DE102014211514A1 - Method for determining the flow rate, the volume flow and the mass flow of particles - Google Patents

Abstract

The invention relates to a method for determining the flow rate of particles through a surface or the volume flow and a measuring device for carrying out the method. Furthermore, the invention relates to a method for operating a paint shop and spray drying plant. For this purpose, a plurality of time-shift measuring devices each having a light source for generating a light beam are used, wherein the light beams of the time-shift measuring devices form skewed straight lines and wherein the area is determined as a sectional area of the parallel projections with respect to the particle flow direction of the light beams of the time shifters that have detected the particles.

Description

The invention relates to a method for determining the flow rate of particles through a surface or the volume flow and a measuring device for carrying out the method. Furthermore, the invention relates to a method for operating a paint shop and spray drying plant.

The determination of various characteristic properties of individual particles whose size is in the range of millimeters and smaller, is of great importance for research as well as for the industrial and commercial use of products or processes. Often, the characteristics of interest relate to the size, shape, speed and refractive index of individual particles. The simultaneous determination of both the size and the velocity of individual particles is of particular interest, since with this information, a flux density such as a mass flow or a volume flow can be determined. In addition, individual particles in a large number of particles can be identified and characterized individually, such as individual droplets in an aerosol or spray.

The determination of characteristic properties of individual droplets is required, for example, for the optimization of injection processes of a fuel into a combustion chamber or for the characterization of a spray of a paint or a paint during a spraying process. The particles whose properties are to be determined, are not only liquid droplets in a gas such as air, but depending on the application solid particles, gas bubbles in a liquid or a droplet emulsion of a first liquid, which is distributed in a second liquid.

From practice, various measuring methods are known. In many cases, optical measuring methods are advantageous because they do not or not significantly affect the individual particles whose properties are to be determined.

The optical measuring methods known from practice and from research include, for example, high-resolution imaging techniques, intensity measurements, interferometry or the evaluation of reflected and refracted or refracted light beams which are scattered by a particle to be measured.

A method of the type mentioned in the introduction is, for example, the time-shifting method, which makes it possible to determine the size of transparent and almost transparent particles. This is for example in N. Damaschke, H. Nobach, N. Semidetnov, C. Tropea (2002) Optical Particle Sizing in Backscatter, Applied Optics 41, 5713-5727 or A. Kretschmer, N. Damaschke, N. Semidetnov, C. Tropea (2006) Application of the Time-Shift Technique for Spray Measurement, 13th Int. Symp to Appl. Laser Techniques to Fluid Mechanics, Lisbon, Portugal, June 26-29, 2006 , described.

In the time-shifting method, a time-shifting device is used which comprises a light source for emitting a usually focused light beam traversed by the particles to be measured, and a number of radiation detectors, usually two radiation detectors, each having a time-resolved intensity distribution of light scattered by the particles at predetermined scattering angles measure the light source. A particle traversing the light beam emits or reflects different scattering portions which are received by the radiation detectors and displayed via the time-resolved intensity distribution. Among other things, these different scattered parts are reflections, surface waves and refractions of various orders and their modes, which reach the radiation detectors with a time delay. In this case, the time between the detected intensity peaks or scattering proportions is proportional to the size and the velocity of the particle.

These methods, which are based on the evaluation of refracted light beams, are hardly or not at all suitable for non-transparent or opaque translucent particles. The intensity of refracted rays is usually so low in opaque particles that no meaningful evaluation of the measurement results is possible. For emulsions such as milk (dissolved fat droplets in water) or for suspensions such as paints (dissolved color pigments in solvents), therefore, a different evaluation of the data is necessary. Opaque translucent particles are considered to be all particles which are neither completely transparent (for example water) nor have an almost completely reflecting interface (for example metal). In these particles, therefore, usually two or more radiation detectors are used, which are each arranged at a certain angle to the light beam. With them, the size of a particle can be calculated from the time delay between equal scattered portions of the intensity profile of several radiation detectors, as long as the velocity of the particles is known or measured.

