\right)},$

wherein L is the axial length of the electrode, a is the diameter of the inner electrode 30 and b the diameter of the outer electrode 32. σ is the conductance of the media. ε.sub.0 and ε.sub.r correspond to the vacuum permeability and the relative dielectric constant of the media, i.e. the glycol-water-mixture inside the chamber 22.

[0051] The resistance R.sub.m can be calculated as follows:

[00006]$R_m=\frac{\ln \left(\frac{b}{a}\right)}{2\pi L\sigma }$

[0052] From this, it follows that the cut-off frequency f.sub.2 could be found independent of the electrode geometry and only depends on the material properties of the media, i.e. the liquid to be analyzed:

[00007]$f_2=\frac{\sigma }{2\pi {\mathrm{.Math.}}_0{ϵ}_{\tau }},$

since the upper cut-off frequency f.sub.2 is much larger than the lower cut-off frequency f.sub.1. The conductivity or resistance, respectively, can be measured in the middle range between the lower cut-off frequency f.sub.1 and the upper cut-off frequency f.sub.2, since in this range the impedance corresponds to the resistance:

|Z|=R.sub.m,f.sub.1<<f<<f.sub.2,

wherein Z is the impedance.

[0053] With measuring the resistance R.sub.m, it is possible to find the capacity of the media by detecting or measuring the upper cut-off frequency f.sub.2:

[00008]$C_m=\frac{1}{2\pi R_m{f}_2}$

[0054] The upper cut-off frequency f2 may be calculated or detected by frequency analysis by measurement of the impedance at several frequencies. This means the cut-off frequency is measured by measuring the impedance at at least two frequencies, one in the area between the two cut-off frequencies, i.e. below the upper cut-off frequency and one above the upper cut-off frequency. Preferably, more than two measurements are carried out and then the uppercut-off frequency is calculated on basis of the measured impedance. In practice, it is preferable that the impedance is measured at a larger number of frequencies, for example 10 to 20 or even more frequencies or a frequency sweep between a lower limit and a higher limit frequency. By this, the upper cut-off frequency and the capacitance C.sub.m of the media can be found with higher accuracy.

[0055] To further improve the measurement and the detection of the capacity C.sub.m of the media, a resonator can be used. A resonator can be created by introducing a inductor L.sub.m into the electrode circuit, as shown in FIG. 7 showing a model of the electrode additional having an inductor L.sub.m. In this model of the electrode R.sub.c, L.sub.c and C.sub.c represent the impedance of the cable. L.sub.m is the inductance in the electrode. C.sub.p is the capacitance resulting from a polarization layer, R.sub.m is the resistance of the media and C.sub.m the capacitance of the media to be detected. By using such a resonator, it is possible to find a resonance frequency above the upper cut-off frequency f.sub.2. The resonance frequency can be detected by frequency analysis, in particular on basis of the phase angle. Preferably, the capacitance C.sub.m is detected at the resonance frequency.

[0056] On basis of the capacitance C.sub.m of the liquid, the concentration can be detected. The concentration for different capacitances is determined experimentally before the relation may be stored in form of a table inside the evaluation unit 28. Furthermore, the relation may be determined temperature-depending, so that a temperature signal from the temperature sensor 26 detecting the temperature of the media inside the sealing chamber 22 may be considered by the evaluation unit 28. On basis of the temperature signal and the capacitance of the media C.sub.m detected, the evaluation unit 28 determines the corresponding concentration from the stored data. The evaluation unit 28 may have a predefined threshold for an allowable glycol concentration and may give an alarm signal, if the concentration falls below this threshold. The alarm signal is an indication fora fault or wear of the shaft sealing.

[0057] While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.

LIST OF REFERENCE NUMERALS

[0058] 2 water housing [0059] 4 pump housing [0060] 6 inlet [0061] 8 outlet [0062] 10 electronic housing [0063] 12 supply cable [0064] 14 shaft [0065] 16 free end [0066] 18, 20 shaft sealings [0067] 22 sealing chamber [0068] 24 impedance sensor [0069] 26 temperature sensor [0070] 28 evaluation unit [0071] 30 inner electrode [0072] 32 outer electrode [0073] 34 opening [0074] 36 sensor electronics [0075] 38 separating wall [0076] 40 sensor cable