Device for determining a property of a fluid

Abstract

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 electromagnetic excitation radiation for the fluid modulated onto a carrier signal and emitting it into the measuring chamber (3) containing the fluid, and a sound measuring device (5) configured to detect sound waves generated by the fluid in the measuring chamber (3). and with an evaluation device (6) which is designed to evaluate the sound signals recorded by the sound measuring device (5) by determining the distribution of the different frequency values ​​of the sound signal with the sound amplitude and the associated frequency and by deriving information about the property of the fluid from this, characterized by that with a control device (7) the radiation source (4) is controlled in a two-stage process (12,22) in such a way that that in a first process stage (12) the excitation radiation is modulated with a frequency band of the same amplitude or with several different individual frequencies of the same amplitude and the distribution of the different frequency values ​​of the sound signal with the associated preliminary sound amplitude and the associated preliminary frequency is determined with the evaluation device (6), and that in a second process stage (22) the modulation of the excitation radiation is repeatedly carried out with the fixed, associated preliminary frequency, wherein the excitation radiation is generated with different radiation intensities and / or different frequencies of the carrier signal and is radiated into the measuring room, and wherein the distribution of the different frequency values ​​of the sound signal with the associated sound amplitude and the associated frequency is determined with the evaluation device (6), and that information about the properties of the fluid is determined from this sound amplitude and the associated frequency.

Description

TECHNICAL AREA

[0001] The invention relates to a device 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.

[0011] Furthermore, international patent application WO 00 2025 185 794 A1 discloses a device and a method for determining a property of a fluid using photoacoustic spectroscopy (PAS). This device comprises a housing enclosing a measuring chamber for receiving the fluid, a radiation source configured to modulate electromagnetic excitation radiation for the fluid and project it into the measuring chamber containing the fluid, a sound measuring device configured to detect sound waves generated by the fluid in the measuring chamber, a control device that controls the radiation source such that the excitation radiation is modulated with a frequency beam consisting of several spaced-apart, simultaneously projected, and narrowband individual frequencies, and an evaluation device configured to...The device evaluates the sound signal captured by the sound measuring device by determining the distribution of the different frequency values ​​using a Fourier transform of the signal and deriving information about the properties of the fluid from this. The device's accuracy is good; however, it exhibits limitations in resolution and accuracy at low signal amplitudes.

[0012] German patent application DE 10 2024 106 208 A1 discloses a device and a method for determining a property of a fluid using photoacoustic spectroscopy (PAS).

[0013] This device comprises a housing enclosing a measuring chamber for the fluid, a radiation source designed to modulate electromagnetic excitation radiation for the fluid and project it into the measuring chamber containing the fluid, and a sound measuring device designed to detect sound waves generated by the fluid within the measuring chamber. The associated control unit controls the radiation source such that the excitation radiation is modulated with a frequency beam containing several spaced-apart individual frequencies. The evaluation unit analyzes the detected sound signal from the sound measuring device by determining the distribution of the different frequency values ​​using a Fourier transform of the signal and deriving information about the properties of the fluid from this analysis.This device allows for a very precise determination of a fluid property, but it can still be improved. DESCRIPTION OF THE INVENTION

[0014] The invention is based on the objective of providing a special, less disruptive device for determining a property of a measuring gas, which is improved compared to the prior art.

[0015] 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.

[0016] Advantageous embodiments of the invention are the subject of the dependent claims.

[0017] The device according to the invention for determining a property of a fluid by means of photoacoustic spectroscopy shows a housing that encloses a measuring chamber for receiving the fluid.

[0018] Furthermore, it contains a radiation source designed to modulate electromagnetic excitation radiation for the fluid and to emit it directly or indirectly into the measurement chamber containing the fluid. Typically, an excitation signal containing or encompassing the expected resonance frequency of the fluid under investigation in the measurement chamber is modulated onto a carrier signal, resulting in the excitation radiation.

[0019] It also includes a sound measuring device designed in such a way that it can detect sound waves generated by the fluid in the measuring room and transmit them to an external device as a sound signal.

[0020] The evaluation unit of the device is designed in such a way that it can evaluate the detected sound signal of the sound measuring device by determining the distribution of the different frequency values ​​of the sound signal with the sound amplitude and the associated frequency, and from this information about the property of the fluid is determined.

