Sensor for determining the concentration of a gas

Abstract

The invention relates to a sensor (1) for determining the concentration of a gas in a gas mixture, comprising a multireflexion cell (2) and at least one acoustic resonator (3), wherein: the multireflexion cell (2) has a light emitter (2.3) so that the light generated by the light emitter (2.3) can be emitted into a chamber (2.1) of the multireflexion cell (2); the light emitter (2.3) is designed to emit light with an intensity according to a first modulation frequency and to emit the light with the intensity according to a second modulation frequency; a part of the acoustic resonator (3) is connected to the chamber (2.1) in such a way that acoustic signals can be coupled in by means of the multireflexion cell (2); the multireflexion cell (2) is designed to couple the acoustic signal of the gas which has been excited by the light emitted according to the first modulation frequency as an acoustic longitudinal mode into the acoustic resonator (3) and to couple the acoustic signal of the gas which has been excited with the light emitted according to the second modulation frequency as an acoustic radial mode into the acoustic resonator (3); the sensor (1) comprises detectors (4) arranged in the acoustic resonator (3) for detecting the acoustic signals and an analyzer (5) connected to the detectors (4) for signal transmission. In this way, a sensor (1) having long-time stability is provided.

Description

[0001] Sensor for determining the concentration of a gas

[0002] The invention relates to a sensor for determining the concentration of a gas in a gas mixture, comprising a multi-reflection cell and at least one acoustic resonator, wherein the multi-reflection cell has a chamber designed to accommodate a gas mixture, the multi-reflection cell further has an inlet for the entry of the gas mixture into the chamber and a light emitter so that the light generated by the light emitter can be emitted into the chamber, a part of the acoustic resonator is connected to the chamber in such a way that acoustic signals can be coupled into the multi-reflection cell, the sensor has detectors arranged in the acoustic resonator for detecting the acoustic signals and an analyzer connected to the detectors for signal transmission.

[0003] Sensors that can detect a specific gas in a gas mixture in an environment are currently used in numerous applications. For example, there are sensors that can detect the gas to monitor environmental pollution or for the early detection of a potential fire. Photoacoustic sensors that are spectrally tuned to the detection of specific gases enable the detection of these gases with high selectivity and precise measurement results. The photoacoustic effect is a physical effect that describes the conversion of light energy into acoustic energy. When a gas is irradiated with light, part of the light energy is absorbed by the gas, the gas molecules enter an excited state and leave this state by radiating thermal energy. Due to heat conduction, the energy is distributed within the gas after a finite time, and an elevated temperature develops in the gas.This heat input causes an increase in volume and a concomitant decrease in density. If this gas is irradiated with light pulses, it periodically heats and cools. The resulting volume change generates sound waves, which in turn can be measured with suitable detectors. Spectrally tuned to the detection of specific gases means that the light emitted with the light flashes requires a specific frequency for the gas to be detected, since gas molecules only absorb light of specific frequencies. The gas in the gas mixture is therefore detected by exciting only the gas to be detected with light of a specific frequency, which emits a signal in the form of sound that is proportional to the concentration of the gas in the gas mixture.EP 4 009 034 A1 describes a detector that enables both the detection and quantification of the concentration of a specific gas in an environment using the photoacoustic effect. A method for manufacturing a detector of the aforementioned type and a method for measuring the concentration of a specific gas in an environment using the aforementioned detector are also described.

[0004] In direct photoacoustic sensors, the output signal measured by the detector scales with the intensity of the emitted light. If the intensity of the emitted light changes, the output signal changes, making it impossible to accurately determine the gas concentration. At the same time, the natural frequency of the acoustic resonators of photoacoustic sensors depends on external influences such as temperature, humidity, and pressure.

[0005] Based on this, the object of the invention is to provide a long-term stable sensor.

[0006] This problem is solved by the subject matter of patent claim 1. Preferred developments can be found in the subclaims.

