Photoacoustic measuring arrangement and method for determining a gas

Abstract

Photoacoustic measuring arrangement (1), comprising - an infrared emitter (3), wherein the infrared emitter (3) is suitable for emitting broadband light with periodically modulated intensity, wherein the infrared emitter (3) is configured to modify an excitation spectrum of the emitted broadband light, wherein the infrared emitter (3) is suitable for heating a gas volume with the broadband light and generating a sound wave in the gas volume, wherein the infrared emitter (3) comprises at least two heaters (4), wherein the infrared emitter (3) is configured such that the excitation spectrum emitted by the infrared emitter (3) is modified by a periodic intensity modulation of at least one heater (4) which is phase-shifted relative to the periodic intensity modulation of at least one other heater (4), and - an acoustic sensor (2) wherein the acoustic sensor (2) is suitable for measuring the generated sound wave, wherein the gas volume and the acoustic sensor (2) are arranged in a photoacoustic cell (6), and wherein the at least two heaters (4) are arranged outside the photoacoustic cell (6).

Description

[0001] The present invention relates to a photoacoustic measuring device. Furthermore, the present invention relates to a method for determining a gas using the photoacoustic measuring device.

[0002] The photoacoustic effect describes the generation of sound waves through the absorption of light in a gaseous, liquid, or solid sample. When light of a specific wavelength is absorbed in such a sample, the molecules are excited into higher rotational, vibrational, or electronic states. The energy stored in the excited states is either released as spontaneous or stimulated emission in the form of radiation, or transferred non-radiatively as kinetic energy through collisions with the surrounding atoms or molecules. Since the lifetime of the rotational and vibrational states is usually much longer than the time until relaxation through collision processes, almost all of the energy introduced into the sample is converted into kinetic energy. This leads to a local increase in temperature and, consequently, to an increase in pressure in the excited volume.

[0003] If the energy / intensity of the incident light beam is periodically modulated, this modulation is transferred to the pressure level. Periodic pressure fluctuations / sound waves are created, whose frequency is determined by the modulations of the energy / intensity of the light beam, and whose amplitude / sound pressure is determined by the amount of absorbed light. Since each molecule has its own specific absorption spectra, a sample can be analyzed using the photoacoustic effect by changing the wavelength of the light source.

[0004] A disadvantage of a measurement using the photoacoustic effect is the requirement to use a monochromatic infrared (IR) light source, which should be tunable to differentially excite the sample or a sample composition on a specific absorption line to determine the sample. Light sources that meet these requirements are either expensive, such as a quantum cascade laser (QCL), or limited to a few wavelengths when filters are used. Furthermore, the mechanical filter change in such a measurement setup, as is often accomplished with a filter wheel, cannot be easily miniaturized.

[0005] It is desirable to find a way to enable photoacoustic measurement setups to do without the need for monochromatic light sources and yet still be able to determine a sample and be suitable for miniaturization.

[0006] EP 0 859 536 A1 relates to an infrared emitter made of metal black and its use in gas sensors. DE 10 2017 129 471 A1 and DE 10 2018 112 341 A1 each show a photoacoustic gas analyzer and a method for operating a photoacoustic gas analyzer. DE 10 2012 217 479 B3 relates to a gas sensor with at least one test gas volume configured such that electromagnetic radiation emanating from a source can penetrate the test gas volume, and with at least one resonator body that can be excited to oscillate by a photoacoustic signal, and with at least one device for detecting the oscillation of the resonator body.

[0007] The object of the present invention is to provide a photoacoustic measuring arrangement that is cost-effective and miniaturizable and enables reliable measurements.

[0008] The object is achieved by a photoacoustic measuring arrangement according to claim 1. Further advantageous embodiments, possible arrangements and favorable methods for measuring can be found in the dependent claims.

[0009] A photoacoustic measuring arrangement is provided, comprising an infrared emitter capable of emitting broadband light with periodically modulated intensity. The infrared emitter is configured to modify an excitation spectrum of an emitted broadband light, and a gas volume is heated by the emitted broadband light to generate a sound wave in the gas volume. The photoacoustic measuring arrangement further comprises an acoustic sensor suitable for measuring the sound wave generated in the gas volume.