[0021] The control unit regulates the radiation source in such a way that, in a first process stage, the excitation radiation is modulated at a fixed carrier frequency with a frequency band of the same amplitude or with several different individual frequencies of the same amplitude. The evaluation unit then determines the distribution of the different frequency values ​​of the sound signal, along with the corresponding preliminary sound amplitude and frequency. This improves the accuracy of the acoustic resonance frequency and thereby reduces the influence of a number of potential disturbances. Thus, in the first process stage, an analysis of the fluid is performed according to the acoustic properties of both the fluid and the measurement setup.

[0022] Furthermore, in a second process stage, controlled by the control unit, the excitation radiation is repeatedly modulated with the fixed, associated preliminary frequency, which corresponds to the resonance frequency. This generates the excitation radiation with varying intensities and / or different carrier signal frequencies, which are then directed into the measurement chamber. This improves the carrier frequency for modulation and thereby reduces the influence of various potential disturbances. Consequently, in this second process stage, the fluid is analyzed spectroscopically based on both the fluid itself and the measurement setup.

[0023] In each subsequent process step, the knowledge regarding the frequency associated with the sound amplitude, which corresponds to the resonance frequency of the fluid in the measuring chamber, is used to carry out this process step in a targeted manner. This can be achieved either by directly optimizing the resonance frequency or by optimizing the carrier frequency. It is not crucial which of the two process steps is performed first. In this way, through a kind of adaptive process, the accuracy of determining the sound amplitude and its corresponding frequency is continuously improved and made more precise, meaning, in particular, less prone to interference. Consequently, the properties of the fluid in the measuring chamber can be determined with increasing accuracy.

[0024] The first process stage effectively reduces or even eliminates disruptive effects caused by narrowband and asynchronous noise, particularly from external sources, or by changes in temperature, fluid mixing ratio, or fluid pressure. The second process stage further reduces or eliminates disruptive effects of the system, such as those caused by defects in or aging of the radiation source (especially a laser light source), or by asynchronous external influences.

[0025] This combination of the different process stages with their varying effects makes it possible to specify a particularly low-interference device for determining the properties of a fluid.

[0026] In particular, the radiation source comprises 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 support points in the Fourier-transformed acoustic signal and thus improving the quality of the determination of a fluid property.

[0027] The device according to the invention is operated with a method according to the invention for determining a property by means of photoacoustic spectroscopy (PAS). In this process, a fluid located in a measuring chamber is excited by means of a radiation source using modulated electromagnetic excitation radiation for the fluid, and the sound waves generated by the excited fluid in the measuring chamber are detected as a sound signal using a sound measuring device. Using an evaluation device, the distribution of the different frequency values ​​of the sound signal generated by the fluid in the measuring chamber is determined with the associated preliminary sound amplitude and the associated preliminary frequency, and information about the property of the fluid is determined from this sound amplitude and the associated frequency.

[0028] This is done in a two-stage process such that in a first process stage, the excitation radiation is modulated at a fixed carrier frequency with a frequency band of the same amplitude or with several different individual frequencies with the same amplitude, and the distribution of the different frequency values ​​of the sound signal generated by the fluid in the measuring chamber is determined with the associated preliminary sound amplitude and the associated preliminary frequency using the evaluation device.

[0029] In a second process stage, the excitation radiation is repeatedly modulated with the fixed, associated preliminary frequency, whereby the excitation radiation is generated with different radiation intensities and / or different frequencies of the carrier signal and is radiated into the measuring room, and whereby the distribution of the different frequency values ​​of the sound signal with the associated sound amplitude and the associated frequency, which corresponds to the varied carrier frequency, in particular the optimal carrier frequency, is determined with the evaluation device.

[0030] This method enables a meaningful determination of the properties of a fluid using photoacoustic spectroscopy, while making this possible in a cost-effective manner.

[0031] It has proven particularly advantageous to further develop the device according to the invention such that, in the second process stage, the excitation radiation with different radiant intensities – also called luminous flux – and / or different frequencies of the carrier signal is generated at a fixed, constant modulation frequency using different operating currents for the radiation source. The different operating currents for the radiation source, which is preferably designed as a laser light source, vary the radiant intensity and, as a secondary effect, also vary the frequency of the carrier signal. This makes it possible to effect a targeted variation of the frequency of the carrier signal or a variation of the luminous intensity of the carrier signal or the excitation signal emitted by the light source.This makes it possible, in a fairly simple and safe way, to at least partially suppress a number of different interfering effects for determining the desired properties of the fluid, and thus to produce a particularly meaningful result, with a given and constant modulation of the carrier signal with the expected or determined resonance frequency of the fluid in the measuring space, which corresponds to the frequency associated with the sound amplitude.