[0007] According to the invention, it is therefore provided to provide a sensor suitable for determining the concentration of a gas in a gas mixture, comprising a multi-reflection cell and at least one acoustic resonator, wherein the multi-reflection cell has a chamber designed to accommodate a gas mixture, the multi-reflection cell further has an inlet for the entry of the gas mixture into the chamber and a light emitter, so that the light generated by the light emitter can be emitted into the chamber, the light emitter is designed to emit light with an intensity according to a first modulation frequency and to emit the light with the intensity according to a second modulation frequency, wherein the first modulation frequency is different from the second modulation frequency, the acoustic resonator is designed such thatthat at least one acoustic longitudinal mode with the first modulation frequency and at least one acoustic radial mode with the second modulation frequency or at least one acoustic radial mode with the first modulation frequency and at least one acoustic longitudinal mode with the second modulation frequency can be excited in the acoustic resonator, a part of the acoustic resonator is connected to the chamber in such a way that acoustic signals can be coupled into the multi-reflection cell, the multi-reflection cell is designed to couple the acoustic signal of a gas excited by the light emitted at the first modulation frequency into the acoustic resonator as an acoustic mode and to couple the acoustic signal of the chamber excited by the light emitted at the second modulation frequency into the acoustic resonator as an acoustic mode,the sensor has detectors arranged in the acoustic resonator for detecting the acoustic signals and an analyzer connected to the detectors for signal transmission, the analyzer being designed to determine the intensity of the emitted light using the detected signal of the acoustic mode of the gas excited by light emitted at the first modulation frequency, which signal is dependent on the gas concentration in the gas mixture and the intensity of the emitted light, and the detected signal of the acoustic mode of the chamber (2.1) excited by light emitted at the second modulation frequency, which signal is dependent on the intensity of the emitted light, and subsequently to determine the gas concentration in the chamber of the multi-reflection cell.

[0008] In this case, gas is understood to mean a specific gas composition that can be detected by the sensor, which is contained in an environment as part of a gas mixture and whose presence is to be detected, and whose concentration in this gas mixture is to be determined. The light emitter formed in the multi-reflection cell emits light at a specific frequency with which the gas molecules of the gas to be detected in the gas mixture can be excited. When a frequency is mentioned here in relation to the light emitter, this means the frequency that is linked to the wavelength of the emitted light via the speed of light. The light emitter is further designed such that it can be used to emit light with the frequency and intensity according to a first and a second modulation frequency.The light is therefore emitted in the form of light pulses, with these light pulses having two different periodicities: the first and the second modulation frequency. Furthermore, it is particularly advantageous if the emitted light exhibits temporal and spatial coherence and is monochromatic. The light emitter can be a laser. The light emitted by the light emitter reacts with the gas to be detected, so that this emitted light is absorbed by the gas in a specific wavelength range determined by the composition of the gas to be detected. This absorption generates acoustic signals with a sound intensity that depends on the intensity of the emitted light and the concentration of the gas in the gas mixture.

[0009] The multi-reflection cell enables additional amplification of the light emitted by the light emitter and a concomitant amplification of the acoustic signal. The sensor relies on the photoacoustic effect generated in the chamber of the multi-reflection cell to detect the gas in the gas mixture and determine its concentration. The chamber in the multi-reflection cell can also emit an acoustic signal when light emitted at a specific modulation frequency strikes the walls of the chamber of the multi-reflection cell, as some of the photons emitted by the light emitter are absorbed by the chamber. Furthermore, the multi-reflection cell is designed to couple the acoustic signals into the acoustic resonator.Coupling in this context means that the chamber is fluidically connected to the acoustic resonator, so that the acoustic signals of the excited gas are transmitted via the gas mixture and the chamber itself to the acoustic resonator and coupled into it in such a way that longitudinal and radial modes are excited. However, it can also be the case that the acoustic signals are coupled into the acoustic resonator via one or more vibrating membranes. The longitudinal mode corresponds to a longitudinal standing acoustic wave in the acoustic resonator with one of the natural frequencies of the acoustic resonator. The radial mode corresponds to a radial standing acoustic wave in the acoustic resonator with one of the natural frequencies of the acoustic resonator.The modes are advantageously coupled into the acoustic resonator in such a way that the antinodes of the modes corresponding to the maximum sound intensities are detected by the detectors. The signals detected by the detectors are acoustic signals of the acoustic modes, the acoustic radial mode, or the acoustic longitudinal mode.