One method for determining the velocity of the particles is the time-of-flight Measuring method in which a plurality of light sources are arranged such that the plurality of emitted light beams are aligned parallel to each other and in the direction of flight of the particles spaced apart. One or more radiation detectors then measure the scattering components already mentioned above when crossing the individual light beams. From the time difference of the same intensity peaks of the different intensity gradients and the knowledge of the distance of the light beams, the airspeed of the particles can be determined.

With a combination of the two measuring methods, a large number of relevant droplet properties can thus be determined. A disadvantage of both methods, however, is that the throughput of the particles, the volume and / or mass flow can not be determined, since no precise knowledge of the location is known at which the particles traverse the light beam or the light rays. The throughput of the particles is defined as the number of particles per time per unit area, while the volume flow indicates the sum of the volumes of the particles per unit time and per unit area. The mass flow is correspondingly the sum of the masses of the particles which pass through a certain unit area per unit of time. At constant density, the mass flow thus simply corresponds to the density multiplied by the volume flow.

The invention is therefore based on the object to provide a method and a measuring device for determining the particle properties, in which also the throughput, the volume and the mass flow can be measured.

• – die Anzahl der Teilchen bestimmt wird, die sowohl in einer ersten und auch mindestens in einer weiteren, zweiten Zeitverschiebungsmesseinrichtung detektiert werden,
• – die Lichtstrahlen der Zeitverschiebungsmesseinrichtungen windschiefe Geraden bilden
• – und die Fläche als Schnittfläche der Parallelprojektionen bezogen auf die Teilchenflussrichtung der Lichtstrahlen der Zeitverschiebungseinrichtungen bestimmt wird, die die Teilchen detektiert haben
• – und anschließend der Durchsatz als Quotient der Anzahl der Teilchen in einem gemessenen Zeitintervall und der Schnittfläche ermittelt wird.

This object is achieved according to the invention in terms of throughput by a plurality of time displacement measuring devices are each provided with a light source for generating a light beam and wherein

• Determining the number of particles which are detected both in a first and also in at least one further second time-displacement measuring device,
• The light beams of the time-shift measuring devices form skewed straight lines
• - And the area is determined as a sectional area of the parallel projections with respect to the particle flow direction of the light beams of the time-shifting devices that have detected the particles
• - And then the throughput is determined as a quotient of the number of particles in a measured time interval and the sectional area.

• – die Anzahl der Teilchen bestimmt wird, die sowohl in einer ersten und auch mindestens in einer weiteren, zweiten Zeitverschiebungsmesseinrichtung detektiert werden,
• – die Lichtstrahlen der Zeitverschiebungsmesseinrichtungen windschiefe Geraden bilden
• – und wobei eine Fläche als Schnittfläche der Parallelprojektionen bezogen auf die Teilchenflussrichtung der Lichtstrahlen der Zeitverschiebungseinrichtungen bestimmt wird, die die Teilchen detektiert haben
• – und wobei die Flugzeit t_FZ der Teilchen vom Lichtstrahl der ersten Zeitverschiebungseinrichtung zum Lichtstrahl der zweiten Zeitverschiebungseinrichtung bestimmt wird, um anschließend aus der Flugzeit und dem Abstand der Lichtstrahlen der ersten und zweiten Zeitverschiebungseinrichtung im Bereich der Schnittfläche die mittlere Geschwindigkeit des Teilchens zu ermitteln,
• – und wobei für jedes Teilchen aus mindestens einer der Zeitverschiebungseinrichtungen, die das Teilchen detektiert hat, die Zeitverschiebung t_ZV bestimmt wird, um anschließend aus der Zeitverschiebung t_ZV und der Geschwindigkeit die Größe des Teilchens zu bestimmen,
• – und wobei aus der Größe der Teilchen das jeweilige Volumen bzw. die Masse bestimmt wird und somit der Volumen- bzw. Massenstrom als Quotient der Summe der Volumina bzw. Massen, die pro Zeitintervall die Fläche durchqueren und der Fläche ermittelt wird.

