Device and method for determining a property of a fluid

Abstract

The invention relates to a device (1) and to a method for determining a property of a fluid using photoacoustic spectroscopy (PAS). The device (1) comprises a housing (2) which encloses a measuring chamber (3) for receiving the fluid, a radiation source (4) which is designed to generate a modulated electromagnetic excitation radiation for the fluid and emit same into the measuring chamber (3) comprising the fluid; a sound measuring device (5) which is designed in such a way that the sound measuring device can detect sound waves generated by the fluid in the measuring chamber (3); a control device (7) which controls the radiation source (4) in such a way that the excitation radiation is modulated with a frequency bundle having a plurality of narrow-band individual frequencies which are spaced apart from one another; and an evaluation device (6) which is designed to evaluate the detected sound signal of the sound measuring device (5) by determining the distribution of the various frequency values using a Fourier transformation of the signal and determining information about the property of the fluid therefrom. The device allows a very reliable conclusion to be made about the desired fluid property to be determined. The device (1) can obviate complex mechanisms for compensating for temperature fluctuations and fluctuations in the purity of the fluid and is therefore very cost-effective.

Description

[0001] Device and method for determining a property of a fluid

[0002] TECHNICAL FIELD

[0003] The invention relates to a device and a method for determining a property of a fluid by means of photoacoustic spectroscopy ( PAS ) .

[0004] STATE OF THE ART

[0005] The spectroscopic detection of molecular components of fluids, i.e., gases or liquids, has long been a part of chemical analysis. Spectroscopic detectors are increasingly being used for trace gas detection or for monitoring the concentration of substances harmful to health and the environment in many application settings.

[0006] Direct absorption spectroscopy is usually used for this purpose. The measured signal is the spectrally dependent attenuation of the electromagnetic radiation from a light source by the substance to be detected. The power of the electromagnetic radiation which falls on a detector behind the sample is measured. In addition to absorption spectroscopy, photoacoustic measuring methods are also generally suitable for the spectroscopic detection of molecular components of gases or liquids. In these methods, the attenuation of the electromagnetic excitation radiation by the sample is indirectly recorded. If the sample absorbs the electromagnetic radiation, a pressure wave is generated in the sample due to the heating of the sample. This pressure or sound wave is recorded using a suitable detection device.The amplitude, i.e. the volume, 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 measuring techniques offer the advantage of a measurement signal close to zero. If the sample does not absorb the electromagnetic radiation, no sound wave is generated by the gas under investigation, and no or only a low background signal can be measured. Furthermore, certain variants of the photoacoustic measuring techniques achieve extreme sensitivity in the detection of trace gases.

[0007] The spectral information provided by a photoacoustic measurement method is, in the methods and devices known from the state of the art, provided by 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, generally only a single species of molecule can be detected, or the narrowband radiation must be tuned. The required laser light sources also represent a considerable cost factor when using such photoacoustic detectors, particularly in the mid-infrared spectral range.

[0008] The absorption of the electromagnetic excitation radiation creates a pressure wave or sound wave which, due to the geometric shape of the measuring cell and the speed of sound in the measuring cell and, if applicable, the distribution of the speed of sound in the measuring cell, undergoes frequency-dependent differentiated attenuation. This means that, based on the recorded sound signal and its spectral distribution, particularly with regard to the formation of room modes, i.e. the positions of the occurring resonance frequencies, a statement can be made about the speed of sound and, derived from this, a statement about the properties of the fluid in the measuring cell. If the quality of the resonance can be kept high, it is generally possible to obtain very meaningful information about the properties of the fluid.