[0010] In this case, the first modulation frequency can be selected such that the acoustic longitudinal mode signal depends on the intensity of the emitted light and the concentration of the gas in the gas mixture. Changes in the intensity of the emitted light lead to changes in the signal, so that the determined gas concentration usually does not correspond to the actual gas concentration in the chamber. The second modulation frequency will be selected such that the acoustic radial mode signal depends on the intensity of the emitted light and is independent of the gas concentration in the chamber, so that the analyzer can determine the intensity of the emitted light with which the gas was excited. For this purpose, the second modulation frequency is selected such that the walls of the chamber of the multi-reflection cell are excited to emit acoustic signals.What is essential to the invention is that the dependence of the acoustic longitudinal mode signal on the intensity of the emitted light can be eliminated with the analyzer, so that the gas concentration can be determined independently of the intensity of the emitted light and the sensor is stable against changes in the light emitter. "Independent of the gas concentration in the chamber" here means that the influence of the gas concentration in the chamber on the acoustic radial mode signal is so small that the light intensity can still be determined with sufficient accuracy, so that the sensor can determine the gas concentration in the gas mixture with stability against changes in the light emitter.Alternatively, the acoustic signal of the gas excited by the light emitted at the first modulation frequency can be coupled into the acoustic resonator as the acoustic radial mode, and the acoustic signal of the chamber excited by the light emitted at the second modulation frequency can be coupled into the acoustic resonator as the acoustic longitudinal mode. Accordingly, the dependence of the acoustic radial mode signal on the intensity of the emitted light can be eliminated with the analyzer. Emitting the light at both the first and second modulation frequencies ensures that the frequency of the longitudinal mode is different from that of the radial mode, so that the respective acoustic signals can be separated with the analyzer.

[0011] In principle, it is possible to design the sensor in different ways.According to a preferred development of the invention, however, it is provided that the sensor further comprises a control device, wherein the light emitter is designed to emit light according to the first modulation frequency with two sidebands, the multi-reflection cell is designed to couple the acoustic signals of the gas excited by the light emitted according to the first modulation frequency with two sidebands into the acoustic resonator as an acoustic mode with two sidebands, the analyzer is designed to determine an absolute frequency shift of the acoustic mode with the acoustic signals of the sidebands, and the control device connected to the analyzer and the light emitter for signal transmission is designed to adapt the first modulation frequency and the second modulation frequency of the light emitter to the frequency-shifted acoustic mode determined with the analyzer.The acoustic resonator is an oscillating system tuned to several specific natural frequencies, so that when excited over a broad band, the acoustic resonator oscillates practically exclusively at these frequencies. The light emitter emits light at the first and second modulation frequencies, which coincide with the natural frequencies of the acoustic resonator, so that the acoustic signals of the excited gas are coupled into the acoustic resonator as resonant longitudinal and radial modes.

[0012] External influences, such as temperature or pressure fluctuations or changes in humidity, can alter the natural frequencies of the acoustic resonator. By mixing the acoustic sideband signals generated in the acoustic resonator, an error signal can be generated that can be used to determine the absolute frequency shift of the acoustic resonator. Absolute frequency shift here refers to the frequency shift and the sign of the frequency shift, not just the magnitude of the frequency shift. Using the control unit, the first modulation frequency and the second modulation frequency can then be adjusted accordingly to the changed natural frequency of the acoustic resonator.

[0013] According to a further preferred development of the invention, it is provided that the light emitter is designed to emit light according to the second modulation frequency with two sidebands, the multi-reflection cell is designed to couple the acoustic signals of the chamber excited with the light emitted according to the second modulation frequency with two sidebands into the acoustic resonator as an acoustic mode with two sidebands, the analyzer is designed to determine an absolute frequency shift of the acoustic mode with the acoustic signals of the sidebands and the control device connected to the analyzer and the light emitter for signal transmission is designed to adapt the first modulation frequency and the second modulation frequency of the light emitter to the frequency-shifted acoustic mode determined with the analyzer.