With respect to the volume and mass flow, the object is achieved according to the invention by providing a plurality of time displacement measuring devices, each with a light source for generating a light beam, and wherein

• Determining the number of particles which are detected both in a first and also in at least one further second time-displacement measuring device,
• The light beams of the time-shift measuring devices form skewed straight lines
• - And wherein a surface is determined as a sectional area of the parallel projections with respect to the particle flow direction of the light beams of the time-shifting devices that have detected the particles
• And wherein the time of flight t _{FZ of} the particles from the light beam of the first time-shifting device to the light beam of the second time-shifting device is determined, in order subsequently to determine the mean velocity of the particle from the time of flight and the distance of the light beams of the first and second time-shifting device in the region of the section surface,
• And wherein for each particle of at least one of the time-shifting devices that detected the particle, the time shift t _{ZV is} determined in order subsequently to determine the size of the particle from the time shift t _ZV and the velocity,
• - And wherein from the size of the particles, the respective volume or the mass is determined and thus the volume or mass flow as a quotient of the sum of the volumes or masses, which traverse the surface per time interval and the surface is determined.

With regard to the measuring device, the stated object is achieved according to the invention by providing a plurality of time displacement measuring devices each having a light source for generating a light beam, wherein the light sources of the time displacement measuring devices are aligned such that the light beams form skewed straight lines and the measuring surface through the sectional surface of the parallel projections is defined based on the particle flow direction of the light beams.

Advantageous embodiments of the invention are the subject of the dependent claims.

The invention is based on the consideration that for a determination of the throughput or of the volume or mass flow, the location or the area at which the pond has traversed the light beam or the light beams has to be determined. However, such a determination of the range is not possible or not without costly further measuring apparatus in the previous time shift measurement or the time-of-flight measurement method, in particular due to the expansion of the light beam in the beam direction. For as possible compact measuring device is resorted to a plurality of the already known time-shifting devices and the light beams not - as for example known from the time of flight measurement - parallel and spaced aligned, but skewed. Windfall are the light rays when they are neither intersecting nor aligned parallel to each other. Projected on a plane perpendicular to the direction of motion of the particles, the light beams thus form an angle φ, with 0 ° <φ <180 °. As a result, the parallel projections cross with respect to the particle flow direction of the light beams of the time shifters and define a sectional area. The size and position of the cut surface can be optimized by aligning the light beams and adapted to the particle flow. The idea is that individual particles detected in multiple time shifters must have traversed the intersection of the light beams of the respective time shifters, thereby enabling localization of the particles and determination of the particles per unit area. In this case, the velocity of the particles and from this the size of the particles and further data can be determined from the spaced-apart light beams, as described above in the context of the time-of-flight measurement method.

In order to determine the time shift t _ZV , the intensity profiles measured by the radiation detectors are evaluated according to characteristic scattered light _peaks in accordance with the methods described above in a preferred embodiment. In this case, from the data of different characteristic scattered light peaks of a measured intensity curve relevant particle data, such as the diameter of the particles, from the time difference of the scattered light peaks can be determined, in alternative or additional embodiment but also from the data of the same characteristic scattered light peaks from several measured intensity gradients of radiation detectors of a light source or a time displacement measuring device.

To determine the velocity of the particles, in a particularly preferred embodiment, the time of flight of the particles is determined, which requires the particle from one light beam to another light beam. For this purpose, the plurality of time-shifting devices preferably comprise a number of radiation detectors which, under given scattering angles, each measure a time-resolved intensity profile of light of the associated light source scattered on the particle. Knowing the distance of the light beams and thus the flight path that travels the particle from one light beam to the other light beam, the speed can be determined by the time shift of the same characteristic scattered light peaks in the measured intensity progressions, the radiation detectors associated with the two light beams.

In order to avoid false measurement data or incorrect evaluations of the measurement data on the basis of measured signals of different particles, but which are assigned to a particle, the measured scattered light peaks are assigned to the particles. In a preferred embodiment, corresponding methods are used for the validation of the particles from the prior art, which are described, for example, in the document WO 2013/139691 A2 for opaque translucent particles or from the pamphlets WO 2013/024167 A1 and WO 2013/024166 A1 are known for transparent particles and their disclosures should also be part of this description. Thus, it is possible to identify and evaluate the characteristic scattered light peaks of a particle, even if there are several particles that are measured in the radiation detectors, thus with several, overlapping intensity profiles of several particles.

