Method for configuring a photoacoustic chamber, and photoacoustic chamber

Abstract

The invention relates to a method for configuring a photoacoustic chamber unit and to a photoacoustic chamber unit that comprises a resonator (30) defining an interior space (31), and at least one acoustic detector (15) adapted for detecting sound waves generated in a medium contained in the interior space (31). One of the modes that are generated in the interior space (31) and being defined by the geometry of the interior space (31) and the acoustic measurement frequency is selected as a measurement mode, and, if there is another disturbing mode that overlaps with it, then an opening (39) or openings (39) are formed for the selective and relative damping of the interfering mode at certain locations in the wall of the resonator (30).

Description

[0001] METHOD FOR CONFIGURING A PHOTOACOUSTIC CHAMBER AND PHOTOACOUSTIC CHAMBER

[0002] TECHNICAL FIELD

[0003] The invention relates to a method for configuring a photoacoustic chamber unit and to a photoacoustic chamber unit. In particular, the invention relates to technical solutions that allow for improved-accuracy detection by selectively and relatively damping the disturbing mode or modes generated in a photoacoustic environment.

[0004] BACKGROUND ART

[0005] The photoacoustic effect is applied for measuring the concentration of gases, vapours, or liquids with high accuracy. The photoacoustic effect is a physical phenomenon in which the energy of light absorbed by a material is converted to sound waves, i.e. , to acoustic waves. The effect is produced when a material is illuminated with modulated - for example, intermittent or pulsating - light, typically with laser light. During this process the illuminated medium absorbs light, the absorbed energy causes local heating, and the rapid heating and consequent thermal expansion generates pressure change, i.e., sound waves, in the medium, i.e., typically gas, vapour, or liquid. These sound waves can be detected utilising an appropriately chosen and placed microphone or another acoustic detector.

[0006] In the case of conventional “closed” photoacoustic chambers, the medium to be measured can proceed through the chamber at severely limited speeds, typically at a speed of less than 1 m / s, because high flow speeds result in such acoustic noise levels that make it impossible to perform high-accuracy measurements. If the medium to be measured is stationary, then a part of the medium, i.e., the gas sample to be measured, must be introduced into the closed photoacoustic chamber in some way. The technique called “extractive sampling” is suitable for that, which for example involves sucking a part of the medium through the photoacoustic chamber by means of a pump. In this case, an extractive sampling unit must be added to the photoacoustic system, which has various disadvantages: it makes the system more expensive, bigger, and heavier, and increases the risk of failure. In addition to that, due to the limit on the flow velocity mentioned above, the application of an extractive sampling unit significantly increases the response time of the measurements, because due to the relatively low flow velocity it takes a significant time for the gas sample to be measured to get through the portion of the gas sampling unit located between the sampling point and the detector. If the flow velocity of the medium to be measured is low, then it is possible to provide a direct gas flow path through the closed photoacoustic chamber. However, in the case of higher flow velocities it becomes necessary to apply the extractive sampling technique, which involves the above-described disadvantages.

[0007] The term “acoustic mode” denotes a characteristic frequency and a spatial oscillation pattern that corresponds to it. Acoustic modes are therefore the natural oscillation modes of a given system, where the system oscillates at a given frequency (the characteristic frequency), with a given spatial pattern (eigenmode or mode pattern), and the oscillations behave as standing waves. The mode patterns can be characterised by the spatial distribution of nodes and antinodes.

[0008] In the technical field of photoacoustics, the term “mode” refers to an acoustic resonance mode of the measuring chamber, for example, in the case of measuring chambers with cylindrical tubular interior spaces, to the azimuthal and the radial modes, resonances that allow the effective amplification of photoacoustic signals, and thereby their effective detection. Thus, the term “mode” here refers to a standing wave or resonance generated in the photoacoustic chamber. In principle, such photoacoustic chambers may exist wherein only a single mode is generated, but in the case of chambers having practically applicable shapes more than one standing waves, i.e., modes, are generated during measurements. For example, with tubular measuring chambers having comparable diameter and length, typically all three of the above-mentioned modes, i.e., the longitudinal, azimuthal, and radial modes may occur. Photoacoustic measurements can be performed accurately if they focus on a single selected mode. If another mode appears at a frequency near the selected mode, then this further mode distorts evaluation at the selected mode, making the measurement less accurate.

[0009] Prior art approaches do not provide such a chamber configuration that would allow for selectively and relatively damping disturbing modes in the case of arbitrary geometries.

[0010] Slits formed in the walls of a tubular photoacoustic chamber are disclosed in US 2009 / 0229345 A1 , but these known slits are adapted for amplifying the own mechanical resonance of the tubular structure and extend also to the edges of the tube, and potential disturbing modes are not taken into consideration for forming the slits.

