Device and method for determining a property of a fluid

Abstract

Device for determining a property of a fluid (1) by means of photoacoustic spectroscopy (PAS) with a housing (2) that encloses a measuring chamber (3) for receiving the fluid, with a radiation source (4) designed in such a way that it modulates electromagnetic excitation radiation for the fluid and shines into the measuring chamber (3) containing the fluid, with a sound measuring device (5) which is designed in such a way that it can detect sound waves generated by the fluid in the measuring chamber (3), with a control device (7) that controls the radiation source (4) in such a way that the excitation radiation is modulated with a frequency beam with several spaced-apart individual frequencies, and with an evaluation device (6) that is designed to evaluate a detected sound signal from the sound measuring device (5) by determining the distribution of the different frequency values ​​by means of a Fourier transform of the sound signal and from this deriving information about the property of the fluid, wherein the individual frequencies of the frequency beam for modulation are very narrowband, in particular with a half-width below 3 Hz.

Description

TECHNICAL AREA

[0001] The invention relates to a device and a method for determining a property of a fluid using photoacoustic spectroscopy (PAS). STATE OF THE ART

[0002] The spectroscopic detection of molecular components of fluids, that is, gases or liquids, has long been a part of chemical analysis. Increasingly, spectroscopic detectors are being used for trace gas detection or for monitoring the concentration of substances harmful to health and the environment in many application environments.

[0003] Direct absorption spectroscopy is most commonly used for this purpose. The measured signal is the spectrally dependent attenuation of the electromagnetic radiation from a light source by the substance being analyzed. The power of the electromagnetic radiation falling onto a detector behind the sample is measured.

[0004] For the spectroscopic detection of molecular components of gases or liquids, photoacoustic measurement methods are also suitable in addition to absorption spectroscopy. These methods indirectly detect the attenuation of the electromagnetic excitation radiation by the sample. If the sample absorbs the electromagnetic radiation, a pressure wave is generated within the sample due to heating. This pressure or sound wave is detected using a suitable detection device. The amplitude, i.e., the loudness, serves, particularly in conjunction with the phase information of the detected sound wave, as a measure of the absorption of the electromagnetic excitation radiation by the sample. Photoacoustic measurement methods offer the advantage of a measurement signal approaching zero.If the sample does not absorb the electromagnetic radiation, the gas under investigation will not generate a sound wave, and no or only a weak background signal will be measurable. Furthermore, certain variations of photoacoustic measurement methods achieve extremely high sensitivities in the detection of trace gases.

[0005] The spectral information provided by a photoacoustic measurement method, in methods and devices known from the prior art, is given by a narrowband excitation radiation whose emission wavelength is tuned to the absorption properties of the substance to be detected. A specific excitation frequency of the excitation radiation is generally selectively sensitive to exactly one molecule. With a single, narrowband laser light source for generating the excitation radiation, it is usually only possible to detect a single species of molecule, or the narrowband radiation must be tuned. Furthermore, the required laser light sources represent a significant cost factor in the use of such photoacoustic detectors, particularly in the mid-infrared spectral range.

[0006] The absorption of electromagnetic excitation radiation generates a pressure wave or sound wave. Due to the geometric shape of the measuring cell and the speed of sound within it, this wave may experience frequency-dependent attenuation, potentially influenced by the distribution of the sound velocity within the cell. This allows for the determination of the sound velocity and, consequently, the properties of the fluid within the measuring cell, based on the detected sound signal and its spectral distribution, particularly regarding the formation of room modes (i.e., the positions of the occurring resonance frequencies). If the quality of the resonance can be maintained at a high level, a very informative picture of the fluid's properties can be obtained.

[0007] The quality of the resonance is helpful here, as the resulting resonance amplification is used to boost the acoustic signal. Typically, a selected resonance frequency of the measuring cell is used to modulate the excitation radiation, and this exact resonance frequency is then used to evaluate the sound signal, for example, using a lock-in technique. However, this has the disadvantage that if there is an unnoticed deviation from the resonance frequency, for example, due to a change in the speed of sound, the measured value, and thus the information at the expected resonance frequency, is subject to significant systematic errors due to the very high quality of the resonance. This results in poor and unreliable information about the properties of the fluid.

