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DE-202026101698-U1 - Device for determining a property of a fluid

DE202026101698U1DE 202026101698 U1DE202026101698 U1DE 202026101698U1DE-202026101698-U1

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 modulation of the excitation radiation with a frequency band of the same amplitude or with several different individual frequencies with the same amplitude and with the evaluation device (6) the distribution of the different frequency values of the sound signal with the associated preliminary sound amplitude and the associated preliminary frequency is determined, 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.

Assignees

  • CS INSTR GMBH & CO KG

Dates

Publication Date
20260513
Application Date
20260325
Priority Date
20260325

Claims (12)

  1. Device for determining a property of a fluid (1) by means of photoacoustic spectroscopy (PAS) comprising a housing (2) enclosing a measuring chamber (3) for receiving the fluid, 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 the radiation source (4) is controlled by a control device (7) 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 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 excitation radiation is repeatedly modulated 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 chamber, 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 property of the fluid is determined from this sound amplitude and the associated frequency.
  2. 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).
  3. 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.
  4. Device for determining a property of a fluid (1) according to one of the Claims 1 until 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).
  5. Device for determining a property of a fluid (1) according to one of the Claims 1 until 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)
  6. Device for determining a property of a fluid (1) according to one of the Claims 1 until 5 , wherein in the first process stage and/or in the second process stage the individual frequencies are 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.
  7. Device for determining a property of a fluid (1) according to one of the Claims 1 until 6 , wherein the individual frequencies of the excitation radiation are chosen to be increasing or decreasing in frequency at different successive individual frequencies.
  8. Device for determining a property of a fluid (1) according to one of the Claims 1 until 7 , wherein the number of individual frequencies of the excitation radiation is chosen between 10 and 1000 individual frequencies, in particular with an integer power of two of individual frequencies.
  9. Device for determining a property of a fluid (1) according to one of the Claims 1 until 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 adjustment function, for example by means of a bell curve.
  10. Device for determining a property of a fluid (1) according to one of the Claims 1 until 9 , where the individual frequencies are chosen such that they are around the expected resonant frequency of the sound signal, in particular evenly or symmetrically distributed around the expected resonance frequency.
  11. Device for determining a property of a fluid (1) according to one of the Claims 1 until 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.
  12. Device for determining a property of a fluid (1) according to one of the Claims 1 until 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.

Description

TECHNICAL AREA The invention relates to a device for determining a property of a fluid using photoacoustic spectroscopy (PAS). STATE OF THE ART 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. 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. 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 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, no sound wave is generated by the gas under investigation, and no or only a weak background signal is measurable. Furthermore, in certain variations, photoacoustic measurement methods achieve extremely high sensitivities in the detection of trace gases. 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. 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. 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. From the German patent DE 10 2007 014 518 B3 and the European patent application EP 4 019 938 A1 Photoacoustic sensors are known that propose increasing efficiency by using reflectors on or inside 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. The European patent application EP 4 009 035 A1 Disclosure reveals a photoacoustic sensor in which excitation radiation with a continuous frequency band is generated using an incandescent lamp and frequency-selectively directed to different areas of the measurement space by means of optical elements. Furthermore, a plurality of acoustic sensors, assigned to individual detection positions within the measurement space, capture the acoustic signals