For a compact arrangement on the one hand and an optimal cut surface of the light beams, the light sources of the time displacement measuring devices are arranged in an advantageous embodiment such that the parallel projections with respect to the particle flow direction of the light beams of the first and second time shifter include an angle of φ = 10 ° -30 °. The detection direction of the radiation detectors also spans an angle of Θ = 150 ° -170 ° with respect to the propagation direction of the light beam for the most compact possible construction.

The method for determining the throughput of particles or the volume or mass flow is used in a preferred embodiment in paint shops. In the process, paint droplets are examined and the paint shop is controlled based on the measured and calculated data. Thus, it can always and directly checked whether a desired amount of paint is used and also a preferred size of the paint particles is applied to the object to be painted, so that a simple and direct optimization or new regulation of the painting process is possible. Corresponding areas of application also arise in injection technology and in spray drying, in which it can be checked by determining the volume flow, for example, whether the spraying process is running properly.

The advantages achieved by the invention are in particular that a cut surface is achieved by a combination of two or more time displacement measuring devices whose light beams are all or partially skewed is defined by which the flow rate and the volume and mass flow can be determined. In addition, due to the spaced, skewed light beams, a determination of the speed is made possible analogously to the time-of-flight measurement method.

An embodiment of an invention will be explained in more detail with reference to a drawing. Show:

1a a schematic representation of a time displacement arrangement according to the prior art,

1b a schematic representation of a measured intensity distribution of the scattered light of a particle in the time-shifting arrangement,

2a a schematic representation of a time-of-flight measurement according to the prior art,

2 B a schematic representation of a measured intensity distribution of the scattered light of a particle in the time of flight measurement,

3a a schematic representation of a time shift arrangement with two crossed light beams,

3b a schematic representation of a measured intensity distribution of the scattered light of a particle in the time shift arrangement with two crossed light beams.

The same parts are provided in all figures with the same reference numerals.

The time shift measuring device 1 to 1 includes a light source 2 that a focused beam of light 4 emits and two radiation detectors 6a . 6b at an angle Θ _{a, b} of about Θ _{a, b} = 150 ° -170 ° to the light beam 4 are arranged. The radiation detectors 6a . 6b thereby measure scattered portions of the light beam 4 by one the light beam 4 traversing particles 8th For example, a droplet or a particle may be reflected or emitted. These scattering components can be, inter alia, reflections, surface waves and refractions of various orders and their modes. At the same time, the different stray portions hit the first radiation detector with a time delay 6a and additionally delayed in the second radiation detector 6b , The two time-resolved intensity gradients 10a . 10b can then be passed through an evaluation unit 12 after characteristic intensity peaks 14a . 14b be evaluated and the time delay between equal intensity peaks 14a . 14b at both radiation detectors 6a . 6b be measured. A schematic representation of such intensity gradients 10a . 10b is in 1b shown.

The time delay t _ZV depends on the diameter of the particles 8th , the speed of the particles 8th and a function generally known from light scattering as a function of the angle of the radiation detectors Θ _{a, b} . For the determination of the diameter of the particles 8th thus still the speed of the particle becomes 8th needed.

A time of flight measuring device 16 for measuring the time of flight of the particles 8th is in the 2 shown. This time of flight measuring device 16 includes one or more light sources 20 comprising a plurality of parallel and spaced light beams 22 emit and send out and a radiation detector 24 , which at an angle Θ ₂ to the rays of light 22 is arranged and the time-resolved intensity distribution 26 the backward scattering of the particles 8th measures. An exemplary intensity distribution 26 of a particle 8th is in 2 B shown. Also with this intensity distribution 26 can with the help of an evaluation unit 28 now characteristic intensity peaks 30 be determined and the scattering of the individual light rays 22 be assigned. From the time difference t _FZ between equal intensity peaks 30 and from the structure of the time-of-flight measuring device 16 known distance of the light beams b, the velocity of the particle can be determined. From a combination of the time-shifting device 1 and the time of flight measuring device 16 it is thus possible the diameter of the particles 8th to determine.