[0032] It has proven particularly advantageous to further develop the device according to the invention such that the control unit is designed to repeatedly execute the two-stage process and determine information about the fluid's properties from the associated sound amplitude and the corresponding frequency, which corresponds to the resonance frequency. By applying the various process stages, and especially by applying them multiple times, it is possible to improve the accuracy of the measurement method in a multi-stage adaptive process and to reduce its dependence on interfering effects. This applies particularly to gliding effects, for example, effects caused by a continuously or steadily changing temperature, which influences the speed of sound in the fluid.Such a constantly changing parameter can be eliminated or reduced using this adaptive multi-stage process, along with other interfering factors, so that, for example, the determination of moisture in a gas as a property of a gaseous fluid can be made particularly reliable and safe.

[0033] It has proven particularly advantageous to further develop the device according to the invention in such a way that the distribution of the different frequency values ​​of the sound signal with the associated sound amplitude and the associated frequency is determined by means of a Fourier transformation using the evaluation device.

[0034] The evaluation of the sound signal can be performed, for example, by means of a Fourier transform from the time domain to the frequency domain. 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 modulated with spaced frequencies, preferably around the fluid's typical resonance frequency in the measuring chamber, for example, using a multi-frequency or multi-tone method. This ensures that excitation, and in particular an amplified 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 does not require elaborate mechanisms to compensate for temperature fluctuations and variations in the purity of the fluid.

[0035] A particularly preferred embodiment of the device according to the invention features an evaluation unit that determines the distribution of the various frequency values ​​of the sound signal with the corresponding sound amplitude and frequency by means of a digital lock-in amplifier, thereby enabling a particularly reliable and rapid determination of the properties of a fluid. Such a lock-in amplifier utilizes the property that, with knowledge of the modulation characteristics, it can enable efficient and rapid synchronous demodulation. This coordinated, synchronous modulation, or modulation using such a lock-in amplifier, also makes it possible to completely or largely eliminate time intervals between the individual process stages, thereby shortening the overall duration of the various process stages.

[0036] A further particularly preferred embodiment of the invention shows a control device designed such that the individual frequencies for modulation are sequentially and / or simultaneously modulated onto the excitation radiation and the modulated excitation radiation is radiated into the measuring room.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] A particularly preferred embodiment of the device according to the invention for determining a property of a measuring gas features individual frequencies for modulation that are very narrowband, in particular with a half-width below 3 Hz, or are designed as monochrome and thus as sinusoidal and overtone-free radiation. The narrowband design of the individual frequencies, especially in the form of discrete individual frequencies, makes it possible to create meaningful reference points, for example in the Fourier-transformed acoustic signal, and thereby improve the accuracy in determining the property of a fluid.

[0041] It has proven particularly advantageous to further develop the device according to the invention such that the control system is designed in such a way that the individual frequencies of the excitation radiation are selected to be increasing or decreasing in frequency at different successive individual frequencies. This sequence of the individual frequencies used, whether increasing or decreasing, makes it possible to achieve a particularly effective, fast, and reliable modulation efficiency and thus a particularly efficient and reliable overall process, thereby reducing the total time required to carry out the various process stages.

[0042] 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.

[0043] It has proven particularly advantageous to further develop the device according to the invention such that the number of individual frequencies of the excitation radiation is selected between 10 and 1000 individual frequencies. 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.

[0044] This particular number of individual frequencies has proven especially effective, as, for example, the Fourier transformation and the subsequent evaluation using a Fast Fourier Transform algorithm can be performed with manageable computational effort, thus enabling a very computationally efficient and reliable determination of the fluid's properties.

[0045] 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.