[0008] Photoacoustic sensors are known from German patent DE 10 2007 014 518 B3 and European patent application EP 4 019 938 A1. These sensors propose increasing efficiency by using reflectors on or in the cell to repeatedly pass the incident monochromatic excitation light through the cell, thereby increasing the signal strength. This also reduces the cost of a photoacoustic sensor.

[0009] European patent application EP 4 009 035 A1 discloses a photoacoustic sensor in which the excitation radiation with a continuous frequency band is generated using an incandescent lamp and is frequency-selectively directed to different areas of the measuring space by means of optical elements. Furthermore, the acoustic signals are detected and evaluated using a plurality of acoustic sensors assigned to individual detection positions in the measuring space. This ensures that different areas of the measuring space are always irradiated with only a single excitation frequency. The use of an incandescent lamp makes it possible to implement a cost-effective photoacoustic sensor.

[0010] Furthermore, various photoacoustic sensors are known from British patent application GB 2 271 181 A and from various articles, in particular from WILDI, Thibault [et al.]: Photo-acoustic dual-frequency comb spectroscopy. In: Nature communications, Vol. 11, 2020, Art.-No. 4164, 6 pp. - ISSN 2041-1723 and from KELLNBERGER, Stephan [et al.]: Optoacoustic microscopy at multiple discrete frequencies. In: Light: science & applications, Vol. 7, 2018, Art.-No. 109, 12 pp. - ISSN 2047-7538, which determine the properties of a sample gas using a photoacoustic method. DESCRIPTION OF THE INVENTION

[0011] The invention is based on the objective of providing a device and a method for determining a property of a measuring gas that are improved compared to the prior art.

[0012] The object is solved according to the invention with a device for determining a property of a measuring gas which has the features specified in claim 1.

[0013] The problem is further solved according to the invention by a method for determining a property of a measuring gas which has the features specified in claim 15.

[0014] Advantageous embodiments of the invention are the subject of the dependent claims.

[0015] The device according to the invention for determining a property of a fluid by means of photoacoustic spectroscopy comprises a housing that encloses a measuring chamber for receiving the fluid. It also includes a radiation source configured to modulate the electromagnetic excitation radiation for the fluid and to emit this radiation directly or indirectly into the measuring chamber containing the fluid. Furthermore, it includes a sound measurement device configured to detect sound waves generated by the fluid in the measuring chamber and to transmit them as an acoustic signal to an external device. The control unit controls the radiation source such that the excitation radiation is modulated with a frequency beam, the frequencies of which, to distinguish them from the excitation frequencies, are hereinafter also referred to as tones. The frequency beam is configured with several frequencies or tones spaced apart from one another.

[0016] The excitation radiation, modulated with several spaced frequencies or multiple tones, is introduced into the measuring chamber in such a way that at least the fluid in certain areas of the chamber is exposed to this modulated excitation radiation. Instead of a single frequency or tone with the presumed resonance frequency, a multi-frequency or multi-tone method with a frequency distribution or tone distribution around the resonance frequency is used for modulating the excitation frequency. The evaluation unit of the device is designed to evaluate the acquired sound signal from the sound measuring device by transforming the sound signal from the time domain to the frequency domain using a Fourier transform.In the frequency domain, the respective amplitudes of the various excited tones are determined, and information about the properties of the fluid is derived from this. The device according to the invention takes advantage of the fact that the excitation of the fluid is achieved using a modulated excitation radiation, which is preferably modulated around the fluid's usual resonance frequency in the measuring chamber with spaced-apart frequencies, i.e., using a frequency or multi-tone method. This ensures that excitation, and in particular an increased excitation of the fluid, is always achieved, and a distinct sound signal is generated, even if the ideal resonance frequency is not part of the frequency beam used for modulation.Shifts in the ideal resonant frequency of the fluid within the measuring chamber depend, among other things, on the fluid's composition, for example, on the degree of variation in humidity of a gaseous fluid, or on the fluid's temperature. Even when different fluids are mixed, it is possible to deduce the mixing ratio from the altered position of the resonant frequency. These shifts are no longer significantly affected by excitation with the frequency beam and by the specific evaluation using the Fourier transform. This allows for both the advantages of efficient sound signal generation through excitation radiation and the simple application or generation of the modulated excitation radiation. This enables a highly reliable determination of the desired fluid property.This design of the device for determining the properties of a fluid makes it possible to create a cost-effective device that can do without elaborate mechanisms for compensating for temperature fluctuations and fluctuations in the purity of the fluid.