[0014] In principle, the light can be emitted in various ways with the intensity corresponding to the first and second modulation frequencies. However, according to a preferred embodiment of the invention, the light emitter is configured to emit the light with the intensity corresponding to the first modulation frequency simultaneously or sequentially with the light with the intensity corresponding to a second modulation frequency. Depending on the modulation frequencies to be emitted, the concentration of the gas in the gas mixture can thus be determined with reduced latency.

[0015] In principle, it is possible to design the light emitter in various ways. However, according to a preferred embodiment of the invention, the light emitter comprises at least one LED. For this purpose, the LEDs can be configured to emit light at different frequencies. According to a further preferred embodiment of the invention, however, the light emitter is configured to emit UV light (ultraviolet light). However, the light emitter can also emit infrared light or light with a frequency between UV and infrared, depending on the gas whose concentration in the chamber is to be determined.

[0016] It is possible to design the acoustic resonator in various ways. However, according to a preferred development of the invention, it is provided that the acoustic resonator extends along a longitudinal direction from a first end to a second end and that part of the acoustic resonator connected to the chamber is arranged within the chamber. For this purpose, it can be provided that the chamber of the multi-reflection cell is annular and open. For example, the part of the acoustic resonator connected to the chamber of the multi-reflection cell can extend into the chamber. In principle, the acoustic resonator can be connected to the chamber in various ways. However, according to a preferred development of the invention, it is provided that at least the first end of the acoustic resonator is arranged within the chamber.In principle, it is possible to design the detectors in various ways and to arrange them in the acoustic resonator. According to a preferred development of the invention, however, the acoustic resonator is hollow cylindrical, one detector is arranged at the second end and another detector is arranged on part of a casing of the acoustic resonator, and the detectors are microphones that measure the sound intensity and sound frequency of the acoustic signals. However, it can also be provided that the acoustic signals are detected using tuning forks or oscillating levers. The hollow cylindrical acoustic resonator can be aligned coaxially with the annular multi-reflection cell, which enables more advantageous coupling of the acoustic signals into the acoustic resonator. The acoustic signal of the radial mode can be detected using the detector on the casing of the hollow cylindrical resonator.It can be provided that several detectors are arranged on the casing of the acoustic resonator. The detector at the second end of the acoustic resonator can detect the acoustic signal of the longitudinal mode.

[0017] According to yet another preferred embodiment of the invention, the sensor is configured with a further hollow-cylindrical acoustic resonator, wherein the further acoustic resonator is arranged on a side of the chamber of the multi-reflection cell opposite the acoustic resonator, wherein the detector is arranged at the second end of the acoustic resonator, and the further detector is arranged on a part of a casing of the further acoustic resonator. This makes it easier to separate the acoustic signals. The radial mode is detected by the further detector in the further acoustic resonator, and the longitudinal mode is detected by the detector at the second end of the acoustic resonator.

[0018] The sensor can be constructed in various ways. However, according to a further preferred embodiment of the invention, the sensor is designed as an integrated circuit. This allows the dimensions of the sensor to be reduced.

[0019] The invention also relates to a method for determining the concentration of a gas in a gas mixture using a sensor as described above, comprising the following steps: introducing a gas mixture into the chamber using the inlet, emitting light having the intensity according to the first modulation frequency and light having the intensity according to the second modulation frequency into the chamber so that the gas in the chamber and the chamber are excited, coupling the acoustic signals of the gas excited by the light emitted according to the first modulation frequency into the acoustic resonator as an acoustic mode and the acoustic signals of the chamber excited by the light emitted according to the second modulation frequency into the acoustic resonator as an acoustic mode, receiving acoustic signals using the detectors, and determining the gas concentration and the intensity of the light by analyzing the signals received by the detectors using the analysis means.

[0020] According to a preferred development of the invention, the method comprises the following further steps: emitting light along two sidebands of the first modulation frequency, coupling the acoustic signals of the gas excited by the light emitted along the two sidebands of the first modulation frequency into the acoustic resonator, determining the absolute frequency shift of the acoustic longitudinal mode and adapting the first modulation frequency and the second modulation frequency to the frequency-shifted acoustic mode.