[0009] The quality of the resonance is helpful here because the resulting resonance enhancement is used to amplify the acoustic signal. For this purpose, a selected resonance frequency of the measuring cell is typically used to modulate the excitation radiation, and precisely this resonance frequency is used to evaluate the sound signal, which is done using a lock-in method, for example. However, this has the disadvantage that in the event of an unnoticed deviation from the resonance frequency, e.g. due to a change in the speed of sound, the measured value and thus the information at the expected resonance frequency is subject to very strong systematic errors due to the very high quality of the resonance, and thus poor, unreliable information about the properties of the fluid is obtained.Photoacoustic sensors are known from German patent DE 10 2007 014 518 B3 and European patent application EP 4 019 938 A1. To increase efficiency, they propose using reflectors on or in the cell to repeatedly guide the irradiated, monochromatic excitation light through the cell, thereby increasing the signal strength. This also allows the cost of a photoacoustic sensor to be reduced.

[0010] European patent application EP 4 009 035 A1 discloses a photoacoustic sensor in which the excitation radiation is generated with a continuous frequency band using an incandescent lamp and is frequency-selectively delivered to different areas of the measuring space using optical elements. Furthermore, the acoustic signals are recorded and evaluated using a plurality of acoustic sensors assigned to individual detection positions in the measuring space. Thus, 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 realize a cost-effective photoacoustic sensor.

[0011] Furthermore, various photoacoustic sensors that determine the properties of a measuring gas using a photoacoustic method 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 communicatios , 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. DESCRIPTION OF THE INVENTION

[0012] The invention is based on the object of specifying a device and a method for determining a property of a measuring gas which is improved compared to the prior art.

[0013] The object is achieved according to the invention with a device for determining a property of a measuring gas, which device has the features specified in claim 1.

[0014] The object is further achieved according to the invention with a method for determining a property of a measuring gas, which has the features specified in claim 15.

[0015] Advantageous embodiments of the invention are the subject of the dependent claims.

[0016] The device according to the invention for determining a property of a fluid by means of photoacoustic spectroscopy has a housing that encloses a measuring chamber for containing the fluid. In addition, it contains a radiation source that is designed such that it generates modulated electromagnetic excitation radiation for the fluid and radiates it directly or indirectly into the measuring chamber containing the fluid. It also contains a sound measuring device that is designed such that it can detect sound waves generated by the fluid in the measuring chamber and transmit them to an external device as a sound signal. The control device controls the radiation source such that the excitation radiation is modulated with a frequency beam, the frequencies of which are also called tones below to distinguish them from the excitation frequencies.The frequency bundle is formed with several frequencies or tones spaced apart from one another.

[0017] The excitation radiation modulated with a plurality of spaced-apart frequencies or in multiple tones is fed into the measuring chamber in such a way that at least the fluid in individual areas of the measuring chamber is exposed to the excitation radiation modulated in this way. For the modulation, instead of a single frequency or a single tone with the presumed resonance frequency, a multi-frequency method or multi-tone method with a frequency distribution or a tone distribution around the resonance frequency is used to modulate the excitation frequency. The evaluation device of the apparatus is designed in such a way that it can evaluate the recorded sound signal from the sound measuring device by transforming the sound signal from the time domain into the frequency domain using a Fourier transformation.In the frequency domain, the respective amplitudes for the distribution 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 occurs with the aid of modulated excitation radiation, which is modulated at spaced frequencies, preferably around the usual resonance frequency of the fluid in the measuring space, i.e. a multi-frequency or multi-tone method. This always achieves excitation, in particular excessive excitation of the fluid, and generates a pronounced sound signal, even if the ideal resonance frequency is not part of the frequency beam for the modulation.Shifts in the ideal resonance frequency of the fluid in the measuring space depend, among other things, on the composition of the fluid, for example, the degree of difference in humidity of a gaseous fluid, or the temperature of the fluid. Even when different fluids are mixed, it is possible to draw conclusions about the mixing ratio from the changed position of the resonance frequency. These shifts no longer have a serious effect due to excitation with the frequency beam and specific evaluation using Fourier transformation, thus enabling the advantage of efficient sound signal generation by the excitation radiation on the one hand and simple irradiation or generation of the modulated excitation radiation on the other. This enables very reliable information to be obtained about the desired property of the fluid to be determined.This design of the device for determining the properties of a fluid makes it possible to create a cost-effective device that does not require complex mechanisms for compensating for temperature fluctuations and fluctuations in the purity of the fluid.