[0017] The device for determining a property of a sample gas displays individual frequencies of the frequency beam for modulation, which are very narrowband, in particular with a half-width below 3 Hz, or are designed as monochrome and thus sinusoidal and overtone-free radiation. The narrowband design of the individual frequencies, especially in the form of discrete frequencies, makes it possible to create meaningful reference points in the Fourier-transformed acoustic signal and thereby improve the accuracy in determining the property of a fluid.

[0018] In the inventive method for determining a property by means of photoacoustic spectroscopy, a fluid located in a measuring chamber is excited by means of modulated electromagnetic excitation radiation, wherein the excitation radiation is modulated by means of a frequency beam with several spaced-apart frequencies. The sound waves generated by the excited fluid in the measuring chamber are detected as an acoustic signal by means of a sound measuring device, and the detected acoustic signal is evaluated by means of an evaluation device by subjecting the detected acoustic signal to a Fourier transform and subsequently determining the intensity distribution of the different frequency values.From the distribution of the amplitudes of the various frequency values, information about the properties of the fluid, in particular the fluid quantity (e.g., the hydrocarbon content in the ambient air), the fluid temperature, a mixing ratio, and / or, specifically, the moisture content of the fluid, can be determined. This method is carried out using a device according to the invention. This method enables a meaningful determination of the properties of a fluid using photoacoustic spectroscopy, and does so cost-effectively.

[0019] It has proven particularly advantageous to further develop the device according to the invention such that the evaluation unit is designed to determine the distribution of the various frequency values ​​using an adaptation function, for example, a bell curve. By using an adaptation function, especially a symmetrical adaptation such as a bell curve, it is possible to determine the position of the maximum amplitude of the Fourier-transformed acoustic frequencies and thereby obtain a meaningful statement about the property of the fluid, particularly the gas under investigation. By using the frequency beam for modulation in conjunction with the Fourier transform and evaluation using an adaptation function, especially a symmetrical adaptation function, a very successful determination of the properties can be achieved in a simple and cost-effective manner.

[0020] It has proven advantageous to further develop the device for determining a property of a fluid, particularly a measuring gas, such that the frequency beam for modulation is configured with equidistant individual frequencies. By using equidistant individual frequencies, it is possible to improve the quality of the evaluation with regard to the fluid's property in a simple and cost-effective manner.

[0021] In a further particularly preferred embodiment of the device for determining a property of a fluid or measuring gas, the frequency beam for modulation with individual frequencies is designed such that the individual frequencies are distributed around the expected resonance frequency of the sound signal in the measuring chamber. It has proven particularly advantageous to distribute the individual frequencies uniformly or symmetrically around the expected resonance frequency.This makes it possible to position individual frequencies both above and below the expected resonance frequency of the sound signal, which is at least approximately known due to the chosen measurement setup, and thereby ensure that sufficient and suitable support points are available after the Fourier transformation for the evaluation of the Fourier-transformed acoustic signals, in order to determine in a very reliable manner, in particular, the position of the maximum as a special measure of the properties of the fluid.

[0022] It has proven advantageous to further develop the device for determining a property of a fluid, particularly a measuring gas, such that the frequency beam for modulation is configured with a number of closely spaced individual frequencies in the range of 50 to 500. The number of individual frequencies used for modulation is preferably an integer power of 2, and thus, in particular, 32, 64, 128, 256, or 512. This specific number of individual frequencies has proven particularly effective because the Fourier transform and the subsequent evaluation using a Fast Fourier Transform algorithm can be performed with manageable computational effort, thus enabling a highly efficient and reliable determination of the fluid's property.

[0023] A particularly preferred embodiment of the invention shows a control device designed such that the individual frequencies of the frequency beam are modulated sequentially and / or simultaneously onto the excitation radiation for modulation and the modulated excitation radiation is radiated into the measuring space.

[0024] Simultaneously modulating the individual frequencies onto the excitation radiation while simultaneously irradiating the modulated excitation radiation leads to a faster determination of the desired property of the fluid, but is more complex and expensive in terms of the effort required for simultaneous modulation and irradiation of the modulated excitation radiation into the measuring space.