[0021] According to a preferred development of the invention, the method comprises the following further steps: emitting light along two sidebands of the second modulation frequency, coupling the acoustic signals of the chamber excited with the light emitted along the two sidebands of the second modulation frequency into the acoustic resonator, determining the absolute frequency shift of the acoustic radial mode and adapting the first modulation frequency and the second modulation frequency to the frequency-shifted acoustic mode.

[0022] According to a preferred development of the invention, it is provided that the light with the intensity according to the first modulation frequency is emitted simultaneously or sequentially with the light with the intensity according to a second modulation frequency.

[0023] According to a further preferred embodiment of the invention, the concentration of the gases NO2, SO2, O3, CEL, CO, CO2, and / or ELS in the gas mixture is determined. These gases are examples of gases that may be present in an environment and that, for various reasons, must be detected and monitored in order to identify potential problems or risk situations such as contamination or flammable or explosive environments. These gases can be excited using a light emitter that emits UV light.

[0024] The invention is described in more detail below with reference to the drawings using preferred embodiments.

[0025] In the drawing show

[0026] Fig. 1 schematically shows a sensor according to a first preferred embodiment of the invention and

[0027] Fig. 2 schematically shows a sensor according to another preferred embodiment of the invention.

[0028] Fig. 1 schematically shows a sensor 1 according to a first preferred embodiment of the invention. The sensor 1 has a multi-reflection cell 2 and a detector 4 arranged in an acoustic resonator 3. The multi-reflection cell 2 is provided with an inlet 2.2 through which a gas mixture from the environment can be admitted into the sensor 1 and a chamber 2.1 formed in the multi-reflection cell 2. A light emitter 2.3 is formed in the multi-reflection cell, which emits UV light in the form of light pulses according to a first modulation frequency with two sidebands and according to a second modulation frequency. The light emitter 2.3 is in this case a laser emitting spatially and temporally coherent UV light. With the emitted light, a gas in the gas mixture is periodically excited and emits acoustic signals of the first modulation frequency with two sidebands. Furthermore, the chamber 2.1 is periodically excited with the light emitted at the second modulation frequency. The acoustic signals are amplified in the multi-reflection cell 2 and coupled into the acoustic resonator 3 as longitudinal and radial modes through an input (not further identified by reference numerals) located on the side opposite the input 2.2. Gases such as NO2, SO2, O3, CH4, CO, CO2, and / or H2S can be excited with the UV light.

[0029] The acoustic signal of the first modulation frequency is coupled into the acoustic resonator 3 as a longitudinal mode with two sidebands, and the acoustic signal of the second modulation frequency is coupled into the acoustic resonator 3 as a radial mode. The modulation frequencies correspond to the resonator's natural frequencies, so that the detector 4 detects resonant acoustic signals of the longitudinal and radial modes. The detector 4 is designed as a microphone with which the sound intensity and sound frequency of the acoustic signals can be detected. The two sidebands generate an error signal, which the analyzer 5 can use to determine an absolute frequency shift of the natural frequency of the acoustic resonator 3.Using a control unit 6, the first and second modulation frequencies can then be adapted to the changed natural frequencies of the acoustic resonator 3, so that the acoustic resonator 3 is stable against external environmental influences that change the natural frequency, such as changes in pressure, temperature and humidity.

[0030] Using an analyzer 5 connected to detector 4 for signal transmission, the acoustic signal of the longitudinal mode can be separated from that of the radial mode. The acoustic signal of the radial mode corresponds to the signal of the acoustic mode of the chamber 2.1 excited by the light emitted at the second modulation frequency and depends on the intensity of the light emitted by the light emitter. The acoustic signal of the longitudinal mode corresponds to the signal of the acoustic mode of the gas excited by the light emitted at the first modulation frequency and depends on both the intensity of the emitted light and the concentration of the gas in the gas mixture.

[0031] Analyzer 5 can determine the intensity of the emitted light and subtract it from the acoustic signal of the longitudinal mode. The acoustic signal of the longitudinal mode is then dependent on the gas concentration.