[0018] The device for determining a property of a measuring gas exhibits individual frequencies of the frequency beam for modulation, which are very narrowband, in particular with a half-width of less than 3 Hz, or are designed as monochrome and thus sinusoidal and overtone-free radiation. The narrowband design of the individual frequencies, particularly in the form of discrete individual frequencies, makes it possible to create meaningful support points in the Fourier-transformed acoustic signal and thus improve the significance when determining the property of a fluid.

[0019] In the inventive method for determining a property using photoacoustic spectroscopy, a fluid located in a measuring chamber is excited by means of modulated, electromagnetic excitation radiation for the fluid, wherein the modulation of the excitation radiation occurs by means of a frequency beam with several spaced-apart frequencies. By means of a sound measuring device, the sound waves generated by the excited fluid in the measuring chamber are recorded as a sound signal, and the recorded sound signal is evaluated by means of an evaluation device in which the recorded sound signal is subjected to a Fourier transformation and the intensity distribution of the various frequency values ​​is subsequently determined.From the distribution of the amplitudes of the various frequency values, information about the properties of the fluid—in particular the amount of fluid, for example the hydrocarbon content in the ambient air, the temperature of the fluid, a mixing ratio, and / or specifically, for example, the humidity in the fluid—can then 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 this is achieved at a low cost.

[0020] It has proven particularly useful to further develop the device according to the invention such that the evaluation unit is designed such that the distribution of the various frequency values ​​is determined by means of an adaptation function, for example by means of a bell curve. Thus, the further evaluation to determine the distribution of the various frequency values ​​is carried out in the evaluation unit on the basis of the used and calculated adaptation function, so that intermediate values ​​of the measured frequency values ​​can also be taken into account in the evaluation, thereby achieving improved informative value for the device for determining a property of a fluid by means of photoacoustic spectroscopy.By using a fitting function, in particular a symmetrical fitting such as a bell curve, it is possible to determine the position of the maximum of the amplitudes of the Fourier-transformed acoustic frequencies and from this to obtain meaningful information about the property of the fluid to be determined, in particular of the gas to be examined. Especially in the area of ​​the maximum of the amplitudes, the measured amplitude values ​​​​are very different and, without the use of an appropriate fitting function, lead to poorer results when determining a property of the fluid. It is precisely by using such a fitting function, in particular a bell-shaped fitting function, that it is possible to achieve greater sensing accuracy with the help of the device, which is regularly accompanied by an improvement and thus a lowering of the detection limit.In addition, it is possible to significantly reduce the costs of a device for determining a property by means of photoacoustic spectroscopy with a comparable accuracy or a comparable detection limit.

[0021] By using the frequency beam for the modulation in conjunction with the Fourier transformation and the evaluation by means of a matching function, in particular the symmetric matching function, it is possible to achieve a very successful determination of the properties in a simple and cost-effective manner.

[0022] Accordingly, it has proven useful to further develop the device for determining a property of a fluid, in particular a measurement gas, such that the frequency beam is designed for modulation with equidistant individual frequencies. By providing equidistant individual frequencies, it is possible to improve the quality of the evaluation with regard to the property of the fluid in a simple and cost-effective manner.

[0023] In a further particularly preferred embodiment of the device for determining a property of a fluid or measurement gas, the frequency beam for modulation with individual frequencies is configured such that the individual frequencies are distributed around the expected resonance frequency of the sound signal in the measurement space. It has proven particularly advantageous to distribute the individual frequencies evenly 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 selected measuring arrangement, and thus to ensure that after the Fourier transformation, sufficient and suitable support points are available for the evaluation of the Fourier-transformed acoustic signals, in particular in order to determine the position of the maximum as a special measure for the properties of the fluid in a very reliable manner.