[0046] It has proven particularly advantageous to further develop the device according to the invention such that the evaluation unit is designed in such a way that the distribution of the various frequency values ​​is determined by means of an adaptation function, for example, a bell curve. Thus, in the evaluation unit, further evaluation is carried out based on the applied and calculated adaptation function to determine the distribution of the various frequency values, so that intermediate values ​​of the measured frequency values ​​can also be taken into account during the evaluation, thereby achieving improved predictive power of the device for determining a property of a fluid by means of photoacoustic spectroscopy.By using a fitting function, particularly a symmetrical fit such as a bell curve, it is possible to determine the position of the maximum amplitude of the Fourier-transformed acoustic frequencies and thus obtain a meaningful statement about the property of the fluid, especially the gas under investigation. Particularly in the region of the maximum amplitude, the measured amplitude values ​​vary considerably and, without the use of a suitable fitting function, lead to poorer results in determining the fluid's property. It is precisely through the use of such a fitting function, especially a bell-shaped one, that the device achieves higher sensing accuracy, which is regularly accompanied by an improvement and thus a lowering of the detection limit.Furthermore, it is possible to significantly reduce the costs of a device for determining a property using photoacoustic spectroscopy with comparable accuracy or a comparable detection limit.

[0047] By using the frequency beam for modulation in conjunction with the Fourier transform and evaluation using an adaptation function, in particular the symmetrical adaptation function, it is possible to achieve a very successful determination of the properties in a simple and also cost-effective way.

[0048] 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, with the associated excitation radiation (modulated, for example, with a frequency beam), operates within the optimal signal range, particularly with regard to the quality of the acoustic sensor, and is used for specific evaluation. This guarantees the high reliability of the measurement result.

[0049] 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.

[0050] 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, density, or moisture content 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 detected influence.

[0051] 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.

[0052] 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 acoustic measuring arrangement, wherein the maximum dimensions are specifically chosen depending on the maximum sensitivity of the acoustic measuring arrangement. Thus, the device for determining a property of a fluid by means of photoacoustic spectroscopy is selected such that the measuring chamber, with its maximum dimensions, depends on the sensitivity of the acoustic 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 further development utilizes this effect in a targeted manner by selecting the resonance frequency of the fluid's sound within the measuring chamber such that the sound measuring device can convert the fluid's acoustic behavior into a highly informative sound signal. 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. As a result, even small changes in the acoustic behavior due to the introduced fluid can be detected particularly effectively and successfully, thereby improving the accuracy of determining the fluid's properties.

[0053] 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 enhanced 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 exactly maintained by the measurement conditions, in particular in the case of a deviation due to changes in the fluid, for example due to changed temperature or humidity, which is reflected in a changed speed of sound and thus in a changed resonance frequency of the sound signal.

[0054] 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.

[0055] 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, 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.

[0056] 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 process of an exemplary method according to the invention with a first and a second process stage for determining a property of a fluid, in particular a measuring gas, by means of photoacoustic spectroscopy (PAS), Fig. Figure 3 shows in a schematic, exemplary diagram a distribution of the individual frequencies of the frequency beam for the modulation of the excitation radiation in the first process stage, and Fig. Figure 4 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 transform of the detected sound signal and the distribution profile of these acoustic frequencies of a first process stage, Fig. Figure 5 shows in a schematic, exemplary diagram the change in the carrier frequency in the second process stage and Fig. Figure 6 shows in a schematic, exemplary diagram the spectral distribution of the acoustic frequencies after the Fourier transformation of the captured sound signal in the second process stage as a result of the change in the carrier frequency. Fig. 5.

[0057] 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.

[0058] 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.

[0059] 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, as well as a controllable discharge line 11 for the controlled discharge of the fluid, in particular the measuring gas, from the measuring chamber 3.

[0060] 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).

[0061] 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.

[0062] 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 device 6 in the form of a sound signal, in particular an electrical sound signal.

[0063] The evaluation unit 6 evaluates the sound signal acquired by the sound measuring device 5 by subjecting the sound signal to a Fourier transform. This transform determines the distribution of the sound signal's frequency values, and then, typically using a matching function, extrapolates or determines, in particular, the location of the maximum frequency distribution and its amplitude. The use of this matching function allows the distribution and the location of the maximum frequencies to be determined outside or adjacent to the specific measured points, thereby yielding significantly more meaningful information about the fluid's properties. From the distribution, and especially the location and amplitude of the maximum, conclusions can be drawn about various properties of the fluid, particularly the moisture content of the sample gas in the measuring chamber 3.

[0064] The device 1 with the radiation source 4, the sound measuring device 5, the evaluation device 6 and the control device 7 are used in such a way that in a two-stage process, i.e. in a first process stage and a subsequent second process stage, an improved determination of a property of a fluid, in particular a measuring gas, is made possible by means of photoacoustic spectroscopy (PAS).