[0010] Such a photoacoustic measurement setup does not require a monochromatic emitter, whose main wavelength and wavelength distribution should also be tunable, like some IR lasers or a QKL, to measure a sound wave generated by the photoacoustic effect. Since such monochromatic emitters are still quite expensive, the proposed photoacoustic measurement setup is much more cost-effective than comparable measurement setups. Furthermore, all components of the proposed measurement setup, such as the infrared emitter and the acoustic sensor, are suitable for miniaturization, or miniaturized components already exist that can be used, such as MEMS microphones and heaters. The acoustic sensor can be sufficiently sensitive to detect small pressure fluctuations. In particular, the acoustic sensor can be capable of detecting and distinguishing between overpressure and underpressure.The infrared light emitted by the infrared radiator may be electromagnetic radiation with a wavelength in a range of 780 nm to 1 mm, preferably in a range of 1 µm to 100 µm.

[0011] The infrared radiator comprises at least two heaters. Any thermal radiator, such as a coil, a wire, a filament, or even a semiconductor, can be used as a heater. By arranging two or more heaters within the infrared radiator, it is possible to influence the emitted broadband light with a greater degree of freedom. For example, a plurality of heaters can be arranged in an array to adjust the intensity of the emitted light by switching selected heaters on or off.

[0012] The excitation spectra emitted by the infrared radiator can also be modified by changing the temperature of the infrared radiator. A preferred temperature range for the infrared radiator is 100°C–1300°C, but depending on the radiator, temperatures up to 3000°C are also possible. Ideally, the radiator emits cavity radiation which, according to Planck's radiation law, depends only on the temperature. By changing the temperature of the infrared radiator or heater, not only the maximum intensity of the emitted radiation but also the intensity distribution of the emitted light can be changed. A higher temperature leads to maximum intensity at shorter wavelengths and to more components of the intensity distribution at shorter wavelengths, whereas the light emitted at lower heater temperatures has a maximum at longer wavelengths with more components of the emitted light at longer wavelengths.Since the temperature of a heater can usually be easily controlled, the emitted excitation spectrum can also be easily adjusted to a desired spectrum. By changing the temperature of the infrared heater, multiple measurement points can be measured in a setup by regulating the temperature. Each measurement point corresponds to a temperature of the infrared heater.

[0013] It is advantageous if the heaters can be controlled individually. This allows each heater to be switched on and off independently, but also allows the emitted excitation spectra of each heater to be regulated as desired by the heater's temperature. If an array of heaters is used, good thermal isolation between adjacent heaters is beneficial to maintain the heaters' independence, as heat transfer would introduce correlation. Furthermore, the temporal modulation of each heater can also be individually adjusted to change the temporal pattern to which the gaseous volume is subjected.

[0014] Accordingly, the excitation spectrum emitted by the infrared radiator is modified by a periodic intensity modulation from at least one heater, which is phase-shifted with respect to the periodic intensity modulation from at least one other heater. Since the gaseous volume is exposed to the phase-shifted and the non-phase-shifted light, the sound wave generated in the gaseous volume depends on the phase-dependent sum of the broadband excitation light from the at least two heaters. In the simplest case, in which the light intensity and spectrum of two heaters are equal, but a phase modulation from one heater has a phase shift of 180° with respect to the other, both sound waves cancel each other out due to destructive interference, and no sound wave can be detected.In general, the extent of a detectable sound wave depends on the phase-dependent sum of both excitation spectra and the absorption spectrum of the gas. A phase shift changes the phase-dependent sum, and consequently, the gaseous volume is subject to a different excitation spectrum.

[0015] Furthermore, the infrared heater can be configured to emit the same light intensity for different broadband excitation spectra by adjusting a surface from which heat is released. Since the heater behaves similarly to an idealized blackbody, the emitted radiation obeys the Stefan-Boltzmann law, according to which the total radiant power depends on the fourth power of the temperature T (in Kelvin) and linearly on the radiating area A. Thus, for example, to achieve the same radiated power at 500°K as at 1000°K, the heater must have a radiating area 16 times larger. An infrared heater whose radiating area can be adjusted to different temperatures and, consequently, different broadband excitation spectra is therefore advantageous because it can deliver the same total radiant power to the gaseous volume.