[0011] DISCLOSURE OF THE INVENTION

[0012] The object of the invention is to provide a method for configuring a photoacoustic chamber unit and a photoacoustic chamber unit that are free from the drawbacks of prior art approaches to the greatest possible extent, and that can be applied for performing the measurements as effectively as possible at the resonance frequency of the selected mode with selective, relative damping of disturbing modes. A further object of the invention is to provide a technical solution that provides selective and relative damping of the disturbing mode(s) simply and cost-effectively, in an easily foreseeable manner.

[0013] The object of the invention has been achieved by the method according to claim 1 and by the photoacoustic chamber unit according to claim 10. Preferred embodiments are defined in the dependent claims.

[0014] In our experiments leading to the invention, we have recognised that the modes or standing waves generated inside the photoacoustic chamber are dependent on the given geometry and the acoustic measurement frequency, so they are temporally constant, as are the spatial locations of the pressure antinodes and nodes. This recognition offers us an opportunity for implementing selective damping at such pressure antinodes of the undesired mode or modes that are located at or relatively near the chamber walls and are located relatively farther from the pressure antinodes of the mode selected for measurement. Selective damping can be implemented by forming an opening or openings in the chamber wall at the given locations.

[0015] BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Preferred embodiments of the invention will be explained referring to the accompanying drawings, where

[0017] Fig. 1 is a side view of a photoacoustic chamber unit according to a first embodiment of the invention,

[0018] Fig. 2 is the front view of the embodiment according to Fig. 1 ,

[0019] Fig. 3 is a schematic depiction of a longitudinal mode generated in a tubular interior space,

[0020] Fig. 4 is a schematic depiction of an azimuthal mode generated in the tubular interior space,

[0021] Fig. 5 is a schematic depiction of a radial mode generated in the tubular interior space,

[0022] Fig. 6 shows a schematic front view of a radial mode of a resonator without openings,

[0023] Fig. 7 shows the resonator and mode according to Fig. 6 in a side view,

[0024] Fig. 8 shows a schematic front view of an azimuthal mode of the resonator without openings,

[0025] Fig. 9 shows the resonator and mode according to Fig. 8 in a side view,

[0026] Fig. 10 shows a side view of a resonator configured with a pair of oppositely disposed longitudinal openings,

[0027] Fig. 11 is a side view of the resonator according to Fig. 10,

[0028] Fig. 12 is a top plan view of the resonator according to Fig. 10,

[0029] Fig. 13 is a spatial view of the resonator according to Fig. 10, Fig. 14 is a schematic front view of a radial mode of the resonator according to Fig.

[0030] 10,

[0031] Fig. 15 shows a side view of the resonator and mode of Fig. 14,

[0032] Fig. 16 is a schematic front view of an azimuthal mode of the resonator according to Fig. 10,

[0033] Fig. 17 shows a side view of the resonator and mode according to Fig. 16,

[0034] Fig. 18 is a theoretical schematic depiction of a cross section across the openings of the resonator according to Fig. 10, and

[0035] Fig. 19 is a frequency-amplitude diagram illustrating the modes of a resonator without openings and a resonator provided with openings.

[0036] MODES FOR CARRYING OUT THE INVENTION

[0037] The invention relates configuring a photoacoustic chamber unit having an arbitrary shape and an arbitrary interior space. The invention preferably relates configuring a so-called “open” photoacoustic chamber with a tube-in-a-tube arrangement that is particularly suited for measuring a quickly varying gas matrix based on the photoacoustic principle.

[0038] A potential application of the invention relates to internal combustion engines, i.e., to their exhaust gases. In various operating states of internal combustion engines the composition of the exhaust gas can vary quickly, so it is necessary to measure the quickly varying gas concentration, even at high flow velocities and with high external noise levels. During the measurement, the change of the temperature of the components not being under test and the temperature of the gas flow to be tested both vary due to the engine’s changing operating states, which results in changes in the acoustic resonance frequency of the chamber. In principle, it is possible to track these changes applying photoacoustic measurements but - due to its geometry - the applied photoacoustic chamber typically has several acoustic modes that can be located close enough to each other to allow the measurement to continue at another (undesired) acoustic mode in the case of a quick change. Because it would be extremely time-consuming to record the resonance curves for all acoustic modes, a solution allowing for supressing or damping acoustic amplification of the undesired modes, and thus for filtering out their disturbance effects - while the quality of the mode applied for the measurement is not reduced substantially - is required. The solution according to the invention comprises forming openings in the wall of the photoacoustic resonator (that is, in the case of a tube-in-a-tube arrangement, in the wall of the inner tube), that preferably dampen the acoustic signal at the antinodes of the mode or modes that are not intended to be excited but are located farther away from the antinodes of the acoustic mode utilised for the measurement.