[0032] Finally, Fig. 2 schematically shows a sensor 1 according to a first preferred embodiment of the invention. The multi-reflection cell 2 is formed in a ring between a hollow-cylindrical acoustic resonator 3 and another hollow-cylindrical acoustic resonator 3. Both extend with their open first ends 3.1 into the likewise open chamber 2.1 of the multi-reflection cell 2. A detector 4 is arranged at each of the second ends 3.2 of the acoustic resonators 3, with which the acoustic signals of the longitudinal modes are primarily detected. A further detector 4 is arranged on the casing of each of the acoustic resonators 3, so that the radial modes can be primarily detected there. The detectors 4 are connected to analyzers 5 arranged at the second ends 3.2 for signal transmission. The analyzers 5 are connected to the control unit 6. The control unit 6 is connected to the light emitter 2.3 in the chamber 2.1 of the multi-reflection cell 2.

[0033] List of reference symbols

[0034] 1 sensor

[0035] 2 Multi-reflection cell 2.1 Chamber

[0036] 2.2 Entrance

[0037] 2.3 Light emitters

[0038] 3 acoustic resonator

[0039] 4 Detector 5 Analyzer

[0040] 6 Control unit

Claims

Patent claims 1. Sensor (1) for determining the concentration of a gas in a gas mixture, comprising a multi-reflection cell (2) and at least one acoustic resonator (3), wherein the multi-reflection cell (2) has a chamber (2.1) designed to accommodate a gas mixture, the multi-reflection cell (2) further has an inlet (2.2) for the gas mixture to enter the chamber (2.1) and a light emitter (2.3), so that the light generated by the light emitter (2.3) can be emitted into the chamber (2.1), the light emitter (2.3) is designed to emit light of an intensity according to a first modulation frequency and to emit the light with the intensity according to a second modulation frequency, wherein the first modulation frequency is different from the second modulation frequency, the acoustic resonator (3) is designed such that at least one acoustic longitudinal mode with the first modulation frequency and at least one acoustic radial mode with the second modulation frequency or at least one acoustic radial mode with the first modulation frequency and at least one acoustic longitudinal mode with the second modulation frequency can be excited in the acoustic resonator (3), a part of the acoustic resonator (3) is connected to the chamber (2) in such a way that1) is connected in such a way that acoustic signals can be coupled into the multi-reflection cell (2), the multi-reflection cell (2) is designed to couple the acoustic signal of the gas excited by the light emitted according to the first modulation frequency as an acoustic modulator into the acoustic resonator (3) and to couple the acoustic signal of the chamber (2) excited by the light emitted according to the second modulation frequency.1) to be coupled into the acoustic resonator (3) as an acoustic mode, the sensor (1) has detectors (4) arranged in the acoustic resonator (3) for detecting the acoustic signals and an analyzer (5) connected to the detectors (4) for signal transmission, the analyzer (5) is designed to use the detected signal, which is dependent on the gas concentration in the gas mixture and the intensity of the emitted light, of the acoustic mode of the gas excited with light emitted according to the first modulation frequency and the detected signal, which is dependent on the intensity of the emitted light, of the acoustic mode of the light emitted according to the second modulation frequency. excited chamber (2.1) to determine the intensity of the emitted light and subsequently to determine the gas concentration in the chamber (2.1) of the multi-reflection cell (2).

2. Sensor (1) according to claim 1 with a control unit (6), wherein the light emitter (2.3) is designed to emit light according to the first modulation frequency with two sidebands, the multi-reflection cell (2) is designed to couple the acoustic signals of the gas excited by the light emitted according to the first modulation frequency with two sidebands into the acoustic resonator (3) as an acoustic mode with two sidebands, the analyzer (5) is designed to determine an absolute frequency shift of the acoustic mode using the acoustic signals of the sidebands, and the control unit (6) connected to the analyzer (5) and the light emitter (2.3) for signal transmission is designed to adapt the first modulation frequency and the second modulation frequency of the light emitter (2.3) to the frequency-shifted acoustic mode determined using the analyzer (5).

