Sensor for determining a measured variable and method for determining a measured variable using a sensor

Abstract

The invention relates to a sensor for determining a measured variable and a method for determining a measured variable using a sensor. A sensor for determining a measured variable which depends on the concentration of a gaseous analyte in a liquid medium comprises a closed measuring chamber which has a gas sensor which is sensitive to the gaseous analyte for generating a measuring signal which depends on the concentration of the gaseous analyte in the measuring chamber, a diffusion membrane which is impermeable to liquids but permeable to the gaseous analyte, which diffusion membrane closes the measuring chamber with a first surface facing the measuring chamber and has a second surface which contacts the medium, an evaluation unit for determining the measured variable on the basis of the measuring signal of the gas sensor, and an ultrasonic emission unit which is designed to introduce ultrasonic waves into the medium such that a mixing of the medium is produced in a volume which adjoins the second surface which contacts the medium. The invention also comprises a method for determining a measured variable which depends on the concentration of a gaseous analyte in a liquid medium using a sensor according to the invention.

Description

Technical Field

[0001] The present invention includes a sensor for determining a measured variable that depends on the concentration of a gaseous analyte in a liquid medium, comprising: a closed measuring chamber having a gas sensor sensitive to the gaseous analyte to generate a measurement signal that depends on the concentration of the gaseous analyte in the measuring chamber. Background Technology

[0002] Gaseous analytes diffuse from a liquid medium into a measurement chamber via a diffusion membrane that is impermeable to liquid but permeable to gaseous analytes. The diffusion membrane here seals the measurement chamber with a first surface facing it and contacts the liquid medium via a second surface opposite to the first surface.

[0003] The measured variable, which depends on the concentration of the gaseous analyte in the liquid medium, can then be determined based on the measurement signal from the gas sensor, for example, by using an evaluation unit and electronic components configured for this purpose. During the generation of the measurement signal in the measurement chamber (as assumed below), a pressure balance preferably exists between the measurement chamber and the liquid medium.

[0004] During diffusion, there exists a diffusion rate describing the diffusion of a gaseous analyte from the liquid medium into the measuring chamber. This diffusion rate determines the sensor's response time. For example, the sensor's response time is defined as the time interval after which, in the event of a change in the concentration of the gaseous analyte in the liquid medium, the sensor exhibits a step response that largely (i.e., 85%, 90%, 95%, etc.) following the changing concentration. For example, a reference time t90 is taken at 90%; this t90 time is determined under laboratory conditions and is generally known for the corresponding sensor and the corresponding gaseous analyte.

[0005] The diffusion rate is theoretically determined by the diffusion rate of the diffusion membrane, which is specific to the corresponding gaseous analyte and can be adjusted through the membrane's special design (material properties, pore size, etc.). However, in practice, this only represents an upper limit. During diffusion, a depletion boundary layer forms on the surface of the second contact medium, where the concentration of the gaseous analyte decreases. In the depletion boundary layer, the proportion (or concentration) of the gaseous analyte decreases; therefore, it is possible to deliver less analyte to diffuse through the diffusion membrane.

[0006] This is particularly true, but not limited to, cases where CO2 is the analyte in gaseous form. Furthermore, during operation, the pores of the diffusion membrane gradually become clogged with impurities. Both of these processes hinder the diffusion process and therefore, in practice, lead to a reduced effective diffusion rate, and consequently, a longer response time. A long response time is generally undesirable, especially if the measured variable is used as a control or regulation variable (e.g., in process and / or laboratory automation). Summary of the Invention

[0007] Therefore, the object of the present invention is to improve the response time of such sensors.

[0008] This objective is achieved through a sensor for determining the concentration of a gaseous analyte in a liquid medium and a method for determining the analyte that depends on the concentration of the gaseous analyte in the liquid medium.

[0009] Regarding the sensor, this objective is achieved through a sensor for determining the analyte variable, which depends on the concentration of the gaseous analyte in the liquid medium.