[0039] The photoacoustic chamber unit according to the invention is preferably configured in such a way that it can be placed directly in the medium flow to be measured, i.e. , the photoacoustic measurement is performed directly in the gas flow, so applying gas sampling is not necessary. Performing measurements without sampling allows for significantly shorter measurement response times. Without sampling, the accuracy of the measurements is also improved, because in the case of gases no adsorption / desorption effects occur in the sampling unit.

[0040] The photoacoustic chamber unit that can be seen in Figs.1 -2 is adapted for performing photoacoustic measurements in a medium flow. The photoacoustic chamber unit comprising a resonator 10 defining an interior space 11 , a medium inlet opening 12 leading to the interior space 11 , and a medium outlet opening 13 leading out from the interior space 11 .

[0041] Modulated laser light determining the acoustic measurement frequency is radiated into the interior space 11 by an irradiation unit 14. In this embodiment, irradiation is performed through one or more optional laser light bores or windows 16, through which the light adapted for generating the photoacoustic signal can be introduced into the resonator 10. One of more light reflection elements 18 can also be arranged in the interior space 11 . The sound waves generated in the medium contained in the interior space 11 are detected by at least one acoustic detector 15. The resonator 10 is preferably a photoacoustic resonator shaped as a tubular (cylindrical) body. The acoustic signal generated by the periodically modulated laser light directed through this resonator 10 is produced in the resonator 10, and, after acoustic amplification in the resonator 10 it is transduced into an electric signal applying one or more acoustic detectors 15, preferably microphones, or even piezoelectric sensors or ultrasonic transducers.

[0042] The photoacoustic chamber unit further comprises at least one retaining unit 17 that is connected to the resonator 10 and is adapted for retaining the resonator 10 in the medium flow, i.e., in the depicted solution, it comprises two retaining elements connected to the outer wall of the resonator 10.

[0043] The resonator 10 has an interior space 11 defining a medium flow-through path extending from the medium inlet opening 12 to the medium outlet opening 13, preferably having an interior space 11 that has a uniform cross-sectional area in a direction transverse to the medium flow-through path.

[0044] According to the invention, at least one through opening 19 that is adapted to connect the interior space 11 with a medium flow space outside the resonator 10 and is separated from the medium inlet opening 12 and from the medium outlet opening 13 by the material of the resonator 10 is formed in the wall of the resonator 10 such that it extends transversely with respect to the medium flow-through path. In the case depicted in Figs. 1 and 2, a single through opening 19 is formed in the wall of the resonator 10, extending parallel to the edges of the tubular resonator 10 in a circumferential direction, and has a rectangular shape when flattened onto a plane. As it will be described in detail below, the at least one opening or cutout has a location and shape (geometry) configured in accordance with the requirements of filtering the acoustic eigenmodes selected for the measurements.

[0045] The photoacoustic chamber unit therefore comprises the open photoacoustic chamber, i.e., the resonator 10, and it also comprises other elements. The unit configured in this way includes the light source and preferably also includes signal processing and forms a part of a more complex photoacoustic measurement system.

[0046] In the photoacoustic chamber unit according to the invention, a location and a shape of the at least one through opening 19 are determined such that for each point within the area of the opening 19 (i.e. the area where there is a gap in the inside wall) the following conditions are met concerning the modes generated in the interior space 11 , the modes being determined by the geometry of the interior space 11 and the acoustic measurement frequency: of which modes a first mode is the selected measurement mode and a second mode is the mode to be dampened:

[0047] - each point within the area opening 19 is closer to the nearest pressure node of a first mode than to the nearest pressure node of a second mode, and

[0048] - each point within the area opening 19 is farther from the nearest pressure antinode of the first mode than from the nearest pressure antinode of the second mode.

[0049] Our experiments leading to the invention indicated that more effective selective and relative dampening of the undesired mode or modes can be achieved by determining a location and a shape of the one or more through openings 19 such that, in comparison with the state without the opening(s) 19, the following hold true for the effects of providing the one or more through openings 19:

[0050] - the quality factor of the first mode is reduced at most by a second threshold extent, said second threshold extent being 20% or lower, and

[0051] - the quality factor of the second mode is reduced at least by a third threshold extent, said third threshold extent being 50% or greater.

[0052] The quality factor or Q-factor is the quotient of the central frequency and the full width at half maximum (FHWM, i.e., the distance measured along the horizontal axis at which the signal falls to half of the maximum value); it also gives the fraction of energy lost over a single period of the oscillation from the energy stored of the resonator. Therefore, with a decreasing Q-factor the resonance curve not only widens but its peak value decreases, too; this is why the Q-factor is applicable for specifying a preferred embodiment of the technical solution according to the invention.