[0016] In addition, the heaters can be silicon-based. A silicon-based microheater can have an edge length of less than 500 µm. Thus, in a heater array of only 5 x 5 mm 2 At least 100 microheaters must be arranged. Consequently, silicon-based heaters represent a suitable approach for miniaturizing the photoacoustic measurement setup.

[0017] In addition, the photoacoustic measurement setup can include a lock-in amplifier capable of amplifying a measurement signal. The measurement signal is the electrical signal emitted by the acoustic sensor. Lock-in amplifiers can amplify a weak signal if the frequency and phase of the signal are known. This significantly improves the signal-to-noise ratio and allows small signals to be measured. Since the frequency and phase of the heating broadband light of the infrared emitter are known, a lock-in amplifier can be used in the proposed photoacoustic measurement setup. The lock-in amplifier could, for example, be implemented in an FPGA to keep the dimensions small.

[0018] Furthermore, the measurement setup could also include a waveguide designed to guide the light from the infrared emitter to the gaseous volume. Since the gaseous volume does not have to be positioned directly in the straight infrared beam, but can be positioned anywhere that reaches the other end of the waveguide, significantly greater design freedom is achieved for the photoacoustic measurement setup. Furthermore, waveguides offer an excellent beam profile, ensuring that a uniform gas volume is heated.

[0019] The gas volume and the acoustic sensor are arranged in a photoacoustic cell. Since the pressure increase in the gas volume in such an arrangement occurs in a confined space where it cannot dissipate, the signal from the acoustic sensor can be significantly amplified.

[0020] In an unclaimed embodiment, the gas volume can be located between the infrared emitter and a photoacoustic cell, with a reference gas and the acoustic sensor arranged in the photoacoustic cell. In this arrangement, the reference gas is a known gas type that generates the sound wave in the photoacoustic cell.

[0021] Some wavelengths of the broadband excitation light are absorbed in the gas volume. Which wavelengths of the broadband excitation light are absorbed and to what extent depends on the composition of the gas in the volume. Each gas has different absorption characteristics, which depend on the molecular structure and the elements it consists of. In the range from 2.5 µm to 4 µm, hydrogen bonds are primarily excited. In the range from 4 to 6, absorption lines of triple and double bonds are primarily found. The wavelength range above 6 µm is called the fingerprint range. Absorption lines of individual bonds are found in this wavelength range.

[0022] If the gas volume between the infrared emitter and the photoacoustic cell contains the same gas type as the reference gas, the wavelengths of the broadband excitation light that are crucial for generating the acoustic signal in the reference gas are absorbed in the gas volume. As a result, less infrared radiation of these wavelengths reaches the reference gas in the photoacoustic cell, and the measurement signal from the acoustic sensor decreases. The reduction in the measurement signal depends on the extent to which the gas in the gas volume resembles the reference gas and on the concentration of the gas in the gas volume. In this way, the reference gas can be detected in the gas volume without having to introduce the gas volume into the photoacoustic cell. The reference gas can be any pure gas or any gas composition.

[0023] Furthermore, the photoacoustic measuring arrangement can comprise a memory in which at least one reference spectrum is stored, as well as a control and evaluation unit which is designed to compare the reference spectrum with the measured values ​​and to assign a content of the gas volume to a gas or gas mixture and / or to determine a concentration of a gas mixture and to control the infrared radiator.

[0024] The reference spectrum can be a function or a curve in which the acoustic sensor signal is plotted against the temperature of the infrared emitter. A reference spectrum for a specific gas can be easily obtained by measuring the acoustic sensor signal for that gas in the photoacoustic measurement setup over the desired temperature range and storing this information in memory. However, the reference spectra could also be obtained by convolving the measured or calculated absorption spectra of gases with measured or calculated emission spectra of the infrared emitter at different temperatures and integrating the convolved spectra for each temperature over the entire wavelength range. In this way, the total absorbed energy is calculated, which is linearly related to the energy of the sound wave.If pure gas is measured, a measured spectrum, where the spectrum is the signal from the acoustic sensor versus temperature, can match a reference spectrum and be determined as the pure gas. If a composition of different gases is measured, the measured spectrum can be a linear combination of the reference spectra of the various gases that make up the composition. The control and evaluation unit can approximate a linear combination of known reference spectra to the measured spectrum, for example, using the least squares method. In this way, the coefficient of the linear combination can be obtained, which can then be used to determine a concentration of the gases in the composition.