[0025] In contrast, the alternative of sequentially modulating individual frequencies onto the excitation radiation with subsequent injection, i.e., the temporally successive modulation and injection of the excitation radiation modulated with different individual frequencies into the measuring space, which requires a simpler device but necessitates summing the audio signals to a total audio signal and subsequent evaluation of the total audio signal, proves to be less advantageous, since the time expenditure is increased with a simpler device.

[0026] It is also possible to combine these two sequential and simultaneous concepts by sequentially applying a group of several individual frequencies used simultaneously for modulation. This leads to a compromise between the two concepts and thus to a faster and simpler overall system.

[0027] It has proven particularly effective to further develop the device for determining a fluid property by equipping it with a fluid supply to the measuring chamber, a fluid outlet from the chamber, and a control unit that regulates the supply and outlet to enable selective filling of the measuring chamber with the fluid. This selective filling allows the quantity of the fluid under investigation, particularly the gas being measured, to be precisely chosen. This ensures that the device, along with the associated application of frequency-band modulated excitation radiation and the corresponding specific evaluation, operates within the optimal signal range, particularly with regard to the quality of the acoustic sensor. This guarantees the high reliability of the measurement result.

[0028] A further, particularly preferred embodiment of the invention, especially of the device for determining a property of a fluid, particularly a measuring gas, comprises a radiation source comprising at least one laser light source and / or one LED light source. These radiation sources enable the irradiation of the radiation modulated according to the invention to excite the fluid or the measuring gas, resulting in highly informative data points in the Fourier-transformed acoustic signal and thus improving the quality of the determination of a fluid property.

[0029] A particularly preferred embodiment of the invention, especially of the device for determining a property of a fluid, particularly a measuring gas, comprises an evaluation unit configured to determine the fluid quantity or concentration from the amplitude of the distribution of the various frequency values. The amplitude, i.e., the maximum of the sampling points or the maximum of the adaptation function determined by the sampling points in the Fourier-transformed acoustic signal, represents the number of molecules of the fluid to be excited in the measuring chamber. This can be used as the basis for determining the fluid fraction in the measuring chamber or the density in the measuring chamber. The device according to the invention, or its embodiments, makes it possible to achieve a very informative result without the device or the method being overly complex or expensive.

[0030] It has proven particularly advantageous to further develop the device according to the invention such that the evaluation unit is designed to determine the temperature or humidity of the fluid from the position of the calculated maximum of the frequency distribution after the Fourier transform, i.e., the instantaneous resonance frequency at the time of measurement. Determining only one of these properties has proven particularly useful, as it is highly reliable and informative. Furthermore, it has also proven effective to infer the influence of temperature or humidity by varying the conditions, for example, by changing the temperature of the fluid in the measuring chamber or, alternatively, by changing the humidity, and thereby to determine the other property with particular reliability and informativeness by eliminating the measured influence.

[0031] Furthermore, it has proven particularly advantageous to further develop the device according to the invention for determining a property of a fluid, especially a measuring gas, such that the measuring chamber is selected such that its maximum dimensions depend on the sensitivity of the sound measuring arrangement, wherein the maximum dimensions are specifically chosen depending on the maximum sensitivity of the sound measuring arrangement. The shape and dimensions of the measuring chamber influence the resonance frequencies of the sound generated by the excitation of the fluid. This is specifically utilized by this further development, in that the resonance frequency of the sound of the fluid in the measuring chamber is selected such that the sound measuring device can convert the acoustic behavior of the fluid into a sound signal with high accuracy.This is preferably the case when the resonance frequency, chosen through the dimensions and shape, lies within the efficiency maximum of the sound measuring device. This allows even small changes in acoustic behavior due to the introduced fluid to be detected particularly effectively and successfully, thereby improving the accuracy of the determination of the fluid's properties.

[0032] Furthermore, it has proven particularly advantageous to further develop the inventive device for determining a property of a fluid, especially a measuring gas, by selecting a measuring chamber with a rotationally symmetrical shape, particularly cylindrical or spherical. This shape of the measuring chamber limits the number of possible resonance frequencies of the sound signal generated by the excited fluid within the chamber. This simplifies the evaluation and modulation of the excitation radiation with the frequency beam. It also allows for the acquisition of a more informative sound signal and thus more meaningful information about the fluid's property. This is further enhanced by limiting and reducing the number of possible resonances and by increasing the quality of the resonance due to the improved symmetry of the measuring chamber's shape.However, the inventive method or device does not lead to the problems known from the prior art with the pronounced drop in the significance of the sound signal when the expected resonance frequency is not precisely maintained by the measurement conditions, particularly in the case of a deviation due to changes in the fluid, for example, due to altered temperature or humidity, which is reflected in a changed speed of sound and thus in a changed resonance frequency of the sound signal.