[0053] The invention can be particularly preferably applied with open resonators having a tube-in-a-tube arrangement, i.e., with open chambers. Schematic depictions of the modes generated in resonators having a cylindrical interior space can be seen in Figs. 3-5.

[0054] Fig. 3 is a schematic depiction of a longitudinal mode generated in a tubular interior space, Fig. 4 is a schematic depiction of an azimuthal mode generated in the tubular interior space, and Fig. 5 is a schematic depiction of a radial mode generated in the tubular interior space,

[0055] If a resonator 20 without openings having a tubular, cylindrical interior space is applied, then for example the modes according to Figs. 6-9 can be obtained. Fig. 6 shows a schematic front view of a radial mode of a resonator 20 without openings, Fig. 7 shows the resonator 20 and mode according to Fig. 6 in a side view, Fig. 8 shows a schematic front view of an azimuthal mode of the resonator 20, and Fig. 9 shows the resonator 20 and mode according to Fig. 8 in a side view. As illustrated in Fig. 9, optimally placed acoustic detectors 15 are applied for detecting the azimuthal mode that by way of example was selected for the measurement; such optimal placement involving disposing the detectors at the antinodes of the azimuthal mode. However, as it is shown in Fig. 7, the radial mode also has significant effects at the locations where the acoustic detectors 15 are arranged, and if there is a significant overlap between the two modes, then the radial mode may significantly distort (deteriorate) the measurement. Therefore, it is necessary to provide major dampening of the radial mode of the example such that it does not result in the significant dampening of the azimuthal mode.

[0056] In the case according to the example, the desired damping is achieved by forming openings 39 or slits in the cylindrical wall of the resonator 30 as shown in Figs. IQ- 13. Fig. 10 shows a side view of a resonator 30 configured with a pair of oppositely disposed longitudinal openings 39, Fig. 11 is a side view of the resonator 30 according to Fig. 10, Fig. 12 is a top plan view of the resonator 30 according to Fig. 10, and Fig. 13 is a spatial view of the resonator 30 according to Fig. 10. Accordingly, Fig. 14 is a schematic front view of a radial mode of the resonator 30 according to Fig. 10, Fig. 15 shows the resonator 30 and mode according to Fig. 14 in a side view, Fig. 16 is a schematic front view of an azimuthal mode of the resonator 30 according to Fig. 10, and Fig. 17 shows the resonator 30 and mode according to Fig. 16 in a side view.

[0057] In the photoacoustic chamber unit according to this example, the resonator 30 therefore has a tubular interior space 31 , and the first mode, i.e. , the measurement mode, is an azimuthal mode generated in the interior space 31 , and the second mode, i.e., the mode to be dampened, is a radial mode generated in the interior space 31 . Like in Figs. 6-9, in this example the irradiation unit 14 is directed to form a laser light path that extends parallel to and spaced apart from a longitudinal axis of the tubular shape. The preferably applied asymmetric arrangement amplifies the azimuthal mode selected for the measurement. In this example the photoacoustic chamber unit comprises two acoustic detectors 15 that are disposed opposite each other at the middle of the length of the tubular shape, and at a plane defined by the longitudinal axis of the tubular shape and the path of the laser light. This essentially means that the detectors are disposed at the antinodes of the azimuthal mode, the antinodes being produced at locations determined by the asymmetric excitation. Directing the laser light in this way is particularly preferable because in such a case it is not necessary to form a dedicated opening or window in the wall of the resonator 30 for letting in the light.

[0058] If the laser light was directed along the axis of symmetry of the cylindrical chamber, it would excite the radial mode much more effectively while only slightly exciting the azimuthal mode. Getting farther away from the axis of symmetry, the azimuthal mode is excited more and more by the laser light, while the radial mode remains excited even further away from the axis of symmetry, this is why the application of the openings according to the invention is necessary. The locations of the nodes and antinodes of the azimuthal modes are primarily determined by the path of the laser light. In the preferred example, two openings 39 are formed in the wall of the tubular shape opposite each other symmetrically to the above-mentioned plane and parallel to the longitudinal axis of the tubular shape, the openings 39 ending spaced apart from both edges of the tubular shape, i.e. , they do not end at the edges. The latter is preferable for improving the mechanical strength of the resonator 30 and, if possible, providing that it is free from eigenresonances.

[0059] As it is adequately illustrated by the arrows in Figs. 14-15 and Figs. 16-17, the application of the openings 39 significantly dampens the intensity of the radial mode, while the intensity of the azimuthal mode remains essentially unchanged. This follows from the feature that the radial mode and the azimuthal mode, respectively have pressure antinodes and pressure nodes at the openings 39.

[0060] Fig. 18 is a theoretical schematic depiction of a cross section across the openings of the resonator according to Fig. 10, illustrating that the radiated energy fraction in respect of the azimuthal mode can be calculated applying the following formulas: W = <pPmaxsin2<p a = 2lrsin<p = 2x1 where r and I are the radius and the length of the cylinder, and x is the width of the cutouts.