[0025] Furthermore, the control and evaluation unit can comprise a neural network. Numerous measured spectra for known gases and gas compositions could be input into the neural network to teach it how to identify a gas. Since applications often only involve specific gases, the possible results can be limited, which is advantageous for a neural network. Since a neural network is useful for pattern recognition and an acoustic signal measured over different temperatures represents a pattern, a neural network is also advantageous for identifying a gas mixture.

[0026] An advantage of the described photoacoustic measuring arrangement is that it can be used for various measuring methods.

[0027] A non-claimed aspect relates to a method for detecting a gas mixture comprising the following steps: a. Detecting a first measurement signal of a gaseous mixed sample, wherein the gaseous sample contains a first gas type which is known and at least one further gas type, with a photoacoustic measuring arrangement as described above, wherein the infrared radiator emits light with a first broadband excitation spectrum at a first temperature. b. Detecting a second measurement signal of the gaseous mixed sample with the photoacoustic measuring arrangement, wherein the infrared emitter emits light with a second broadband excitation spectrum at a second temperature. c. Calculating an expected second measurement signal based on the first measurement signal and the reference spectrum, assuming that the gaseous sample consists exclusively of the first gas type. d. Comparing the expected second measurement signal with the measured second measurement signal. e. Determine whether at least one other gas species is present in the gaseous sample.

[0028] The second temperature is different from the first temperature.

[0029] The proposed method makes it possible to easily determine, using only two measurement points, whether a gaseous sample is pure or a combination of different gases. If the gaseous sample is known to contain two known gas species, the method is also suitable for determining the concentration of both gases in the gaseous sample. This requires solving a system of two equations, each of which is a linear combination of the known acoustic sensor signals for the first and second gas species at the first and second measurement temperatures. The coefficients of these linear combinations are the concentrations of the two gas species in the gaseous sample.

[0030] Without limiting the method, the proposed method can be extended by a third, or n-th, measurement point, where n is a real number greater than 3.

[0031] Thus, in step b., a third measurement signal of the gaseous mixed sample can be detected with the photoacoustic measurement arrangement, wherein the infrared emitter emits light with a third excitation spectrum at a third temperature, and wherein in step c. a third expected measurement signal can be calculated from the first and second measurement signals and the reference spectrum under the assumption that the gaseous sample consists exclusively of a first and a second gas type, and wherein in step d. the third expected measurement signal can be compared with the measured third measurement signal, and wherein in step e. it is determined whether at least one further gas type is contained in the gaseous sample.

[0032] Analogous to the previous method, this can be used to easily determine with just three measuring points whether a gaseous sample is pure, contains two gas types, or contains at least one other gas type. However, this method is not limited to just two or three gas types in the sample, but can be extended to numerous gas types. This method is therefore suitable for determining the number of gases in a composition. If the number of gas types contained in the gaseous sample and the gas types themselves are known, the method is also suitable for determining the concentration of the gases in the gaseous sample. To do this, a system of equations must be solved, each equation being a linear combination of the known signals from the acoustic sensor for the known gas types at the measuring temperatures.The number of gas species in the gaseous sample determines the smallest number of equations needed to determine the concentration. The coefficients of these linear combinations are the concentrations of gas species in the gaseous sample.

[0033] A gas mixture can be determined using another method.

[0034] The method for determining a gas mixture comprises the following steps: a. heating a gaseous sample with an infrared radiator in a photoacoustic measuring arrangement, wherein the infrared radiator has at least two heaters, each heater having a broadband excitation spectrum that is different from that of the other heater, and where the intensities of the heaters are periodically modulated with a same frequency, and where the modulations of the heaters have a phase shift to each other. b. Detecting a measurement signal, wherein the measurement signal is negative, positive or zero depending on the phase-dependent sum of the broadband excitation spectra and an absorption line of the gaseous sample. c. Change the phase shift between the heaters and repeat step b., whereby the change should bring the measurement signal closer to zero. d. Repeat step c. until the measurement signal becomes zero, minimal or less than a threshold value. e. Calculating the wavelength at which the phase-dependent sum of the output spectra becomes zero, minimal, or less than a threshold value, taking as the phase shift the phase shift at which the measurement signal is zero, minimal, or less than a threshold value. f. Determining the gaseous sample by comparing the wavelength calculated in step e. with known gas spectra or reference spectra.