3. Sensor (1) according to claim 1 or 2 with the control unit (6), wherein the light emitter (2.3) is designed to emit light according to the second modulation frequency with two sidebands, the multi-reflection cell (2) is designed to couple the acoustic signals of the chamber (2.1) excited by the light emitted according to the second modulation frequency with two sidebands into the acoustic resonator (3) as an acoustic mode with two sidebands, the analyzer (5) is designed to determine an absolute frequency shift of the acoustic mode using the acoustic signals of the sidebands, and the control unit (6) connected to the analyzer (5) and the light emitter (2.3) for signal transmission is designed to adapt the first modulation frequency and the second modulation frequency of the light emitter (2.3) to the frequency-shifted acoustic mode determined using the analyzer (5).

4. Sensor (1) according to one of the preceding claims, wherein the light emitter (2.3) is designed to emit the light with the intensity according to the first modulation frequency simultaneously or sequentially with the light with the intensity according to a second modulation frequency.

5. Sensor (1) according to one of the preceding claims, wherein the light emitter (2.3) comprises at least one LED.

6. Sensor (1) according to one of the preceding claims, wherein the acoustic resonator (3) extends along a longitudinal direction from a first end (3.1) to a second end (3.2) and the part of the acoustic resonator (3) connected to the chamber (2.1) is arranged within the chamber (2.1).

7. Sensor (1) according to claim 6, wherein at least the first end (3.1) of the acoustic resonator (3) is arranged within the chamber (2.1).

8. Sensor (1) according to one of claims 6 or 7, wherein the acoustic resonator (3) is hollow cylindrical, a detector (4) is arranged at the second end (3.2) and a further detector (4) is arranged at a part of a casing of the acoustic resonator (3), and the detectors (4) are microphones measuring sound intensity and sound frequency of the acoustic signals.

9. Sensor (1) according to one of claims 6 to 8, with a further hollow cylindrical acoustic resonator (3), wherein the further acoustic resonator (3) is arranged on a side of the chamber (2.1) of the multi-reflection cell (2) opposite the acoustic resonator (3), wherein the detector (4) is arranged at the second end (3.2) of the acoustic resonator (3), and the further detector (4) is arranged on a part of a casing of the further acoustic resonator (3).

10. A method for determining the concentration of a gas in a gas mixture with a sensor (1) according to one of claims 1 to 9, comprising the following steps: 51) Introducing a gas mixture into the chamber (2.1) with the inlet (2.2), 52) Emitting light with the intensity according to the first modulation frequency and light with the intensity according to the second modulation frequency into the chamber (2.1), so that the gas in the chamber (2.1) and the chamber (2.1) are excited, 53) coupling the acoustic signals of the gas excited by the light emitted according to the first modulation frequency into the acoustic resonator (3) as an acoustic mode and the acoustic signal of the chamber (2.1) excited by the light emitted according to the second modulation frequency into the acoustic resonator (3) as an acoustic mode, S4) Receiving acoustic signals with the detectors (4) and 55) Determining the gas concentration and the intensity of the light by analyzing the signals received by the detectors (4) with the analysis means (5).

11. The method according to claim 10 comprising the following further steps: S3a) Emitting light to two sidebands of the first modulation frequency, S4a) coupling the acoustic signals of the gas excited by the light emitted according to the two sidebands of the first modulation frequency into the acoustic resonator (3), 56) Determine the absolute frequency shift of the acoustic mode and 57) Adapting the first modulation frequency and the second modulation frequency to the frequency-shifted acoustic mode.

12. Method according to one of claims 10 or 11, with the following further steps: S3a) Emitting light to two sidebands of the second modulation frequency, S4a) coupling the acoustic signals of the chamber (2.1) excited with the light emitted according to the two sidebands of the second modulation frequency into the acoustic resonator (3), S6) Determine the absolute frequency shift of the acoustic mode and S7) Adapting the first modulation frequency and the second modulation frequency to the frequency-shifted acoustic mode.

13. The method according to any one of claims 10 to 12, wherein the light having the intensity according to the first modulation frequency is emitted simultaneously or sequentially with the light having the intensity according to a second modulation frequency.

14. Method according to one of claims 10 to 13, wherein the concentration of the gases NO2, SO2, O3 or H2S in the gas mixture is determined.

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