[0024] It has proven useful to further develop the device for determining a property of a fluid, in particular a measuring gas, in such a way that the frequency bundle for the modulation is designed with a number in the range of 50 to 500 closely spaced individual frequencies. The number of individual frequencies used for the modulation is preferably chosen to be an integer power of 2 and thus in particular to be 32, 64, 128, 256 or 512. This selected number of individual frequencies has proven particularly useful because the Fourier transformation and the subsequent evaluation using a fast Fourier transformation algorithm are carried out with manageable computational effort and a meaningful and reliable determination of the property of the fluid can therefore be achieved in a very computationally efficient manner.

[0025] A particularly preferred development of the invention shows a control device which is designed such that the individual frequencies of the frequency bundle for the modulation are modulated sequentially and / or simultaneously onto the excitation radiation and the modulated excitation radiation is radiated into the measuring space.

[0026] The simultaneous modulation of the individual frequencies onto the excitation radiation with the irradiation of 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 the simultaneous modulation and irradiation of the modulated excitation radiation into the measuring room.

[0027] In contrast, the alternative of sequentially modulating individual frequencies onto the excitation radiation with subsequent irradiation, i.e. sequentially modulating and irradiating the excitation radiation modulated with different individual frequencies into the measuring room, which requires a simpler device but which requires summing the audio signals to form an overall audio signal and subsequent evaluation of the overall audio signal, proves to be less advantageous since the time required is increased with a simpler device.

[0028] It is also possible to combine these two sequential and simultaneous concepts by sequentially applying a group of several individual frequencies simultaneously for modulation. This results in a compromise between the two concepts and thus in a faster and simpler overall system.

[0029] It has proven particularly useful to develop the device for determining a property of a fluid in such a way that the device is provided with a supply for the fluid to the measuring chamber, with a discharge for the fluid from the measuring chamber, and with a control device which controls the supply and discharge in such a way that a selective filling of the measuring chamber with the fluid is possible. This selective filling allows the quantity of fluid to be examined, in particular measuring gas, to be selected in a targeted manner, so that the device is guided with the associated irradiation of the excitation radiation modulated with a frequency bundle, with associated specific evaluation, in the optimum signal range, in particular with regard to the quality of the acoustic sensor. This ensures that the measuring result is particularly meaningful.

[0030] A further, particularly preferred development of the invention, in particular of the device for determining a property of a fluid, in particular a measurement gas, features a radiation source that has at least one laser light source and / or one LED light source. These radiation sources in particular enable the radiation modulated according to the invention to be irradiated to excite the fluid or the measurement gas, which leads to highly meaningful support points in the Fourier-transformed acoustic signal and thus increases the quality of the determination of a property of a fluid.

[0031] A particularly preferred development of the invention, in particular of the device for determining a property of a fluid, in particular a measuring gas, shows an evaluation device which is designed in such a way that it determines the amount of fluid or the concentration of the fluid from the amplitude of the distribution of the various frequency values. The amplitude, i.e. the maximum of the support points or the maximum of the adaptation function determined by the support points in the Fourier-transformed acoustic signal, represents the number of molecules of the fluid to be excited in the measuring chamber, which can be used as a basis for determining the proportion of fluid in the measuring space or the density in the measuring space. The device according to the invention and its developments make it possible to achieve a very meaningful result without the device or the method being very complex and expensive.

[0032] It has proven particularly useful to develop the device according to the invention in such a way that the evaluation device is designed in such a way that it determines the temperature or the moisture content of the fluid from the position of the calculated maximum of the distribution of the frequency values ​​after the Fourier transformation, i.e. the instantaneous resonance frequency at the time of the measurement. It has proven particularly useful to determine a single one of these properties, which is very reliable and very meaningful. In addition, it has also proven useful to draw conclusions about the influence of the temperature or the humidity by varying the conditions, for example by changing the temperature of the fluid in the measuring space or alternatively by changing the humidity, and in this way to determine the other property particularly reliably and meaningfully by eliminating the detected influence.