[0035] Since the gaseous sample is exposed to phase-shifted and non-phase-shifted light, the sound wave generated in the gaseous volume depends on the phase-dependent sum of the broadband excitation light from at least two heaters. The extent of a detectable sound wave depends on the phase-dependent sum of both excitation spectra and the absorption spectrum of the gas. If the phase-dependent sum spectrum of both excitation spectra is positive at an absorption line of the gas, the signal of the acoustic sensor will be positive. If the phase-dependent sum spectrum of both excitation spectra is negative at an absorption line of the gas, the signal of the acoustic sensor will be negative. If the phase-dependent sum spectrum of both excitation spectra is zero at an absorption line of the gas, the signal of the acoustic sensor will also be zero.By shifting the phase, the phase-dependent sum and thus the acoustic sensor signal are tuned. When the acoustic sensor signal becomes zero or minimal, the zero point of the phase-dependent sum of the excitation spectra coincides with an absorption line of the gas. Consequently, an absorption line of the gaseous sample can be determined by calculating the zero points of the phase-dependent sum of the excitation spectra at a given phase where the acoustic signal is minimal. The gas type can thus be determined by comparing the detected absorption line with known absorption spectra.

[0036] As an alternative to the above method, in step c., instead of the phase shift, an excitation spectrum of the heaters can be changed by changing the temperature of at least one heater.

[0037] Changing the temperature of a heater also changes the broadband excitation light from the heater. Consequently, a phase-dependent sum of both broadband excitation spectra differs even when the temperature of a heater is changed. Analogous to the previous method, a zero point of the phase-dependent sum can be tuned by changing the temperature across the wavelength. When the zero point of the phase-dependent sum coincides with an absorption line of the gas, the acoustic sensor signal becomes zero or minimal. If the incremental changes cannot be made small enough, a lower threshold for the acoustic sensor signal can be set, which is considered zero.

[0038] The invention and a manufacturing method are described below based on embodiments with reference to the figures. Identical parts or parts with the same effect are designated by the same reference numerals.

[0039] The figures serve only to illustrate the invention and are therefore only schematic and not drawn to scale. The dimensions of some parts may be exaggerated or distorted. Therefore, neither absolute nor relative dimensions can be derived from the figures. Identical or equivalent parts are provided with the same reference numerals. Fig. 1 shows a simplified photoacoustic measuring arrangement in a first embodiment. Fig. Figure 2 shows a simplified photoacoustic measurement setup of a comparison example. Fig. Figure 3 shows exemplary broadband excitation spectra at different temperatures. Fig. Figure 4 shows normalized broadband excitation spectra at different temperatures. Fig. Figure 5 shows a simplified infrared radiator comprising two heaters. Fig. Figure 6 shows a simplified infrared heater comprising 324 heaters. Fig. Figure 7 shows two exemplary phase-dependent sums, each of which is a sum of two broadband excitation spectra at two different temperatures.

[0040] Fig. Figure 1 shows a schematic photoacoustic measuring arrangement 1 in a first embodiment. An infrared radiator 3 emits broadband light, which is periodically modulated, through a window into a photoacoustic cell 6. The infrared light heats a gas volume in the photoacoustic cell 6, causing the volume to expand. Since the intensity of the broadband excitation light is periodically modulated, the periodic modulation is transferred to the volume expansion of the gas volume. As a result, a sound wave is generated with the same frequency as the periodic modulation of the broadband excitation light. An acoustic sensor 2, arranged in the photoacoustic cell 6, measures the sound wave as a sound detector signal. The acoustic sensor 2 is phase-sensitive to the sound wave and can measure positive and negative pressure. Since the sound wave is enclosed in the photoacoustic cell 6, it is not dissipated.Therefore, the sound detector signal is improved compared to an embodiment without photoacoustic cell 6.