[0061] Based on practical experience, for example in the case of a 25 mm-diameter cylindrical resonator the preferred width of the cutouts, i.e., the openings 39 is x ~ 4 mm, for which it holds true that for the useful azimuthal mode the radiated energy is W < 1 %, while in respect of the radial mode the radiated energy is about 10%.

[0062] The openings 39 or cutouts may extend along almost the full length of the resonator chamber, the only constraint being the mechanical stability of the chamber body, and because of that the openings 39 do not join the medium flow openings.

[0063] The width of the openings 39 or cutouts should preferably be chosen such that the losses induced by them are smaller than the radiated loss in the direction of the microphones in the case of the useful mode (in this case, the azimuthal mode). For example, with 2 microphones, other losses - without cutouts - amount to about 1 % of the energy stored in the resonator. The energy radiated through the cutouts is W~P2*Da / A, where A is the surface area of the resonator, Da is the area of the radiating surface, and P is the maximum pressure amplitude along this surface. A possible goal to be set for configuration can therefore be that for the useful mode the energy radiated through the cutouts must not exceed other losses.

[0064] According to the invention it is expedient if the combined surface area of the longitudinal openings 39 is between 5% and 10%, preferably between 6% and 9%, and more preferably it is approximately 8% of the cylindrical surface area of the of the tubular shape, which is also reflected in the example included above.

[0065] Fig. 19 is a frequency-amplitude diagram illustrating the modes of a resonator without openings, for example the resonator 20 according to Figs. 6-9 and a resonator provided with openings, for example the resonator according to Figs 14- 17. As can be seen in the diagram, in the case of the resonator without cutouts the signal peak of the radial mode appears near the signal peak of the azimuthal mode; there is significant overlap between the signal curves of the two modes. In the case of a resonator provided with cutouts, the signal peak of the radial mode completely disappears, and there is no disturbing peak near the signal peak of the azimuthal mode.

[0066] The method for configuring a photoacoustic chamber unit according to the invention is carried out on a chamber unit having a resonator 10, 30 defining an interior space 11 , 31 , a medium inlet opening 12, 32 leading to the interior space 11 , 31 , a medium outlet opening 13, 33 leading out from the interior space 11 , 31 , an irradiation unit 14 for radiating modulated laser light determining the acoustic measurement frequency into the interior space 11 , 31 , and at least one acoustic detector 15 adapted for detecting sound waves generated in the medium contained in the interior space 11 , 31. The method comprises selecting, as a measurement mode, one of the modes generated in the interior space 11 , 31 and being defined by the geometry of the interior space 11 , 31 and the acoustic measurement frequency, and then checking whether there is a mode among the modes of which the full width at half maximum on a frequency-amplitude diagram overlaps, at least by a first threshold extent, with the full width at half maximum of the measurement mode. The first threshold extent is 10% or greater.

[0067] Our experiments indicated that in case an overlap of at least such an amount is not present, the other mode does not significantly deteriorate the measurement accuracy provided by the selected measurement mode, so it is not necessary to include openings providing selective damping.

[0068] If, however, there is an overlap by an amount that exceeds the first threshold value (threshold extent), then at least one wall area is defined along an inside wall of the resonator 10, 30 for which it holds true that, by replacing the wall area with an opening 19, 39 connecting the interior space 11 , 31 with a space outside of the resonator 10, 30,

[0069] - the quality factor of the measurement mode is reduced at most by a second threshold extent, said second threshold extent being 20% or lower, and

[0070] - the quality factor of the overlapping mode is reduced at least by a third threshold extent, said third threshold extent being 50% or greater, and, in the at least one wall area defined in this way, the opening 19, 39 is formed in the wall of the resonator 10, 30.

[0071] In our experiments leading to the invention, we have found that it is necessary to achieve the threshold values (threshold extents expressed as percentages) included above for the beneficial effects to be achieved by invention to occur to a significant extent, while under these threshold values the desired effects essentially cannot be detected.

[0072] In the method, the at least one wall area is defined

[0073] - empirically, - based on determining, by measurement or by calculation, a wall-adjacent pressure antinode or pressure antinodes of the overlapping mode,

[0074] - based on determining, by measurement or by calculation, a wall-adjacent pressure node or pressure nodes of the measurement mode,

[0075] - or by any combination of the above.