[0033] In addition, it has proven particularly advantageous to further develop the inventive device for determining a property of a fluid, especially a measuring gas, such that the measuring chamber is selected so that its maximum dimensions are limited to ensure that the detected sound signal lies in the ultrasonic range. Besides the possibility of aligning the shape and dimensions of the measuring chamber with a view to maximizing the efficiency of the sound measuring device, it has also proven advantageous to implement this in such a way that the detected sound signal lies in the ultrasonic range and thus outside the audible frequency range. This prevents interference from a multitude of possible interfering signals and thereby enables a highly informative determination of the property of a fluid.This is especially true if the sound measuring device is chosen so that its maximum efficiency lies in the ultrasound range, thereby combining the two advantageous designs and achieving a particularly meaningful result.

[0034] It has proven particularly advantageous to further develop the device according to the invention for determining a property of a fluid, especially a measuring gas, in such a way that the control unit and the evaluation unit are designed in such a way that measurements are carried out at different pressures of the fluid and individual measurements are used to determine static disturbances and to correct the determination of a property of the fluid.For example, by changing the pressure of the fluid under investigation in the measuring chamber, it is possible to determine the static error component or static disturbances, such as those caused by the influence of the chamber walls on the sound signal. This is achieved, for instance, by extrapolating to the level of zero pressure or the absence of the fluid under investigation in the measuring chamber. This influence is then taken into account during the evaluation, particularly by subtracting the disturbance component generated independently of the pressure, such as a background signal, from the measured values. This further improves the reliability of the measurement result in determining the properties of a fluid.

[0035] A further, particularly preferred embodiment of the invention or of the device for determining a property of a fluid, especially a measuring gas, comprises a temperature compensation unit that compensates for changes in the temperature of the measuring gas in the measuring chamber. This can be achieved, for example, by a controlled temperature control unit that maintains the fluid or measuring gas in the measuring chamber at a desired temperature, or by appropriately taking the effects of temperature into account during the subsequent evaluation. Since many properties of the measuring gas depend on temperature, and especially on changes in temperature, the temperature compensation unit in the device according to the invention makes it possible to further improve the accuracy of the statements regarding the property of a fluid, especially the measuring gas.

[0036] The invention is explained below by way of example with reference to a preferred embodiment and the illustration. The invention is not limited to this preferred embodiment. Fig. Figure 1 shows a schematic representation of the structure of an exemplary device according to the invention for determining a property of a fluid, in particular a measuring gas, using photoacoustic spectroscopy (PAS). Fig. Figure 2 shows in a schematic, exemplary diagram the distribution of the individual frequencies of the frequency beam for the modulation of the excitation radiation. Fig. Figure 3 shows in a schematic, exemplary diagram the distribution of the individual frequencies of the frequency beam for the modulation of the excitation radiation, the distribution of the acoustic frequencies after the Fourier transformation of the detected sound signal and the distribution profile of these acoustic frequencies.

[0037] In Fig. Figure 1 schematically shows a device for determining a property of a fluid using photoacoustic spectroscopy (PAS), in particular a device for determining the residual moisture of a measuring gas.

[0038] The device 1 is provided with a housing 2 in which a measuring chamber 3 is arranged for receiving the fluid to be examined, in particular the measuring gas.

[0039] The measuring chamber 3 is equipped with a controllable supply line 10 for the controlled supply of the fluid, in particular the measuring gas, to the measuring chamber 3, and a controllable discharge line 11 for the controlled discharge of the fluid, in particular the measuring gas, from the measuring chamber 3.