[0033] Furthermore, it has proven particularly useful to further develop the device according to the invention for determining a property of a fluid, in particular a measuring gas, in such a way that the measuring chamber is selected such that its maximum extent is selected as a function of the sensitivity of the sound measuring arrangement, wherein the maximum extent is selected in particular as a function of the maximum sensitivity of the sound measuring arrangement. The device for determining a property of a fluid by means of photoacoustic spectroscopy is thus selected such that the measuring chamber with its maximum extent is selected as a function of the sensitivity of the sound measuring arrangement or vice versa.The shape and dimensions of the measuring chamber influence the resonance frequencies of the sound generated by the excitation of the fluid. This is exploited in a targeted manner by this further development, in that the resonance frequency of the sound of the fluid in the measuring chamber is selected so that the sound measuring device can convert the acoustic behavior of the fluid into a sound signal in a very meaningful way. This is preferably always the case when the resonance frequency selected by the dimensions and shape lies in the range of the efficiency maximum of the sound measuring device. This means that even small changes in the acoustic behavior due to the introduced fluid can be recorded particularly effectively and successfully, thereby increasing the quality of the determination of the properties of the fluid.Furthermore, it has proven particularly useful to further develop the device according to the invention for determining a property of a fluid, in particular a measuring gas, in such a way that the measuring chamber is selected such that its shape is rotationally symmetrical, in particular cylindrical or spherical. By shaping the measuring chamber in this way, the number of possible resonance frequencies of the sound signal generated by the excited fluid in the measuring chamber can be limited. This can simplify the evaluation and modulation of the excitation radiation with the frequency beam. It is also possible to obtain a more meaningful sound signal and thus more meaningful information about the property of the fluid. This is supported in particular by the fact that the number of possible resonances is limited and thus reduced and, on the other hand, the quality of the resonance is increased by the increased symmetry of the shape of the measuring chamber.However, the method according to the invention and the device according to the invention do not lead to the problems known from the prior art with the pronounced drop in the significance of the sound signal if the expected resonance frequency is not exactly maintained by the measuring conditions, in particular in the case of a deviation due to changes in the fluid, for example due to changes in temperature or humidity, which is reflected in a changed speed of sound and thus in a changed resonance frequency of the sound signal.

[0034] In addition, it has proven particularly useful to further develop the device according to the invention for determining a property of a fluid, in particular a measuring gas, in such a way that the measuring chamber is selected such that its maximum extent is limited so that the recorded sound signal lies in the ultrasonic range. In addition to the possibility of aligning the shape and dimensions of the measuring chamber with a view to the maximum efficiency of the sound measuring device, it has also proven useful to implement this in such a way that the recorded sound signal lies in the ultrasonic range and thus outside the audible frequency range, which prevents influence from a large number of possible interference signals and thus enables a very meaningful determination of the property of a fluid.This is all the more true if the sound measuring device is selected in such a way that its maximum efficiency is in the ultrasonic range, thus combining the two advantageous designs and thus achieving a particularly meaningful result.

[0035] It has proven particularly useful to further develop the device according to the invention for determining a property of a fluid, in particular a measuring gas, in such a way that the control device and the evaluation device 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 obtain information about the static error component or the static disturbances, for example due to the influence of the walls of the measuring chamber on the sound signal. This can be done by extrapolating to the level of zero pressure or of the non-existent fluid under investigation in the measuring chamber and then taking this influence into account in the evaluation, in particular by subtracting the disturbance component generated independently of the pressure, for example a background signal, from the measured values. This makes it possible to further improve the significance of the measurement result in determining the properties of a fluid.