[0076] The modes and the related pressure antinodes and pressure nodes can be determined by calculation using various theoretical, numerical, or analytical methods. If the cavity has simple geometry (e.g., a box with flat walls, a cylinder or sphere), then the modes can be derived from the solutions of the corresponding wave equation with the right boundary conditions. In the case of more complex cavities, analytical solution is not possible, so for these cavities numerical techniques can be applied, for example FEM (Finite Element Method), FDTD (Finite- Difference Time-Domain), or BEM (Boundary Element Method), but theoretical oscillation models, acoustic impedance and boundary condition analysis or modal analysis may also be applied. All calculation methods lead to the same result: the individual oscillation patterns and resonance frequencies of the specific interior space are obtained.

[0077] According to the invention, preferably the mode that generates a signal with the greatest amplitude is selected as a measurement mode by the at least one acoustic detector 15. The mode that can be best detected by the acoustic detector 15, for example microphone, may in some case also be dependent on the characteristics of the detector. The at least one acoustic detector 15 is preferably placed at the wall- adjacent pressure antinode or pressure antinodes of the measurement mode, or at locations determined by empirically maximising the measurement signal-to-noise ratio.

[0078] As can be understood from some of the examples above, it is also possible to amplify the signal of a given mode by appropriately directing the laser light, which can also be performed by way of calculations or empirically. Therefore, a laser light path that maximises the photoacoustic measurement signal of the selected measurement mode is preferably adjusted by means of the laser light irradiation unit 14.

[0079] The lowest flow resistance for the medium flowing through the flow path can be provided by applying a resonator 10, 30 having an interior space 11 , 31 that defines a medium flow-through path extending from the medium inlet opening 12, 32 to the medium outlet opening 13, 33 and has a uniform cross-sectional area in a direction transverse to the medium flow-through path.

[0080] As it has been described in relation to Figs. 14-17, if the resonator 30 has a tubular shaped interior space 31 , the measurement mode is an azimuthal mode generated in the interior space 31 , and the overlapping mode is a radial mode generated in the interior space 31 , then the method comprises

[0081] - directing the laser light irradiation unit 14 to form a laser light path that extends parallel to and spaced apart from a longitudinal axis of the tubular shape,

[0082] - applying two acoustic detectors 15 that are disposed opposite each other at the middle of the length of the tubular shape, and at a plane defined by the longitudinal axis of the tubular shape and the path of the laser light, and

[0083] - forming two openings 39 in the wall of the tubular shape opposite each other, symmetrically to this plane and parallel to the longitudinal axis of the tubular shape, the openings 39 ending spaced apart from both edges of the tubular shape.

[0084] It is also preferable if the photoacoustic chamber unit is equipped with a bandpass filter that performs bandpass filtering matched to the frequency of the measurement mode on the signal supplied by the at least one acoustic detector 15.

[0085] Further components may also be included in the methods and photoacoustic chamber unit according to certain embodiments of the invention.

[0086] The configuration process may also include testing protection against external noise, and determining the operating frequency based on the tests. The photoacoustic chamber unit according to the invention is expediently capable of performing measurements without extractive sampling based on photoacoustic spectroscopy in the medium to be measured, without applying a forced flow of the medium to be tested. Because the resonator, i.e., the chamber, lets through the medium with as small a resistance as possible, it is also extremely sensitive to external acoustic noise. One of the most practicable ways of filtering out this noise is to apply such an acoustic frequency for the measurement at which the disturbance due to ambient noise is the lowest.

[0087] Configuration may further include determining the surface area-to-volume ratio, taking into consideration the highest signal amplification and best spatial detection. In the case of measurements performed in a flowing medium, for example in a pipe system, it is the flow area that can principally be used for determining the geometric dimensions. For atmospheric measurements, the surface area / length can be determined on the basis of the spatial angle to be tested.

[0088] Sampling errors can be reduced by appropriately determining the measurement and integration times to comply with requirements, and, on the basis of that, significant noise filtering can be achieved.

[0089] The appropriate acoustic mode can be selected at the frequency or frequencies obtained according to the above or based on the geometry determined according to the above.

[0090] The photoacoustic signal generated at the acoustic mode corresponding to the chosen geometry and operating frequency can be maximised by optimising photoacoustic signal generation, i.e., the laser light path. To achieve the optimal signal-to-noise ratio of the selected acoustic mode, optimal detector (microphone) placement and the optimal light path can be determined and adjusted empirically or by calculation.

[0091] According to the invention it is also possible to damp or eliminate undesired acoustic modes through the application of suitable openings or cutouts. The (multiple) excitable acoustic modes occurring with the given geometry at the measurement frequency cannot be eliminated by applying excitation and detection selected according to the above. To eliminate disturbing modes, such cutouts are included in the resonator that only minimally affect the chosen acoustic mode, but at the same time provide significant damping of the other (disturbing) acoustic modes.