[0040] Measuring chamber 3 has a rotationally symmetric shape, namely a cylindrical form. This cylindrical shape has a significant influence on the resonance properties of measuring chamber 3, because its shape and dimensions severely limit the multitude of possible acoustic resonance frequencies of the fluid under investigation within it. Furthermore, the remaining resonance frequencies, particularly the resonance frequency determined by the rotational symmetry, are characterized by a high quality factor. A high quality factor is associated with a favorable, high amplitude and thus high efficiency. However, with a high quality factor, deviations from the resonance frequency lead to a significant drop in the corresponding frequency value, thereby substantially impairing the reliability of the frequency value.The acoustic resonance frequencies are a result of the speed of sound of the fluid in the measuring chamber 3 and the mean free path lengths in the measuring chamber 3. Therefore, by choosing the shape and dimensions of the measuring chamber 3, it is possible to influence the acoustic resonance frequencies of the fluid in the measuring chamber 3. The relevant speed of sound of the fluid under investigation must be taken into account in each case, which depends, among other things, on the temperature, pressure, type of fluid, and the condition of the fluid (impurities, especially moisture).

[0041] The housing 2 with the measuring chamber 3 is connected to a radiation source 4 for generating modulated electromagnetic excitation radiation for the fluid. The radiation source 4 is arranged such that it emits the excitation radiation into the measuring chamber 3 for the fluid under investigation, in particular for the gas under investigation. The frequency of the emitted excitation radiation is selected such that it selectively excites the fluid under investigation, causing it to heat up. Due to the modulation and the duration of the excitation radiation, the fluid under investigation is heated significantly, resulting in the generation of a pressure or sound wave in the measuring chamber 3.

[0042] Furthermore, the housing 2 with the measuring chamber 3 is connected to a sound measuring device 5, which is suitable and intended to acoustically detect a pressure or sound wave formed in the measuring chamber 3 by the excited fluid and to supply it to an evaluation unit 6 in the form of a sound signal, in particular an electrical sound signal.

[0043] The evaluation unit 6 evaluates the sound signal acquired by the sound measuring device 5 by subjecting the sound signal to a Fourier transform. From this, the distribution of the sound signal's frequency values ​​is determined, and, using an adaptation function, the location of the maximum of the frequency distribution and its amplitude are extrapolated or determined. From the distribution, and especially the location of the maximum and its amplitude, various properties of the fluid, particularly the moisture content of the measuring gas in the measuring chamber 3, can be inferred.

[0044] The device 1 further includes a control unit 7, which controls the radiation source 4 via a control line 7a such that the excitation radiation is modulated using a frequency beam consisting of several spaced-apart individual frequencies. Via further control lines 7a, the control unit 7 controls the supply line 10 and the discharge line 11 by selectively opening or closing the respective associated control valve, thereby enabling a controlled supply or discharge of the fluid or the measuring gas from the measuring chamber 3. Furthermore, the control unit 7 can selectively control the sound measuring device 5 and the evaluation device 6, thereby influencing their function.

[0045] In Fig. Figure 2 shows a schematic diagram illustrating an exemplary distribution of the individual frequencies of a frequency beam used for modulating the excitation radiation. In the example shown, equidistant individual frequencies are realized in the frequency range between 1450 Hz and 1500 Hz, each with a spacing of a few Hertz. To determine the individual frequencies for modulating the excitation radiation using the radiation source 4, an expected resonance frequency was first determined based on the shape and dimensions of the measuring chamber 3 and knowledge of the fluid under investigation. The expected resonance frequency that promises the best quality factor and thus the best efficiency is preferably selected. In the present example, a resonance frequency of approximately 1475 Hz was calculated to determine the moisture content in toluene.

[0046] Based on the expected resonant frequency, the individual frequencies of the frequency beam for modulation were arranged symmetrically around it, so that the individual frequencies were chosen to be in the range between 1450 Hz and 1500 Hz. Accordingly, approximately the same number of individual frequencies lie below the expected resonant frequency as there are individual frequencies above it. This results in the generated sound signal, after the Fourier transform, exhibiting roughly the same number of points below the expected resonant frequency as above it. Therefore, determining the shape of the resonance curve using a matching function is particularly reliable and informative.

