EP-4735877-A1 - SENSOR FOR DETERMINING THE CONCENTRATION OF A GAS

EP4735877A1EP 4735877 A1EP4735877 A1EP 4735877A1EP-4735877-A1

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.

Inventors

PALZER, STEFAN
ORTIZ PEREZ, Alvaro
Rodríguez Gutiérrez, Gabriel
Kuczius, Vincent

Assignees

Technische Universität Dortmund

Dates

Publication Date: 20260506
Application Date: 20240628

Claims (14)

1. Sensor (1) for determining the concentration of a gas in a gas mixture with 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 with the light emitter (2.3) can be emitted into the chamber (2.1), 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, 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 (2.1) 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 with the light emitted according to the first modulation frequency into the acoustic resonator (3) as an acoustic mode and to couple the acoustic signal of the chamber (2.1) excited with the light emitted according to the second modulation frequency 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 be connected to the gas concentration in the gas mixture and the intensity of the emitted Light-dependent signal of the acoustic mode of the gas excited with light emitted at the first modulation frequency and the detected signal of the acoustic mode of the light emitted at the second modulation frequency, dependent on the intensity of the emitted light 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 device (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 with 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 with the acoustic signals of the sidebands and the control device (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 with the analyzer (5).
3. Sensor (1) according to claim 1 or 2 with the control device (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 with 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 with the acoustic signals of the sidebands and the control device (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 with 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) has 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. Method for determining the concentration of a gas in a gas mixture with a sensor (1) according to one of claims 1 to 9 with 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 with 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 with 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 along two sidebands of the first modulation frequency, S4a) coupling the acoustic signals of the gas excited with the light emitted according to the two sidebands of the first modulation frequency into the acoustic resonator (3), 56) Determining 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) Determining 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 H2 S in the gas mixture is determined.

Description

Sensor for determining the concentration of a gas The invention relates to a sensor for determining the concentration of a gas in a gas mixture, with 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 gas mixture to enter 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. 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 the pollution of an environment or for the early detection of a possible fire. Photoacoustic sensors that are spectrally set to detect 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. If a gas is irradiated with light, part of the light energy is absorbed by the gas, the gas molecules change into an excited state and leave this by radiating thermal energy. Due to heat conduction, the energy is distributed in the gas after a finite time and an increased temperature is established in the gas. This heat supply causes an increase in volume and an associated decrease in density. If this gas is irradiated with light pulses, it is periodically heated and cooled. The resulting change in volume generates sound waves, which in turn can be measured with suitable detectors. In this case, being spectrally adjusted to the detection of particular gases means that the light emitted with the light flashes requires a certain frequency for the gas to be detected, since the gas molecules only absorb light of certain frequencies. The gas in the gas mixture is therefore detected by only exciting the gas to be detected with light of a certain frequency, whereby a signal proportional to the concentration of the gas in the gas mixture is emitted in the form of sound. EP 4 009 034 A1 describes a detector which enables both the detection and the quantification of the concentration of a specific gas in an environment using the photoacoustic effect. A method for producing a detector of the above-mentioned type and a method for measuring the concentration of a specific gas in an environment using the above-mentioned detector are also described. With direct photoacoustic sensors, the output signal measured with the detector scales with the intensity of the emitted light. If the intensity of the emitted light changes, the output signal changes, which means that the concentration of the gas cannot be determined precisely. At the same time, the natural frequency of acoustic resonators of the photoacoustic sensors depends on external influences such as temperature, humidity and pressure. Based on this, the object of the invention is to provide a long-term stable sensor. This object is achieved by the subject matter of patent claim 1. Preferred further developments can be found in the subclaims. According to the invention, it is therefore provided to provide a sensor suitable for determining the concentration of a gas in a gas mixture, with 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 gas mixture to enter the chamber and a light emitter, so that the light generated with the light emitter can be emitted into the chamber, the light emitter 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 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 can be excited with the second modulation frequency 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 with the light emitted according to the first modulation frequency into the acoustic resonator as an acoustic mode and to couple the acoustic signal of the cha