[0010] include:

[0011] - A closed measurement chamber equipped with a gas sensor sensitive to gaseous analytes to generate a measurement signal that depends on the concentration of the gaseous analyte in the measurement chamber;

[0012] - A diffusion membrane that is impermeable to liquids but permeable to gaseous analytes, the diffusion membrane sealing the measuring chamber with a first surface facing the measuring chamber and having a second surface in contact with the medium;

[0013] - An evaluation unit, which is used to determine the measured variable based on the measurement signal from the gas sensor, and

[0014] - An ultrasonic transmitting unit, which is designed to introduce ultrasonic waves into a medium, thereby creating a mixture of the medium in a volume adjacent to a second surface of the contact medium.

[0015] Ultrasonic waves initiate flow processes within the medium. These flow processes can include non-directional flow, such as turbulence, and / or directional flow, causing mixing in the volume adjacent to the second surface of the contact medium. As a result of this mixing, fresh medium is delivered to the second surface of the contact medium, or depletion of the boundary layer is achieved. Depending on the type of mixing and / or the type of ultrasonic waves introduced, the diffusion membrane itself also begins to move to some extent. In the case of a moving diffusion membrane, pore clogging and / or pore reopening are reduced, or even diffusion of gaseous analytes from the measurement chamber to the gas sensor is facilitated. Therefore, all these processes increase the effective diffusion rate and reduce the actual response time.

[0016] The ultrasound here is used only for mixing the medium in the volume adjacent to the second surface of the contact medium and is explicitly not used in the context of this invention to determine the concentration of gaseous analytes in the medium by means of an ultrasonic emission unit (e.g., via a resonant frequency).

[0017] For example, an ultrasonic transmitting unit includes at least one ultrasonic transducer for converting an excitation signal into ultrasonic waves and electronic components for generating the excitation signal.

[0018] In addition to the ultrasonic emitting unit, preferably no further means are required to generate mixing, such as sensors for the operation of the flow cell and / or another mechanical mixing by a stirrer.

[0019] In one embodiment of the invention, the ultrasonic transmitting unit is designed to transmit ultrasonic waves with a frequency of at least 10 kHz (kilohertz), and especially at least 20 kHz.

[0020] In one embodiment of the invention, the ultrasonic transmitting unit includes a transmitting surface for transmitting ultrasonic waves, the transmitting surface being arranged substantially parallel to the diffusion film.

[0021] In the development of this invention, the ultrasonic transmitting unit can operate in a resonance mode in which a standing wave is formed in the volume between the ultrasonic transmitting unit and the diffusion membrane.

[0022] The ultrasonic emission unit is adjusted to resonate, for example, by the sensor evaluation unit and / or the excitation unit described above.

[0023] For example, the emitting surface of the ultrasonic emitting unit is arranged substantially parallel to the diffusion film, and the standing wave travels perpendicular to the emitting surface and the second surface of the diffusion film.

[0024] In one embodiment of the aforementioned development, the ultrasonic transmitting unit is thus designed to generate standing wave ultrasonic waves in a resonant mode within the volume, the standing wave ultrasonic waves traveling perpendicular to the transmitting surface and the diffusion membrane.

[0025] For this purpose, an ultrasonic wave with a frequency of at least 50 kHz, preferably at least 70 kHz, is suitable. This frequency is used to generate a standing wave on a second surface of a diffusion film that is perpendicular to and parallel to the emitting surface, and the distance between the emitting surface and the second surface is matched to each other. For example, this distance is from 0.1 cm to 6 cm, especially from 0.5 cm to 3 cm.

[0026] However, alternatively, standing waves that travel parallel to the emitting surface of the ultrasonic emitting unit can also be generated, for example, by a so-called "transverse resonator".

[0027] In one embodiment of the aforementioned development, the ultrasonic transmitting unit is therefore designed to generate standing wave ultrasonic waves in a resonant mode within the volume, the standing wave ultrasonic waves traveling parallel to the diffuser membrane. For this purpose, a frequency of at least 10 kHz is suitable, wherein the aforementioned considerations related to frequency dependence or modulation also apply.

[0028] By appropriately designing the ultrasonic transmitting unit, such as through a special resonator geometry, standing wave ultrasonic waves can be generated that travel parallel to the diffusion membrane. For this purpose, the ultrasonic transmitting unit preferably includes a conduit traveling parallel to the diffusion membrane, particularly having a rectangular cross-section and openings in the contact medium arranged at opposite ends of the conduit.