[0036] A further, particularly preferred development of the invention or of the device for determining a property of a fluid, in particular a measuring gas, shows a temperature compensation unit which compensates for a change in the temperature of the measuring gas in the measuring chamber. This can be done, for example, by a regulated temperature control unit which keeps the fluid or the measuring gas in the measuring chamber at a desired temperature or by appropriate subsequent consideration of the influences of the temperature in the context of the evaluation. Since many properties of the measuring gas are dependent on the temperature, in particular on changes in temperature, the temperature compensation unit in the device according to the invention makes it possible to further increase the quality of the statements about the properties of a fluid, in particular the measuring gas.

[0037] The invention is explained below using a preferred embodiment with reference to the figure. The invention is not limited to this preferred embodiment.

[0038] Fig . 1 shows a schematic representation of the

[0039] Structure of an exemplary device according to the invention for determining a property of a fluid, in particular a measuring gas, by means of photoacoustic spectroscopy (PAS), Fig. 2 shows in a schematic, exemplary

[0040] Diagram showing the distribution of the individual frequencies of the frequency beam for the modulation of the excitation radiation,

[0041] Fig. 3 shows in a schematic, exemplary

[0042] Diagram showing 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 recorded sound signal and the distribution profile of these acoustic frequencies.

[0043] Fig. 1 schematically shows a device for determining a property of a fluid by means of photoacoustic spectroscopy (PAS) 1, in particular a device for determining the residual moisture of a measuring gas 1.

[0044] 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.

[0045] The measuring chamber 3 is provided 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.

[0046] The measuring chamber 3 has a rotationally symmetrical shape, namely a cylindrical shape. This cylindrical shape has a significant influence on the resonance properties of the measuring chamber 3, because the shape and dimensions of this cylindrical measuring chamber 3 lead to a severe limitation of the number of possible acoustic resonance frequencies of a fluid to be examined in the measuring chamber 3.

[0047] In addition, the remaining resonance frequencies, in particular the resonance frequency caused by the rotational symmetry, are characterized by a high quality factor. On the one hand, a high quality factor goes hand in hand with an advantageous, high amplitude and thus high efficiency. On the other hand, with a high quality factor, a deviation from the resonance frequency leads to an enormous drop in the associated frequency value and thus to a significant deterioration in the meaningfulness 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 free paths in the measuring chamber 3. This is why the choice of shape and size of the measuring chamber 3 makes it possible to influence the acoustic resonance frequencies of the fluid in the measuring chamber 3.In each case, the relevant speed of sound of the fluid to be examined must be taken into account, which depends, among other things, on the temperature, the pressure, the type of fluid and the condition of the fluid (contamination, especially due to moisture).

[0048] The housing 2 with the measuring chamber 3 is connected to a radiation source 4 for generating modulated, electromagnetic excitation radiation for the fluid, wherein the radiation source 4 is arranged such that it radiates the excitation radiation into the measuring chamber 3 for the fluid to be examined, in particular for the measuring gas to be examined. The frequency of the radiated excitation radiation is selected such that it selectively excites the fluid to be examined and leads to heating of the fluid. Due to the modulation and the duration of the irradiation of the excitation radiation, the fluid to be examined is heated considerably, which leads to the creation of a pressure or sound wave in the measuring chamber 3.

[0049] The housing 2 with the measuring chamber 3 is furthermore 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.

[0050] The evaluation device 6 evaluates the sound signal detected by the sound measuring device 5 by subjecting the sound signal to a Fourier transformation, from which the distribution of the frequency values ​​of the sound signal is determined, and from this, with the aid of an adaptation function, in particular the position of the maximum of the distribution of the frequency values ​​and the amplitude of the frequency distribution are extrapolated or determined. It is precisely by using the adaptation function that the distribution or the position of the maximum of the frequencies outside or next to the concrete measured support points can be determined, and in this way much more meaningful information on the properties of the fluid can be obtained. In this case, conclusions can be drawn about various properties of the fluid, in particular the moisture content of the measuring gas in the measuring chamber 3, from the distribution, in particular the position of the maximum or its amplitude.