[0041] A lock-in amplifier 5 is connected between the acoustic sensor 2 and a control and evaluation unit 8, which controls the infrared radiator 3. Since the control and evaluation unit 8 supplies the lock-in amplifier 5 with frequency and phase information of the modulation of the infrared radiator 3, the lock-in amplifier 5 can improve the signal-to-noise ratio of the arrangement.

[0042] In addition, a memory 7 is connected to the control and evaluation unit 8. The memory 7 contains reference spectra of various pure gases or gas mixtures, the simplest of which would be a plot of the signal from the acoustic sensor 2 against the temperature of the infrared radiator 3. The control and evaluation unit 8 uses the reference spectra to determine a gas in the photoacoustic cell 6. The control and evaluation unit 8 could contain an artificial neural network for determining the gases, since neural networks are suitable for pattern recognition. Reference spectra can be used to train the neural network.

[0043] The Fig. The arrangement shown in Figure 2 is similar to the arrangement in Fig. 1. The main differences are that the gas volume is located between the infrared emitter 3 and a photoacoustic cell 6, and that a reference gas is located in the photoacoustic cell 6. The reference gas is a known gas type. The gas volume contains the gas to be analyzed. Fig. The arrangement shown in Figure 2 makes it possible in particular to check whether the known reference gas is present in the gas volume.

[0044] The gas volume is located in an additional chamber 9 in front of the photoacoustic cell, but the gas volume could also be located in free space within the beam of the infrared emitter 3. The photoacoustic cell 6 is sealed gas-tight from the additional chamber 9, and the additional chamber 9 has windows that allow the infrared light to enter and pass through the photoacoustic cell 6. The arrangement checks whether the gas type, which is the reference gas, is present in the measured gas volume.

[0045] The infrared light passes through the gas volume on its way to the photoacoustic cell 6. If the gas type that serves as the reference gas is present in the gas volume, certain wavelengths of infrared light that are characteristic of that gas type are absorbed. Depending on the concentration in the gas volume and the length of the light path in the absorbing gas volume, more or less light of the specific wavelengths is absorbed according to the Lambert-Beer law. The reference gas in the photoacoustic cell 6, on the other hand, is heated by exactly the same wavelengths that are absorbed in the gas volume. Thus, if the gas type that serves as the reference gas is present in the gas volume, less light reaches the reference gas in the photoacoustic cell 6, the reference gas is heated less, and the signal from the acoustic sensor 2 decreases significantly. The extent of the signal reduction is determined by the gas concentration.If the gas type that is the reference gas is not present in the gas volume, no characteristic wavelengths are absorbed in the gas and the signal of the acoustic sensor 2 is not attenuated by the gas volume.

[0046] In Fig. Figure 3 shows three broadband excitation spectra of a thermal infrared radiator (3) at 320°C, 420°C, and 520°C. The graphs are normalized to the spectrum peak at 520°C. With increasing temperature, the peak height increases. Since the graphs are normalized to the peak value of 520°C, the peak value for 520°C is equal to 1. The peak value of the graph for 320°C, in contrast, is only ~0.2.

[0047] Furthermore, the peak value shifts toward shorter wavelengths with increasing temperature. Furthermore, the slope of the graphs from the peak value toward longer wavelengths is steeper at higher temperatures. Overall, the broadband excitation spectra can be significantly altered by changing the temperature.

[0048] Furthermore, the diagram in Fig. Figure 3 shows two idealized perpendicular absorption lines corresponding to two different gases, at ~3.8 µm and ~5 µm. Comparing the intersection points of the absorption lines with the broadband excitation spectra at 320°C and 520°C, it can be seen that at 320°C, the gas with the absorption line at 5 µm absorbs more energy than the gas with the absorption line at 3.8 µm, while at 520°C, the gas with the absorption line at 5 µm absorbs less energy than the gas with the absorption line at 3.8 µm. Since there is a linear relationship between the absorbed energy and the signal from acoustic sensor 2, this difference would also be detectable in the signal from acoustic sensor 2.