[0029] In a further development of the invention, the ultrasonic transmitting unit can operate in a mode other than resonance. Depending on the need, the same ultrasonic transmitting unit can operate in both the aforementioned resonance mode and the mode other than resonance. In the context of this application, resonance mode or mode other than resonance always refers to resonance within the medium. Therefore, in the case of a mode other than resonance, this means that there is no resonant vibration, such as a standing wave, in the medium itself; of course, resonance phenomena can still occur in the sensor itself or its components, such as in the ultrasonic transmitting unit.

[0030] In one further embodiment of this development, the ultrasonic transmitting unit is designed to emit ultrasonic waves at (multiple) intensities and / or (multiple) frequencies in which cavitation exists in the volume.

[0031] In a further developed embodiment, the ultrasonic transmitting unit is designed to transmit ultrasonic waves with a frequency between 1 MHz and 100 MHz.

[0032] At these high frequencies, so-called Eckart flows are generated. Eckart flows are described, for example, in the scientific publication LabChip, 2012, 12, 2438–2451 by M. Wiklund et al., which is incorporated herein by reference. Eckart flows are directional flows away from the ultrasonic emitting element. For this purpose, the emitting surface is preferably parallel to the diffusion film.

[0033] In one embodiment of the invention, the ultrasonic transmitting unit has a piezoelectric element. The piezoelectric element is used, for example, as an ultrasonic transducer of the ultrasonic transmitting unit.

[0034] In one embodiment of the present invention, the gas sensor is selected from the group consisting of:

[0035] -CO2 gas sensor, O2 gas sensor, CH4 gas sensor.

[0036] Of course, the sensor is not limited to the gas sensor described herein. The precise manner in which the gas sensor functions in the measurement chamber is irrelevant to this invention. For example, the gas sensor is designed as an optical gas sensor, in which the concentration of the gaseous analyte in the measurement chamber can be determined by spectroscopic analysis methods, such as nondispersive infrared spectroscopy (NDIR or nondispersive infrared spectroscopy).

[0037] In one embodiment of the invention, the sensor's response time is less than 5 minutes, and especially less than 2 minutes.

[0038] In one embodiment of the invention, the sensor has a response time that is at most half that of a sensor otherwise designed in the same manner (but without the ultrasonic emitting unit generating media mixing in the media volume adjacent to the second surface of the contact medium). By mixing the medium through the ultrasonic emitting unit, the response time is thus at least halved or even shorter.

[0039] Regarding the method, the objective is achieved by a method for determining the analyte variable, which depends on the concentration of a gaseous analyte in a liquid medium, comprising the following steps:

[0040] - Introducing ultrasound into the medium, thereby generating mixing of the medium in the volume adjacent to the second surface of the contact medium;

[0041] - Diffusion of gaseous analytes into the measurement chamber through a diffusion membrane;

[0042] - Generates a measurement signal that depends on the concentration of the gaseous analyte in the measurement chamber; and

[0043] - Determine the measured variable based on the measured signal. Attached Figure Description

[0044] The invention and further advantageous embodiments are explained in more detail below with reference to exemplary embodiments. In all the drawings, the same parts are denoted by the same reference numerals; reference numerals already mentioned are omitted in subsequent drawings for reasons that are clear or, if otherwise seem reasonable.

[0045] The attached diagram shows:

[0046] Figure 1 This is a first embodiment of the sensor according to the present invention;

[0047] Figure 2 This is a further embodiment of the sensor according to the present invention;

[0048] Figure 3 This is a further embodiment of the sensor according to the present invention;

[0049] Figure 4 This is a further embodiment of the sensor according to the present invention. Detailed Implementation

[0050] Figure 1 A cross-sectional view of a schematic structure of a sensor according to a first embodiment of the present invention is shown, the sensor being used to determine a measured variable that depends on the concentration of a gaseous analyte in a liquid medium.