[0051] The device 1 further contains a control device 7, which controls the radiation source 4 via a control line 7a in such a way that the excitation radiation is modulated using a frequency beam comprising a plurality of spaced-apart individual frequencies. Via further control lines 7a, the control device 7 controls the supply line 10 and the discharge line 11 by opening or closing the respective associated control valve in a targeted manner, thereby enabling the fluid or the measuring gas to be fed into or discharged from the measuring chamber 3 in a targeted manner. Furthermore, the control device 7 can control the sound measuring device 5 or the evaluation device 6 in a targeted manner and thereby influence their function.

[0052] Fig. 2 shows a schematic diagram of an example distribution of the individual frequencies of a frequency beam that is used to modulate the excitation radiation. In the example shown, equidistant individual frequencies are realized in the frequency range between 1450 Hz and 1500 Hz, each of which is spaced apart by a few Hertz. In determining 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 with knowledge of the fluid to be tested. Preferably, the expected resonance frequency is selected that is expected to have the best quality and thus the best efficiency. In the present example, a resonance frequency of approximately 1475 Hz was calculated to determine the moisture content in toluene.

[0053] In accordance with the expected resonance frequency, the individual frequencies of the frequency beam for modulation were arranged symmetrically around this, so that the individual frequencies were selected in the range between 1450 Hz and 1500 Hz. Accordingly, there are approximately the same number of individual frequencies below the expected resonance frequency as there are individual frequencies above this expected resonance frequency. This means that the generated sound signal, after the Fourier transformation, has approximately the same number of support points below the expected resonance frequency as above, and therefore the determination of the course of the resonance curve using an adaptation function is particularly reliable and meaningful.

[0054] Using an inverse Fourier transformation with appropriate resolution, these individual frequencies of the frequency beam are translated into the time domain, mathematically creating the signal of a beat of a frequency mixture, which in spectroscopy is referred to as 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. As a result, all individual frequencies of the frequency beam are simultaneously superimposed by modulation of the excitation radiation. The modulated excitation radiation is then fed to the fluid to be examined, in particular the measurement gas, in the measurement chamber 3 of the device 1. The molecules of the fluid or the measurement gas are excited by the excitation radiation.Depending on the geometric extent or the shape of the measuring cell 3 and as a function of the temperature of the fluid or the 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 adjusted. The acoustic resonance profile of the measuring chamber 3 and the properties of the fluid in the measuring chamber 3 are decisive 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 acoustic signal of the fluid simultaneously, at least as long as these frequencies lie within the reception range of the sound measuring device.It proves to be 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.

[0055] The recorded sound signal is transformed into the frequency domain by means of a Fourier transformation with the aid of the evaluation device 6. Since the modulation of the excitation radiation has taken place with the specific individual frequencies, the Fourier transformation into the frequency domain results in individual amplitude values ​​for the individual frequencies, which are shown as frequency values ​​in Fig. 3. Each of these amplitude values ​​of the individual frequencies of the acoustic signal is assigned the corresponding individual frequency for the modulation of the excitation radiation. It can be seen that with a greater frequency difference from the expected resonance frequency or the actual resonance frequency, the amplitude value of the acoustic signal drops significantly, which represents a measure of the quality of the resonance curve.By determining the half-width or a comparable property of the matching profile, the quality of the resonance can be determined from the measurement and, if the quality changes, a conclusion can be drawn, for example, about a potential contamination of the cell.

[0056] Since the typical course of a resonance curve is known, it is possible to determine the course of the resonance curve using an approximation function based on the various amplitude values ​​for the individual frequencies of the acoustic signal and to use this course to calculate the amplitude and in particular the resonance frequency of the amplitude value. This amplitude or the calculated resonance frequency forms the basis for determining various properties of the fluid or the measuring gas. For example, using appropriate calibration tables, the amount of fluid in the measuring chamber, the proportional amount of the measuring gas in a fluid in the measuring chamber, the water content in a fluid in the measuring chamber, for example in toluene in the measuring chamber, or even the temperature or density changes of the fluid in the measuring chamber can be determined from this calculated information.