[0092] The measuring body, i.e. , the photoacoustic chamber unit according to the invention is preferably suited for performing measurements without extractive sampling, primarily (but not exclusively) for measurements of gases with quickly changing acoustic parameters, i.e., among others exhaust gases, more particularly mainly for measurements with the unit built into the exhaust system of motor vehicles. To achieve this goal, as a first step, the ambient noises need to be examined, because due to its “open” geometry the measuring body reacts sensitively to such noises. In the next step, the free surface area and length of the measuring body are determined for a given exhaust system geometry. Thereafter, the signal-to-noise ratio of the measurements can be maximised according to the above. Maximisation is hindered by possible overlaps between the acoustic modes resulting from the geometry of the measuring body that may make it impossible to select an optimal solution; the overlapping acoustic modes may be active simultaneously in the gas having a quickly varying composition contained in the resonator, which makes it impossible to follow the characteristic frequency of the selected acoustic mode. The technical solution according to the invention allows for suppressing acoustic modes that, due to overlapping with the mode applied for measurements, prevent or make difficult the selection of optimal parameters and continuous and appropriate operation utilising the selected mode.

[0093] The above-described optimisation must be carried out at the frequency of photoacoustic signal generation / signal detection, i.e., these steps should be directed mainly at providing the best signal-to-noise ratio possible for the measurement at this frequency (or frequencies).

[0094] Thus, the measuring body is suited for photoacoustic measurements in freely flowing mediums without forced sample extraction, primarily for measuring exhaust gases, being either built into the exhaust system or being a part of a mobile sampling unit, among others (but not exclusively) located behind a vehicle, i.e. on a trailer, or mounted on drones or other UAV-s. Medium flow through the measuring body can be brought about in various ways. In addition to being applied in a medium flowing in a pipe system, in which case the measuring body may form a part of the pipe system as an extension piece, the measuring body can however also be utilised fixed to ground objects for performing atmospheric measurements.

[0095] However, the invention can be applied not only with the chamber, interior space, resonator, and modes disclosed above but also in the case of other chambers, interior spaces, resonators, and modes.

Claims

CLAIMS1 . A method for configuring a photoacoustic chamber unit, wherein the steps of the method are carried out on a chamber unit having a resonator (10, 30) defining an interior space (11 , 31 ), a medium inlet opening (12, 32) providing an inlet to the interior space (11 , 31 ), a medium outlet opening (13, 33) providing an outlet from the interior space (11 , 31 ), an irradiation unit (14) adapted for radiating a modulated laser light determining an acoustic measurement frequency into the interior space (11 , 31 ), and at least one acoustic detector (15) adapted for detecting sound waves generated in a medium contained in the interior space (11 , 31 ), characterised in that the method comprises the following steps:- selecting, as a measurement mode, one of the modes generated in the interior space (11 , 31 ) and being defined by the geometry of the interior space (11 , 31 ) and the acoustic measurement frequency,- checking whether there is a mode among the modes of which the full width at half maximum on a frequency-amplitude diagram overlaps, at least by a first threshold extent, with the full width at half maximum of the measurement mode, said first threshold extent being 10% or greater, and, in the negative case, considering the configuration of the chamber unit to be completed, whereas, in the affirmative case,- defining at least one wall area along an inside wall of the resonator (10, 30) for which it holds true that, by replacing the wall area with an opening (19, 39) connecting the interior space (11 , 31 ) with a space outside of the resonator (10, 30),- the quality factor of the measurement mode is reduced at most by a second threshold extent, said second threshold extent being 20% or lower, and- the quality factor of the overlapping mode is reduced at least by a third threshold extent, said third threshold extent being 50% or greater, and- forming, in the at least one wall area defined in this way, the opening (19, 39) in the wall of the resonator (10, 30).

2. The method according to claim 1 , characterised by defining the at least one wall area- empirically,- based on determining, by measurement or by calculation, a wall-adjacent pressure antinode or pressure antinodes of the overlapping mode,- based on determining, by measurement or by calculation, a wall-adjacent pressure node or pressure nodes of the measurement mode,- or by any combination of the above.

3. The method according to claims 1 or 2, characterised by selecting as a measurement mode the mode that generates a signal with the greatest amplitude by the at least one acoustic detector (15).

4. The method according to any of claims 1 to 3, characterised by placing the at least one acoustic detector (15)- at the wall-adjacent pressure antinode or pressure antinodes of the measurement mode, or- at locations determined by empirically maximising the measurement signal- to-noise ratio.

5. The method according to claim 4, characterised by adjusting, by means of the laser light irradiation unit (14), a laser light path that maximises a photoacoustic measurement signal of the selected measurement mode.

6. The method according to any of claims 1 to 5, characterised by applying a resonator (10, 30) having an interior space (11 , 31 ) that defines a medium flow- through path extending from the medium inlet opening (12, 32) to the medium outlet opening (13, 33) and has a uniform cross-sectional area in a direction transverse to the medium flow-through path.