[0049] Fig. Figure 4 also shows broadband excitation spectra of an infrared radiator 3, but in this figure each spectrum is normalized and the temperatures of the infrared radiator 3 are 120°C, 520°C and 1220°C. The diagram shows the same vertical absorption lines corresponding to two different gases as in Fig. 3 at ~3.8 µm and ~5 µm. Since the temperatures of the plotted spectra show a larger difference and are normalized, the peak shift and the differences in the slopes are even more pronounced.

[0050] The differences in the absorption of two gases at different temperatures can already be exploited when it is necessary to ensure that a known pure gas is present in the gas volume. The gas volume is first measured at a first temperature and then at a second temperature. Based on the first measurement signal, an expected second measurement signal can be calculated, which depends on the broadband excitation spectrum at the second temperature, assuming that only the pure gas is present in the gas volume. If the expected signal matches the measured signal, only the pure gas is present in the gas volume. Otherwise, at least one other component is present in the gas volume.

[0051] If it is known that the gaseous sample contains two known gas species, the method is also suitable for determining the concentration of both gases in the gaseous sample. This requires solving a system of two equations, each of which is a linear combination of the known signals from the acoustic sensor 2 for the first and second gas species at the first and second measurement temperatures. The coefficients of these linear combinations are the concentrations of two gas species in the gaseous sample. The proposed method can be extended to a third or nth gas with a third or nth measurement point corresponding to a different temperature.

[0052] It is a challenging task for a thermal infrared heater 3 to operate at different temperatures, according to the standardization in Fig. 4, to radiate the same energy. The infrared radiator 3 can be considered a blackbody that obeys the Stefan-Boltzmann law. Since the radiated power depends on the fourth power of the temperature, it must be adjusted for different temperatures. This can be achieved by adjusting the radiating area, which is linearly related to the radiated power.

[0053] Fig. Figure 5 shows an infrared radiator 3 comprising two heaters 4 that emit broadband excitation spectra in the infrared range and, similar to a blackbody, emit temperature-dependent spectra. The larger heater 4 is designed to have a radiating area 16 times larger than that of the smaller heater 4 in the middle. Since the heaters 4 obey the Stefan-Boltzmann law, both heaters 4 radiate the same power when the temperature of the smaller heater 4 is twice that of the larger heater 4, where the temperature is specified in Kelvin.

[0054] A more adaptable embodiment of the infrared heater 3, consisting of 324 heaters 4 in an 18x18 array, is shown in Fig. 6. In such an infrared radiator 3, the radiating surface can be provided with a Fig. 5 higher degree of freedom to different temperatures and consequently different broadband excitation spectra.

[0055] To keep the infrared emitter 3 relatively small, the heaters 4 can be silicon-based to take advantage of MEMS technology and processes. The silicon-based microheaters can have an edge length of less than 500 µm. The heaters 4 in a heater array should be individually controllable to ensure that the broadband excitation spectrum, the radiated power, and also the temporal modulation can be adjusted as desired.

[0056] Fig. Figure 7 shows two example phase-dependent sums, each of which is a sum of two broadband excitation spectra at two different temperatures. For both sums, the phase shift is 180°, so only one spectrum needs to be subtracted from the other. The temperatures of heaters 4 are 620°C and 220°C for the dark line and 420°C and 220°C for the lighter line. A vertical idealized absorption line of a gas lies at ~5 µm.

[0057] A gas volume interacts with both the out-of-phase and out-of-phase light. Consequently, the generated sound wave depends on the phase-dependent sum of the broadband excitation light. The sign of the sound detector signal depends on whether the phase-dependent sum is positive or negative at the wavelength of the gas's absorption line. If the phase-dependent sum is zero at the wavelength of the absorption line, the sound detector signal also assumes a value of zero, because the out-of-phase and out-of-phase sound waves cancel each other out.

[0058] The phase-dependent sum is tuned by shifting the phase or by changing the excitation spectrum by changing the temperature. Fig.For example, in Figure 7, the temperature of one of the heaters 4 was lowered from 620°C to 420°C while maintaining the same phase shift. As a result, the zero point, or equilibrium point, of the phase-dependent sum was shifted from ~4.2 µm to ~5 µm. Since the absorption line of the measured gas is at 5 µm, the acoustic sensor 2 would have a positive signal in the case of the dark line. The signal would assume a value of zero in the second case, where the second heater 4 has a temperature of 420°C.