[0051] The sensor includes a gas sensor 2, which is arranged in a closed measurement chamber 1 and connected to an evaluation unit 4. The upper end of the measurement chamber 1 is sealed by a diffusion membrane 3 that is impermeable to liquids but permeable to gaseous analytes, with the measurement chamber 1 facing a first surface 31 of the diffusion membrane 3. Conversely, a second surface 32 of the diffusion membrane 3 is in contact with the medium, i.e., in contact with the medium when determining the measured variable, which depends on the concentration of the gaseous analyte in the liquid medium 6. The gaseous analyte diffuses from the liquid medium into the measurement chamber 1 through the diffusion membrane 3. The liquid medium 6 (not shown here) remains outside the measurement chamber 1. The gas sensor 2 is designed to determine the concentration of the gaseous analyte in the gas contained in the measurement chamber 1.

[0052] According to the invention, the sensor includes an ultrasonic transmitting unit 5. This ultrasonic transmitting unit includes, for example, at least one transducer unit 52 having an ultrasonic transducer for converting an excitation signal into ultrasonic waves, and an excitation unit 51 for generating an excitation signal, for example, by means of electronic components configured for this purpose. Figure 1 In the illustrated embodiment, the ultrasonic transducer of transducer unit 52 is designed to include or surround a transducer unit of the membrane at its edge. For example, the transducer unit is annular in the case of a circular membrane, or rectangular in the case of a rectangular membrane, and so on. Transducer unit 52 includes a piezoelectric element and / or another transducer element known from the prior art and designed to convert an electronic excitation signal into ultrasonic waves, such as a magnetostrictive transducer.

[0053] Figure 1 The sensor embodiment shown is suitable for cases where the ultrasonic transmitting unit 5 operates in a mode other than resonance. Here, cavitation KA is achieved in the volume 7 adjacent to the second surface 32 by means of an appropriate ultrasonic intensity of the ultrasound waves introduced into the medium 6. Cavitation KA typically exists at an ultrasonic frequency between 10 kHz and 30 kHz and an appropriate intensity. In the case of cavitation KA, at sufficient ultrasonic intensity, the strong pressure fluctuations of the ultrasound waves generate bubbles, which then grow and collapse. As a result of bubble collapse, turbulent (i.e., non-directional) flow occurs, which causes the medium 6 to mix in the volume 7 near the second surface 32. Thus, the vapor bubbles generated during cavitation KA ensure turbulence, and thereby ensure that the liquid medium 6 is mixed in the volume 7.

[0054] Additionally, it can simultaneously counteract the clogging of the pores in the diffusion membrane 3. The ultrasonic intensity should be selected to ensure sufficient mixing and cleaning of the diffusion membrane 3, but damage to the diffusion membrane 3 due to excessive vibration should be avoided. The latter also depends on the corresponding embodiment of the diffusion membrane 3, therefore any further explanation here is meaningless.

[0055] In comparison, Figure 2A further embodiment of the sensor is shown, wherein the ultrasonic emitting unit 5 in resonant mode generates a standing wave between the ultrasonic emitting unit 5 and the diffusion film 3. Figure 2 In the illustrated embodiment, the emitting surface AF of the ultrasonic emitting unit 5 is arranged substantially parallel to the diffusion membrane 3. In this embodiment, the medium 6 is provided, for example, through the space formed between the diffusion membrane 3, which is open at its side, and the emitting surface AF. Unlike the measuring chamber 1, this space is therefore not... Figure 2 or Figure 1 (not) Figure 3 and Figure 4 The enclosed chamber in the following exemplary embodiments.

[0056] The ultrasonic frequency used to generate the standing wave SW is matched to the distance between the emitting surface and the second surface, wherein the standing wave SW travels perpendicular to the emitting surface AF and perpendicular to the second surface 32 of the diffusion film 3 parallel to it. For example, this distance is a few centimeters, such as 2 cm, so as to generate a standing wave SW with at least one node between the two surfaces.

[0057] The velocity of sound in liquid medium 6 also depends on the physical and / or chemical state variables of the medium, such as temperature and / or pressure, and also on the concentration of gaseous analytes in medium 6 itself. Therefore, given a resonator geometry, these state variables also affect the ultrasonic frequency required to generate the standing wave SW.

[0058] Therefore, for example, when used within the sensor's measurement range as expected (e.g., for CO2, from 0.02–20 mmol CO2 / L) and under normal conditions (i.e., atmospheric pressure near approximately 10¹³ hPa and ambient temperature near approximately 25 °C), this ultrasonic frequency is first evaluated and adjusted via the predicted concentration of the gaseous analyte. In cases of process automation where the sensor is exposed to typical process conditions (e.g., temperatures from 0–80 °C), this estimate of the ultrasonic frequency will change accordingly.