[0057] It has proven particularly useful to use a typical bell curve or an adjustment according to the Levenberg-Marquardt method as an approximation function.

[0058] The resonance curve shown in Fig. 3 was determined using an adjustment according to the Levenberg-Marquardt method. In this case, an amplitude of 0.84 was obtained at a resonance frequency of 1479.5 Hz. These values, for example, yield a concentration of 3.7 ppm (vol.) of toluene in air.

[0059] Since the amplitude or the resonance frequency is determined mathematically using an approximation function, without the need for modulation with the exact resonance frequency, this invention enables a device 1 and a method for determining a property of a fluid using photoacoustic spectroscopy (PAS), which prove to be particularly simple and also very informative. In particular, constant checking of the actual position of the resonance frequency is not necessary, and adjusting the modulation frequencies or the modulation frequency to changing conditions of the fluid is also unnecessary thanks to this inventive implementation, which greatly simplifies the effort required to determine a property of a fluid. CS Instruments GmbH & Co. KG Straub

[0060] P823010 / PCT / 1 03 . 03 . 2025

[0061] Reference symbol list

[0062] 1 Device for determining a property of a fluid, in particular a measuring gas, by means of photoacoustic spectroscopy (PAS)

[0063] 2 Housing of the device

[0064] 3 Measuring chamber for the gas to be measured

[0065] 4 Radiation source

[0066] 5 Sound measuring device

[0067] 6 Evaluation device

[0068] 7 Control device

[0069] 7a Control line

[0070] 10 Supply line for the fluid into the measuring room

[0071] 11 Derivation for the fluid from the measuring chamber

Claims

Claims 1. A 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, a radiation source (4) configured to generate modulated electromagnetic excitation radiation for the fluid and radiate it into the measuring chamber (3) containing the fluid, a sound measuring device (5) configured to detect sound waves generated by the fluid in the measuring chamber (3), a control device (7) controlling the radiation source (4) such that the excitation radiation is modulated with a frequency beam comprising a plurality of spaced-apart individual frequencies, and an evaluation device (6) configured to evaluate a detected sound signal from the sound measuring device (5),by determining the distribution of the various frequency values ​​by means of a Fourier transformation of the sound signal and determining information about the properties of the fluid therefrom, whereby the individual frequencies of the frequency beam for the modulation are very narrow-band, in particular with a half-width of less than 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 adaptation 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 bundle is designed for modulation with 50 to 500 individual frequencies, in particular with an integer power of two of 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 for the modulation are modulated sequentially and / or simultaneously.

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 line (11) for the fluid from the measuring chamber (3), wherein the control device (7) controls the supply line (10) and discharge line (11) in such a way that a selective filling of the measuring chamber (3) with the fluid is possible.

8. Device for determining a property of a fluid (1) according to one of claims 1 to 7, wherein the radiation source (4) comprises at least one 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 various 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, the density or the moisture content of the fluid from the position of the maximum of the distribution of the various 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 selected depending on the sensitivity of the sound measuring device (5), wherein the maximum extent is selected in particular 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 space (3) is selected such that its maximum extent is limited such that the detected sound signal lies 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 ones of these measurements are used to determine pressure-independent disturbances and can be used 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) by means of a device for determining a property of a fluid (1) according to one of claims 1 to 14, wherein the fluid 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 measuring device (5) Sound waves generated by the fluid in the measuring chamber (3) are recorded as a sound signal, and the recorded sound signal is evaluated by means of the evaluation device (6) in that the recorded sound signal is subjected to Fourier transformation, the distribution of the various frequencies is then determined, and from this the information about the property 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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