[0059] Therefore, a gas can be identified by tuning the phase-dependent sum, whether by temperature, phase, or intensity, to find a detector signal that is zero or nearly zero. The zero point of the found phase-dependent sum corresponds to an absorption line of the gas thus identified.

Claims

[1] Photoacoustic measuring arrangement (1), comprising - an infrared radiator (3), wherein the infrared radiator (3) is suitable for emitting broadband light with periodically modulated intensity, wherein the infrared radiator (3) is designed to change an excitation spectrum of the emitted broadband light, wherein the infrared radiator (3) is suitable for heating a gas volume with the broadband light and generating a sound wave in the gas volume, wherein the infrared radiator (3) comprises at least two heaters (4), wherein the infrared radiator (3) is designed such that the excitation spectrum emitted by the infrared radiator (3) is changed by a periodic intensity modulation of at least one heater (4) which is phase-shifted with respect to the periodic intensity modulation of at least one other heater (4), and - an acoustic sensor (2), wherein the acoustic sensor (2) is suitable for measuring the generated sound wave, wherein the gas volume and the acoustic sensor (2) are arranged in a photoacoustic cell (6), and wherein the at least two heaters (4) are arranged outside the photoacoustic cell (6). [2] Photoacoustic measuring arrangement (1) according to the preceding claim, wherein the excitation spectra emitted by the infrared radiator (3) are changed by changing a temperature of the infrared radiator (3). [3] Photoacoustic measuring arrangement (1) according to claim 1 or claim 2, wherein the heaters (4) can each be controlled individually. [4] Photoacoustic measuring arrangement (1) according to one of claims 1 to 3, wherein the heaters (4) are based on silicon. [5] Photoacoustic measuring arrangement (1) according to one of the preceding claims, wherein the infrared radiator (3) is adapted to emit the same light intensity for different broadband excitation spectra by adjusting an area from which heat is radiated. [6] Photoacoustic measuring arrangement (1) according to one of the preceding claims, further comprising a lock-in amplifier (5) which is suitable for amplifying a measuring signal provided by the acoustic sensor (2). [7] Photoacoustic measuring arrangement (1) according to one of the preceding claims, wherein a waveguide is arranged to guide the light from the infrared radiator (3) to the gas volume. [8] Photoacoustic measuring arrangement (1) according to one of the preceding claims, further comprising - a memory (7) in which at least one reference spectrum is stored, - a control and evaluation unit (8) which is designed to compare the reference spectrum with the measured values and to assign a content of the gas volume to a gas or gas mixture and / or to determine a concentration of a gas mixture and to control the infrared radiator (3). [9] Photoacoustic measuring arrangement (1) according to the preceding claim, - wherein the control and evaluation unit (8) comprises a neural network. [10] A method for determining a gas, comprising the following steps: a. Heating a gaseous sample with an infrared radiator (3) in a photoacoustic measuring arrangement (1) according to one of claims 1 to 9, wherein the infrared radiator (3) has at least two heaters (4), wherein the heaters (4) each have a broadband excitation spectrum which is different from that of the other heater (4), and wherein the intensities of the heaters (4) are periodically modulated at the same frequency, and wherein the modulations of the heaters (4) have a phase shift relative to one another, b. Detecting a measurement signal, wherein the measurement signal is negative, positive or zero depending on the phase-dependent sum of the broadband excitation spectra and an absorption line of the gaseous sample, c. Changing the phase shift between the heaters (4) and repeating step b., whereby the change should bring the measurement signal closer to zero, d. Repeat step c. until the measurement signal becomes zero, minimal or less than a threshold value, e. Calculating the wavelength at which the phase-dependent sum of the output spectra becomes zero, minimal or less than a threshold value, taking as the phase shift the phase shift at which the measurement signal is zero, minimal or less than a threshold value, f. Determining the gaseous sample by comparing the wavelength calculated in step e. with known gas spectra or reference spectra. [11] Method for determining a gas according to claim 10, wherein in step c., instead of the phase shift, an excitation spectrum of the heaters (4) is changed by changing the temperature of at least one heater (4).

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