[0059] Then, as needed, the actual ultrasonic frequency of the sensor in a specific measurement operation under a specific state variable is tracked or adjusted by means of modulation. This is achieved, for example, by excitation unit 51 and / or evaluation unit 4, the evaluation unit being configured for adjusting the excitation signal. If the state variable is detected by other sensors, the measured value of that variable can also be used to estimate, adjust, and / or regulate the ultrasonic frequency.

[0060] The flow of the mixing medium 6 is generated in volume 7 by a standing wave SW (see the circle with the arrow). For the purpose of mixing and removing the depleted boundary layer according to the invention, the standing wave here need not be in… Figure 2 The resonance mode shown travels perpendicular to the diffusion film 3.

[0061] exist Figure 3 In the further embodiment shown, in the resonant mode, for example, a standing wave SW is generated, which travels parallel to the emitting surface AF and the diffuser 3. In this context, we also talk about a so-called lateral resonator. This "laterally traveling" standing wave SW also ensures the corresponding mixing of the medium 6 in the volume 7 adjacent to the second surface 32. In the case shown here, the emitting surface AF and the diffuser 3 are parallel to each other. However, this is not necessarily the case; for example, in another embodiment, the emitting surface AF can also be tilted such that it travels parallel to the plane of the paper. In this case, the emitting surface AF and the diffuser 3 will therefore be perpendicular to each other.

[0062] Figure 4 It shows (with) Figure 1 (Completely the same) Further modes beyond resonance. In this exemplary embodiment, the so-called Eckart flow ES, as described above, is generated by emitting ultrasonic waves at higher frequencies between 1 MHz and 100 MHz. The Eckart flow is a directional flow away from the ultrasonic emitting unit; see the middle arrow. For this purpose, the emitting surface AF is preferably parallel to the diffusion film 3. At the same time, there is still a lateral backflow from the diffusion film 3 to the emitting surface AF; see the outer arrow. Even with such an Eckart flow ES, the mixing of the medium 6 according to the invention in volume 7 is thus achieved by ultrasonic waves.

[0063] The various operating modes (resonance mode / non-resonance mode) shown in the exemplary embodiments can, of course, be combined, i.e., the same sensor is designed to operate in both resonance mode and non-resonance mode. For example, in Figure 2 or Figure 3 and Figure 4 In this configuration, the arrangement of the emitting surface AF with respect to the diffusion film 3 remains essentially unchanged. In order to combine the resonant mode with modes other than the resonant mode (e.g., alternately or even simultaneously), the ultrasonic emitting unit 5 or its excitation unit 51 and transducer unit 52 may accordingly include multiple transducer elements and / or excitation elements.

[0064] Of course, the present invention is not limited to the form of the ultrasonic transmitting unit 5 with a direct emission surface AF shown herein. There is no fundamental limitation on the exact form of the ultrasonic transmitting unit 5. For example, the ultrasonic transmitting unit can be probe-shaped, i.e., specifically designed as a probe. In this case, the probe will be immersed in the medium 6 to introduce ultrasonic waves, as in the case of ultrasonic welding electrodes.

[0065] Regardless of the corresponding embodiment, the mixing of medium 6 significantly reduces the sensor response time. Compared to comparable sensors without ultrasonic transmitting unit 5, the sensor response time is at least halved by introducing ultrasonic waves. For example, in the case of a CO2 sensor, the t90 time is expected to be less than 5 minutes, and in particular, less than 2 minutes. The response time of a comparable CO2 sensor, which does not explicitly affect the mixing by other means (e.g., by operating as a flow sensor), is approximately 10 minutes.