[0047] These individual frequencies of the frequency beam are translated into the time domain via an inverse Fourier transform with appropriate resolution, mathematically generating the signal of a beat frequency mixture, which in spectroscopy is called an interferogram. This interferogram represents all individual frequencies of the frequency beam and is modulated onto the excitation radiation in the radiation source 4 using a modulator. This superimposes all individual frequencies of the frequency beam simultaneously by modulating the excitation radiation. The modulated excitation radiation is then introduced into the measuring chamber 3 of the device 1, where it is directed to the fluid under investigation, in particular the sample gas. The excitation radiation excites the molecules of the fluid or sample gas.Depending on the geometric dimensions or shape of the measuring cell 3, as well as the temperature or moisture content of the fluid in the measuring chamber 3, the amplitudes of the respective frequencies of the resulting acoustic signal in the measuring chamber 3 are selectively determined. The acoustic resonance profile of the measuring chamber 3 and the properties of the fluid within it are crucial for the resulting frequencies of the acoustic signal, which is generated by the excited fluid and detected by the sound measuring device 5. The sound measuring device 5 detects all frequencies of the fluid's acoustic signal simultaneously, at least insofar as these frequencies lie within the detection range of the sound measuring device.It proves particularly advantageous that the detection of the frequencies of the acoustic signal by the sound measuring device 5 can take place simultaneously with the irradiation of the modulated excitation radiation.

[0048] The captured sound signal is transformed into the frequency domain using the evaluation unit 6 via a Fourier transform. Since the excitation radiation is modulated with specific individual frequencies, the Fourier transform into the frequency domain yields individual amplitude values ​​corresponding to these frequencies, which are expressed as frequency values ​​in Fig. Figure 3 illustrates this. Each of these amplitude values ​​of the individual frequencies of the acoustic signal is assigned the corresponding individual frequency for modulating the excitation radiation. It is evident that with a larger frequency difference from the expected or actual resonance frequency, the amplitude value of the acoustic signal drops significantly, which represents a measure of the quality factor of the resonance response. Furthermore, by determining the half-width or a comparable property of the matching profile, the quality factor of the resonance can be inferred from the measurement, and a change in the quality factor can, for example, indicate potential contamination of the cell.

[0049] Since the typical shape of a resonance curve is known, it is possible to use an approximation function to determine the shape of the resonance curve based on the different amplitude values ​​for the individual frequencies of the acoustic signal and, based on this shape, to calculate the amplitude and, in particular, the resonance frequency of the amplitude value. This amplitude, or rather the calculated resonance frequency, forms the basis for determining various properties of the fluid or the sample gas.

[0050] For example, the amount of fluid in the measuring chamber, the proportion of the measuring gas in a fluid in the measuring chamber, the water content in a fluid in the measuring chamber (e.g., in toluene in the measuring chamber), or the temperature or density changes of the fluid in the measuring chamber can be determined from this calculated information using appropriate calibration tables.

[0051] It has proven particularly useful to use a typical bell curve or an adjustment according to the Levenberg-Marquardt method as an approximation function.

[0052] The in Fig. Figure 3 of the resonance curve was determined using the Levenberg-Marquardt method. In this case, an amplitude of 0.84 Hz was obtained at a resonance frequency of 1479.5 Hz. These values, for example, correspond to a concentration of 3.7 ppm (vol.) of toluene in air.

[0053] Since the amplitude or the resonance frequency is determined mathematically using an approximation function, without requiring modulation at exactly the resonance frequency, this invention provides a device 1 and a method for determining a property of a fluid using photoacoustic spectroscopy (PAS) that prove to be particularly simple and also very informative. In particular, continuous monitoring of the actual position of the resonance frequency is unnecessary, as is adjusting the modulation frequencies or the modulation frequency to changing fluid conditions, thus greatly simplifying the process of determining a fluid property. Reference symbol list 1 Device for determining a property of a fluid, in particular a measuring gas, using photoacoustic spectroscopy (PAS) 2 Housing of the device 3 Measuring chamber for the gas to be measured 4 Radiation source 5 Sound measuring device 6 Evaluation unit 7 Control unit 7a Control line 10 Supply line for the fluid into the measuring chamber 11. Drainage for the fluid from the measuring chamber