[0065] In Fig. Figure 2 shows a schematic diagram illustrating an exemplary sequence of an exemplary method according to the invention, comprising a first and a second process stage for determining a property of a fluid, in particular a measuring gas, using photoacoustic spectroscopy (PAS). This exemplary method according to the invention can be combined with the method described in Figure 2. Fig. The exemplary device shown in 1 is used.

[0066] The device 1 includes a control unit 7 which controls the radiation source 4 via a control line 7a such that, in a first process stage 12, the excitation radiation 15 is modulated at a fixed, constant carrier frequency using a frequency beam consisting of several spaced-apart individual frequencies of the same amplitude. The individual frequencies are preferably selected such that they are arranged symmetrically around an estimated resonance frequency F0 of the fluid in the measuring chamber or around a corresponding frequency F*0 determined from a previous run of the method according to the invention.The modulated excitation radiation is coupled into the measuring chamber 3 in control of the control unit 7 16, the induced sound signal is detected by the sound measuring device 5 in control of the control unit 7 17 and the sound signal detected by the sound measuring device 5 is evaluated in control of the control unit 7 in such a way as to determine a preliminary sound amplitude and the associated preliminary frequency F*0 after the first process stage 12.

[0067] In a second process stage 22, the excitation radiation is repeatedly modulated 25 with the fixed, associated preliminary frequency F*0, which corresponds to the resonance frequency, using the control unit 7. The excitation radiation is generated by varying the radiation intensity and the frequency of the carrier signal 25. This is achieved primarily by varying the operating current of the radiation source, which is preferably a laser light source. The generated excitation radiation is then directed into the measuring chamber containing the fluid under investigation 26.The sound signal generated by the excitation radiation is detected by the sound measuring device 5 27 and evaluated by the evaluation device 6 28 such that the distribution of the different amplitude values ​​of the sound signal is determined as a function of the varied carrier frequency at a constant, fixed resonance frequency, and the corresponding frequency F**0 is determined as the optimal carrier frequency in the second process stage 22. This optimal carrier frequency F**0 enables a very informative subsequent determination of the resonance frequency within the framework of determining a property of a fluid by means of photoacoustic spectroscopy (PAS). In addition, information about the property of the fluid E is determined from this.

[0068] This information on the properties of fluid E is characterized by the fact that the impact of a multitude of disturbances (temperature changes, changes in the composition of the fluid, impurities in the fluid, noise or vibrations, etc.) is significantly reduced, and thus, by applying both process stages with their different effects, very meaningful information on the properties of the fluid can be obtained.

[0069] To mitigate the effects of fluctuating and changing disturbances, it has proven effective to use information such as the distribution of the various frequency values ​​of the sound signal with its corresponding sound amplitude and frequency F**0, which corresponds to the optimal carrier frequency, from the second process stage 22 as a starting point for the first process stage 12. This information is then used for adapted and optimized modulation. This leads to a stepwise optimization of the information, including information on the fluid property to be determined.

[0070] This repeated execution of the two process stages 12,22 can be carried out multiple times, in particular more than 20 times, thereby optimizing the accuracy of the device or compensating for changes in the fluid such as temperature changes.

[0071] Via further control lines 7a, the control unit 7 controls the supply line 10 as well as the discharge line 11 by selectively opening or closing the respective associated control valve, thereby enabling a targeted 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 or the evaluation unit 6 and thereby influence their function.

[0072] In Fig. Figure 3 shows a schematic diagram illustrating an exemplary distribution of the individual frequencies of a frequency beam used to modulate the excitation radiation in the first process stage 12 to the fixed carrier frequency. 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. When determining the individual frequencies for modulating the excitation radiation using the radiation source 4, an expected resonance frequency is 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 is calculated to determine the moisture content in toluene.

[0073] Based on the expected resonant frequency, the individual frequencies of the frequency beam for modulation are arranged symmetrically around it, so that the individual frequencies are chosen 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.

[0074] 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 as far 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.

[0075] The captured sound signal is transformed into the frequency domain in the first process stage 12 using the evaluation unit 6 and a Fourier transform. Since the excitation radiation is modulated with specific individual frequencies, the Fourier transform into the frequency domain yields individual amplitude values ​​for these frequencies, which are expressed as frequency values ​​in Fig. Figure 4 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 full width at half maximum (FWHM) 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.

[0076] 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.

[0077] 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.