[0066] List of reference numerals

[0067] 1 Measurement Room

[0068] 2 gas sensors

[0069] 3. Diffusion membrane

[0070] 31 First Surface

[0071] 32 Second Surface

[0072] 4 evaluation units

[0073] 5 ultrasonic transmitting units

[0074] 51 Excitation Unit

[0075] 52 transducer units

[0076] 6 media

[0077] 7 volumes

[0078] AF Emitting Surface

[0079] KA cavitation

[0080] SW standing wave

[0081] ES Eckart Stream

Claims

1. A sensor for determining a measured variable that depends on the concentration of a gaseous analyte in a liquid medium (6), include: - A closed measurement chamber (1) having a gas sensor (2) sensitive to the gaseous analyte for generating a measurement signal that depends on the concentration of the gaseous analyte in the measurement chamber (1); - A diffusion membrane (3) that is impermeable to liquids but permeable to the gaseous analyte, the diffusion membrane (3) sealing the measuring chamber (1) with a first surface (31) facing the measuring chamber (1) and having a second surface (32) in contact with the medium; - Evaluation unit (4), the evaluation unit (4) is used to determine the measured variable based on the measurement signal of the gas sensor (2), and - An ultrasonic transmitting unit (5) is designed to introduce ultrasonic waves into the medium (6) such that mixing of the medium (6) is generated in a volume (7) adjacent to the second surface (32) of the contact medium.

2. The sensor according to claim 1, in, The ultrasonic transmitting unit (5) is designed to emit ultrasonic waves with a frequency of at least 10 kHz.

3. The sensor according to claim 2, in, The ultrasonic transmitting unit (5) is designed to emit ultrasonic waves with a frequency of at least 20 kHz.

4. The sensor according to claim 1, in, The ultrasonic transmitting unit (5) includes an transmitting surface (AF) for transmitting ultrasonic waves, the transmitting surface (AF) being arranged substantially parallel to the diffusion membrane (3).

5. The sensor according to any one of claims 1-4, in, The ultrasonic emitting unit (5) is capable of operating in a resonance mode in which a standing wave (SW) is formed in the volume (7) between the ultrasonic emitting unit (5) and the diffusion membrane (3).

6. The sensor according to claim 4, in, The ultrasonic transmitting unit (5) is designed to generate standing wave ultrasonic waves in the volume (7) in a resonant mode, the standing wave ultrasonic waves traveling perpendicular to the transmitting surface (AF) and the diffusion film (3).

7. The sensor according to claim 5, in, The ultrasonic transmitting unit (5) is designed to generate standing wave ultrasonic waves in the volume (7) in a resonant mode, the standing wave ultrasonic waves traveling parallel to the diffusion membrane (3).

8. The sensor according to any one of claims 1-4, in, The ultrasonic transmitting unit (5) can operate in modes other than resonance.

9. The sensor according to claim 8, wherein, The ultrasonic transmitting unit (5) is designed to emit ultrasonic waves at an intensity and / or frequency at which cavitation (KA) exists in the volume (7).

10. The sensor according to claim 8, in, The ultrasonic transmitting unit (5) is designed to emit ultrasonic waves with a frequency between 1 MHz and 100 MHz.

11. The sensor according to any one of claims 1-4, wherein, The ultrasonic transmitting unit (5) has a piezoelectric element.

12. The sensor according to any one of claims 1-4, wherein, The gas sensor (2) is selected from the following group: -CO2 gas sensor, O2 gas sensor, CH4 gas sensor.

13. The sensor according to any one of claims 1-4, in, The sensor's response time is less than 5 minutes.

14. The sensor according to claim 13, in, The sensor's response time is less than 2 minutes.

15. The sensor according to any one of claims 1-4, in, The sensor has a response time that is at most half the response time of a sensor designed in the same manner in other ways, wherein the sensor designed in the same manner in other ways does not have an ultrasonic emitting unit that generates mixing of the medium in the volume of the medium adjacent to the second surface of the contact medium.

16. A method for determining an analyte variable that depends on the concentration of a gaseous analyte in a liquid medium (6), The method uses a sensor according to any one of claims 1-15. The method includes the following steps: - Introducing ultrasonic waves into the medium (6) thereby generating mixing of the medium (6) in a volume (7) adjacent to the second surface (32) of the contact medium; - The gaseous analyte is diffused through the diffusion membrane (3) into the measuring chamber (1); - A measurement signal is generated using a gas sensor (2) that depends on the concentration of the gaseous analyte in the measurement chamber (1); as well as - The measured variable is determined based on the measured signal.

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