7. The method according to claim 6, characterised in that if the resonator (30) has a tubular shaped interior space (31 ), the measurement mode is an azimuthal mode generated in the interior space (31 ), and the overlapping mode is a radial mode generated in the interior space (31 ), then- the irradiation unit (14) is directed to form a laser light path that extends parallel to and spaced apart from a longitudinal axis of the tubular shape,- two acoustic detectors (15) are applied that are disposed opposite each other at the middle of the length of the tubular shape, and at a plane defined by the longitudinal axis of the tubular shape and the path of the laser light, and- symmetrically to this plane and parallel to the longitudinal axis of the tubular shape, two openings (39) are formed in the wall of the tubular shape opposite each other, the openings (39) ending spaced apart from both edges of the tubular shape.

8. The method according to claim 7, characterised by choosing a combined surface area of the longitudinal openings (39) such that it is between 5% and 10%, preferably between 6% and 9%, and more preferably it is approximately 8% of the cylindrical surface area of the tubular shape.

9. The method according to any of claims 1 to 8, characterised by equipping the photoacoustic chamber unit with a bandpass filter that performs bandpass filtering matched to the frequency of the measurement mode on the signal supplied by the at least one acoustic detector (15).

10. A photoacoustic chamber unit for performing a photoacoustic measurement in a medium flow, comprising a resonator (10, 30) defining an interior space (11 , 31 ), a medium inlet opening (12, 32) providing an inlet to the interior space (11 , 31 ), a medium outlet opening (13, 33) providing an outlet from the interior space (11 , 31 ), an irradiation unit (14) adapted for radiating a modulated laser light determining an acoustic measurement frequency into the interior space (11 , 31 ), and at least one acoustic detector (15) adapted for detecting sound waves generated in a medium contained in the interior space (11 , 31 ), characterised in that- it further comprises at least one retaining unit (17) that is connected to the resonator (10, 30) and adapted for retaining the resonator (10, 30) in the medium flow,- the resonator (10, 30) has an interior space (11 , 31 ) that defines a medium flow-through path extending from the medium inlet opening (12, 32) to the medium outlet opening (13, 33), and- at least one through opening (19, 39) that is adapted to connect the interior space (11 , 31 ) with a medium flow space outside the resonator (10, 30) and is separated from the medium inlet opening (12, 32) and from the medium outlet opening (13, 33) by the material of the resonator (10, 30) is formed in the wall of the resonator (10, 30) such that it extends transversely with respect to the medium flow-through path.

11. The photoacoustic chamber unit according to claim 10, characterised in that a location and a shape of the at least one through opening (19, 39) are determined such that for each point within an area of the opening (19, 39) the following conditions are met concerning the modes generated in the interior space (11 , 31 ), the modes being determined by the geometry of the interior space (11 , 31 ) and the acoustic measurement frequency:- each point within the area of the opening (19, 39) is closer to a nearest pressure node of a first mode than to a nearest pressure node of a second mode, and- each point within the area of the opening (19, 39) is farther from a nearest pressure antinode of the first mode than from a nearest pressure antinode of the second mode.

12. The photoacoustic chamber unit according to claims 10 or 11 , characterised in that a location and a shape of the at least one through opening (19, 39) is determined such that, in comparison with a state without the at least one through opening (19, 39), it holds true that- the quality factor of the first mode is reduced at most by a second threshold extent, said second threshold extent being 20% or lower, and- the quality factor of the second mode is reduced at least by a third threshold extent, said third threshold extent being 50% or greater.

13. The photoacoustic chamber unit according to any of claims 10-12, characterised in that it comprises a resonator (10, 30) having an interior space (11 , 31 ) that has a uniform cross-sectional area in a direction transverse to the medium flow-through path.

14. The photoacoustic chamber unit according to claim 13, characterised in that the resonator (30) has a tubular interior space (31 ), the first mode is an azimuthal mode generated in the interior space (31 ), and the second mode is a radial mode generated in the interior space (31 ), and- the irradiation unit (14) is directed to form a laser light path that extends parallel to and spaced apart from a longitudinal axis of the tubular shape,- it comprises two acoustic detectors (15) that are disposed opposite each other at the middle of the length of the tubular shape, and at a plane defined by the longitudinal axis of the tubular shape and the path of the laser light, and- symmetrically to this plane and parallel to the longitudinal axis of the tubular shape, two openings (39) are formed in the wall of the tubular shape opposite each other, the openings (39) ending spaced apart from both edges of the tubular shape.

15. The photoacoustic chamber unit according to claim 14, characterised in that the combined surface area of the longitudinal openings (39) is between 5% and 10%, preferably between 6% and 9%, and more preferably it is approximately 8% of the cylindrical surface area of the tubular shape.

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