Claims

[1] Device for determining a property of a fluid (1) by means of photoacoustic spectroscopy (PAS) comprising a housing (2) enclosing a measuring chamber (3) for receiving the fluid, with a radiation source (4) designed in such a way that it modulates electromagnetic excitation radiation for the fluid and shines into the measuring chamber (3) containing the fluid, with a sound measuring device (5) which is designed in such a way that it can detect sound waves generated by the fluid in the measuring chamber (3), with a control device (7) that controls the radiation source (4) in such a way that the excitation radiation is modulated with a frequency beam with several spaced-apart individual frequencies, and with an evaluation device (6) that is designed to evaluate a detected sound signal from the sound measuring device (5) by determining the distribution of the different frequency values ​​by means of a Fourier transform of the sound signal and from this deriving information about the property of the fluid, wherein the individual frequencies of the frequency beam for modulation are very narrowband, in particular with a half-width below 3 Hz. [2] Device for determining a property of a fluid (1) according to claim 1, wherein the evaluation device (6) is designed such that the distribution of the different frequency values ​​is determined by means of an adjustment function, for example by means of a bell curve. [3] Device for determining a property of a fluid (1) according to one of claims 1 to 2, wherein the frequency beam is designed for modulation with equidistant individual frequencies. [4] Device for determining a property of a fluid (1) according to one of claims 1 to 3, wherein the frequency beam is formed with individual frequencies which are distributed around the expected resonance frequency of the sound signal, in particular uniformly or symmetrically around the expected resonance frequency. [5] Device for determining a property of a fluid (1) according to one of claims 1 to 4, wherein the frequency beam for modulation is designed with 50 to 500 individual frequencies, in particular with an integer power of two individual frequencies. [6] Device for determining a property of a fluid (1) according to one of claims 1 to 5, wherein the control device (7) is designed such that the individual frequencies of the frequency bundle are modulated sequentially and / or simultaneously for modulation. [7] Device for determining a property of a fluid (1) according to one of claims 1 to 6, wherein the device is provided with a supply line (10) for the fluid to the measuring chamber (3) and with a discharge (11) for the fluid from the measuring chamber (3), wherein the control device (7) controls the supply line (10) and discharge (11) in such a way that selective filling of the measuring chamber (3) with the fluid is enabled. [8] Device for determining a property of a fluid (1) according to any one of claims 1 to 7, wherein the radiation source (4) comprises at least a laser light source and / or an LED light source. [9] Device for determining a property of a fluid (1) according to one of claims 1 to 8, wherein the evaluation device (6) is designed such that it determines the amount of fluid from the amplitudes of the distribution of the different frequency values. [10] Device for determining a property of a fluid (1) according to one of claims 1 to 9, wherein the evaluation device (6) is designed such that it determines the temperature, density, or moisture content of the fluid from the position of the maximum of the distribution of the different frequency values. [11] Device for determining a property of a fluid (1) according to one of claims 1 to 10, wherein the measuring space (3) is selected such that its maximum extent is chosen depending on the sensitivity of the sound measuring device (5), wherein the maximum extent is in particular chosen depending on the maximum sensitivity of the sound measuring device (5). [12] Device for determining a property of a fluid (1) according to one of claims 1 to 11, wherein the measuring chamber (3) is selected such that its shape is rotationally symmetrical, in particular cylindrical or spherical. [13] Device for determining a property of a fluid (1) according to one of claims 1 to 12, wherein the measuring chamber (3) is selected such that its maximum extent is limited so that the detected sound signal is in the ultrasonic range. [14] Device for determining a property of a fluid (1) according to one of claims 1 to 13, wherein the control device (7) and the evaluation device (6) are designed such that measurements are carried out at different pressures of the fluid in the measuring chamber (3) and individual measurements of these measurements can be used to determine pressure-independent disturbances and to correct the determination of a property of the fluid. [15] Method for determining a property of a fluid by means of photoacoustic spectroscopy (PAS) using a device for determining a property of a fluid (1) according to any one of claims 1 to 14, wherein the fluid located in the measuring chamber (3) is excited by means of the radiation source (4) by means of modulated electromagnetic excitation radiation for the fluid, wherein the excitation radiation is modulated by means of the frequency beam with several spaced-apart individual frequencies, wherein the sound waves generated by the fluid in the measuring chamber (3) are detected as a sound signal by means of the sound measuring device (5). and wherein the detected sound signal is evaluated by means of the evaluation device (6), by subjecting the captured sound signal to the Fourier transformation, The distribution of the different frequencies is then determined. and from this information about the properties of the fluid, in particular the amount of fluid, the temperature, the density of the fluid and / or the moisture content in the fluid, is determined.

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