[0078] It has proven particularly useful to use a typical bell curve or an adjustment according to the Levenberg-Marquardt method as an approximation function.

[0079] The in Fig. Figure 4 of the resonance curve was determined using the Levenberg-Marquardt method. In this case, an amplitude of 0.84 at a resonance frequency of 1479.5 Hz was obtained. From these values, for example, a concentration of 3.7 ppm (vol.) of toluene in air can be derived as information E about the properties of the fluid in the measuring chamber.

[0080] 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.

[0081] In Fig. Figure 5 shows a schematic, exemplary diagram illustrating the change in the carrier frequency during the second process stage. The change in radiation intensity and / or carrier frequency is shown as a function of the operating currents applied to the radiation source. The operating currents are varied in steps between 0 and approximately 75 mA, and the radiation intensity and / or carrier frequency change accordingly in an approximately linear relationship within the normalized range of 17–19.3 Ω.

[0082] As explained previously, in the second process stage, with a constant resonance frequency F*0 modulated to the carrier frequency, the carrier frequency and additionally the radiation intensity are varied in the manner shown.

[0083] The selected resonance frequency F*0 is either an estimated resonance frequency or a resonance frequency F*0 obtained from the evaluation of the first process stage.

[0084] The modulated, varied carrier frequency thus forms the excitation signal, which is coupled into the measuring chamber 3 containing the fluid by means of the radiation source. Due to this coupling, an acoustic signal is generated, which is recorded by the acoustic measuring device 5 and evaluated by the evaluation device 6 in such a way that the spectral distribution of the acoustic frequencies is recorded and analyzed according to signal strength and the associated resonance frequency or associated carrier frequency.

[0085] In Fig. Figure 6 shows, in a schematic, exemplary diagram, the spectral distribution of the acoustic frequencies after the Fourier transformation of the captured sound signal in the second process stage as a result of the change in the carrier frequency (spectral or spectroscopic analysis). Fig. 5 shown.

[0086] The carrier frequency is varied between approximately 151.91 terahertz and 151.99 terahertz by changing the operating current of the radiation source, which is a laser light source. The spectral distribution of the sound signal amplitude at each carrier frequency shows three maxima, spaced approximately 0.2 terahertz apart. Of these three maxima, one is very pronounced and lies at approximately 151.68 terahertz with a normalized signal amplitude of approximately 0.035. In contrast, the two less pronounced maxima are at approximately 151.48 terahertz and approximately 151.28 terahertz, with normalized signal amplitudes of approximately 0.015 and approximately 0.014, respectively.

[0087] The pronounced maximum allows the identification of the most suitable carrier frequency, which best represents a resonance frequency. Based on this finding, the carrier frequency is adjusted to the new, optimized carrier frequency during the next first process stage. Following the process sequence of the first stage, the resonance frequency is varied while maintaining a constant carrier frequency and excitation signal amplitude (acoustic analysis).

[0088] Furthermore, this spectral distribution of acoustic frequencies enables spectral analysis and thus the determination of various properties of the fluid under investigation. In the example shown, the water content of the carbon dioxide gas can be determined. With this spectral analysis, the specific analysis of the amplitude, i.e., the maximum loudness of the sound signal, can even be omitted.

[0089] In this way, an improved carrier frequency and an improved resonance frequency can be determined step by step within an adaptation process. This is characterized by the fact that the influences of various disturbances are reduced or even eliminated when determining a property of a fluid, particularly a measuring gas, using photoacoustic spectroscopy (PAS). It has proven particularly advantageous to repeat the two process steps several times in succession, thereby achieving increasingly better values ​​for the resonance frequency and the signal amplitude at the resonance frequency. From this, the desired statement about a property of the fluid being determined can be calculated. For example, the water content in a carbon dioxide gas can be determined with high accuracy using this method. 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 12 First process stage 15 Generation of modulated electromagnetic excitation radiation 16. Radiation of the excitation radiation into the measuring chamber 17. Capturing the sound signal with the sound measuring device 18 Evaluation of the sound signal for sound amplitude and associated frequency at the sound amplitude 22 Second process stage 25 Generation of modulated electromagnetic excitation radiation 26. Radiation of the excitation radiation into the measuring chamber 27. Capturing the sound signal with the sound measuring device 28 Evaluation of the sound signal for sound amplitude and associated frequency at the sound amplitude as well as determination of a property of a fluid F0 estimated resonance frequency of the fluid in the measuring room F*0 preliminary sound amplitude and the associated preliminary frequency after the first process stage F**0 preliminary sound amplitude and the associated preliminary frequency after the second process stage E Information on the properties of the fluid in the measuring chamber QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2007 014 518 B3

[0008] EP 4 019 938 A1

[0008] EP 4 009 035 A1

[0009] GB 2 271 181 A

[0010] WO 00 2025 185 794 A1

[0011] DE 10 2024 106 208 A1

[0012] Cited non-patent literature

[0000] WILDI, Thibault [et al.]: Photo-acoustic dual-frequency comb spectroscopy. In: Nature communicatios, Vol. 11, 2020, Art.-No. 4164, 6 S. - ISSN 2041-1723 und aus KELLNBERGER, Stephan [et al.]: Optoacoustic microscopy at multiple discrete frequencies. In: Light: science & applications, Vol. 7, 2018, Art.-No. 109, 12 S. - ISSN 2047-7538

[0010]

Claims

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 electromagnetic excitation radiation for the fluid modulated onto a carrier signal and emitted 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), and an evaluation device (6) configured to evaluate the sound signals detected by the sound measuring device (5) by determining the distribution of the different frequency values ​​of the sound signal with the sound amplitude and the associated frequency, and thereby deriving information about the property of the fluid, characterized in that it is connected to a control device (7),The radiation source (4) is controlled in a two-stage process (12, 22) such that in a first process stage (12) the excitation radiation is modulated with a frequency band of the same amplitude or with several different individual frequencies of the same amplitude, and the distribution of the different frequency values ​​of the sound signal with the corresponding preliminary sound amplitude and the corresponding preliminary frequency is determined by the evaluation device (6); and that in a second process stage (22) the excitation radiation is repeatedly modulated with the fixed, corresponding preliminary frequency, whereby the excitation radiation is generated with different radiation intensities and / or different frequencies of the carrier signal and is emitted into the measuring chamber, and wherein the distribution of the different frequency values ​​of the sound signal with the corresponding sound amplitude and the corresponding frequency is determined by the evaluation device (6).and that information about the properties of the fluid is determined from this sound amplitude and the associated frequency. Device for determining a property of a fluid (1) according to claim 1, wherein in the second process stage (22) the excitation radiation is generated with different radiation intensities and / or different frequencies of the carrier signal using different operating currents for the radiation source (4). Device for determining a property of a fluid (1) according to claim 1 or 2, wherein the control device (7) is designed such that the two-stage process is carried out repeatedly and information about the property of the fluid is determined from the associated sound amplitude and the associated frequency. Device for determining a property of a fluid (1) according to one of claims 1 to 3, wherein the distribution of the different frequency values ​​of the sound signal with the associated sound amplitude and the associated frequency is determined by means of Fourier transformation using the evaluation device (6). Device for determining a property of a fluid (1) according to any one of claims 1 to 3, wherein the evaluation unit (6) uses a digital lock-in amplifier to determine the distribution of the different frequency values ​​of the sound signal with the corresponding sound amplitude and frequency. (synchronous demodulation technique) Device for determining a property of a fluid (1) according to one of claims 1 to 5, wherein in the first process stage and / or in the second process stage the individual frequencies are designed to be very narrowband, in particular with a half-width below 3 Hz, and wherein the different individual frequencies are irradiated for a predetermined analysis duration and with a predetermined time interval to each other. Device for determining a property of a fluid (1) according to one of claims 1 to 6, wherein the individual frequencies of the excitation radiation are selected to be increasing or decreasing in frequency at different successive individual frequencies. Device for determining a property of a fluid (1) according to one of claims 1 to 7, wherein the number of individual frequencies of the excitation radiation is selected between 10 and 1000 individual frequencies, in particular with an integer power of two of individual frequencies. 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 the distribution of the different frequency values ​​of the sound signal is determined by means of an adaptation function, for example by means of a bell curve. Device for determining a property of a fluid (1) according to one of claims 1 to 9, wherein the individual frequencies are selected such that they are distributed around the expected resonance frequency of the sound signal, in particular uniformly or symmetrically around the expected resonance frequency. Device for determining a property of a fluid (1) according to one of claims 1 to 10, wherein the evaluation device (6) is designed in such a way 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 ​​of the sound signal. Device for determining a property of a fluid (1) according to one of claims 1 to 11, 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.

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