A measuring device 32 for determining the flow rate and the volume and mass flow is the embodiment of the 3 refer to. In this case, two time displacement measuring devices are exemplary 1a . 1b arranged such that the emitted light beams 4a . 4b form skewed straight lines, which means that the light rays 4a . 4b neither cut nor aligned parallel to each other. The rays of light 4a . 4b are further aligned such that their parallel projections span an angle φ with respect to the particle flow direction and a sectional area in the region of the particle flow 34 define. This parallel projection is schematically in an enlarged partial view of 1 shown. Particles from both time-shift meters 1a . 1b be detected, thus have both light beams 4a . 4b traverses and can be in the area of the cut surface 34 be located. To do this, the intensity distributions become 36a . 36b . 36c . 36d that is schematically in 3b are shown, both time shifting devices 1a . 1b with an evaluation unit, not shown, for characteristic intensity peaks 38a . 38b . 38c . 38d - evaluated and these intensity peaks 38a . 38b . 38c . 38d corresponding particles 8th so as to identify the particles 8th to be able to make. In doing so, appropriate procedures for the validation of the particles 8th used from the prior art, for example, from the document WO 2013/139691 A2 for opaque translucent particles or from the pamphlets WO 2013/024167 A1 and WO 2013/024166 A1 are known for transparent particles and their disclosures should also be part of this description.

Due to the combination of two or more time shift arrangements 1a . 1b , where all emitted focused light beams 4a . 4b or only part of the light beams form skewed straight lines, a combination of the time shift measurement and the time of flight measurement is achieved. By an appropriate arrangement of skewed light rays 4a . 4b , these form in the area of the cut surface 34 and in the direction of the particle flow a distance b to each other, through which the time of flight t _FZ and thus the speed can be determined. On the basis of the further data of the radiation detectors then, as usual in the time shift measurement, the diameter of the particles can be determined.

When evaluating the data over a certain period of time, the number of particles and their respective size and thus the volume and mass can be determined, which have crossed the defined intersection within the measurement period. From this, the desired data for throughput and volume and mass flow can be determined. This data can be used in a further step to monitor the particle flow and to control and regulate depending on the application. In particular, when operating a paint shop, a spray drying plant or injection system can be optimized and monitored by monitoring the characteristic properties of the particle flow processes.

LIST OF REFERENCE NUMBERS

1, 1a, 1b

Time difference measuring device

2

light source

4, 4a, 4b

beam of light

6a, 6b

Strahlungsdetekor

8th

particle

10a, 10b

Intenstiätsverteilung

12

evaluation

14a, 14b

characteristic intensity peaks

16

Flight time measuring device

20

light sources

22

light rays

24

radiation detector

26

Intenstiätsverteilung

28

evaluation

30

characteristic intensity peaks

32

measuring device

34

section

36a, 36b, 36c, 36d

Intenstiätsverteilung

38a, 38b, 38c, 38d

characteristic intensity peaks

Θ _{a, b}

Angle of the radiation detectors in the ZV

Θ2

Angle of the radiation detector in the FZ

b

Distance of the light rays in the FZ

φ

Angle of light rays

t _ZV

time shift

t _FZ

flight time

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2013/139691 A2 [0019, 0034]
• WO 2013/024167 A1 [0019, 0034]
• WO 2013/024166 A1 [0019, 0034]

Cited non-patent literature

• N. Damaschke, H. Nobach, N. Semidetnov, C. Tropea (2002) Optical Particle Sizing in Backscatter, Applied Optics 41, 5713-5727 [0006]
• A. Kretschmer, N. Damaschke, N. Semidetnov, C. Tropea (2006) Application of the Time-Shift Technique for Spray Measurement, 13th Int. Symp to Appl. Laser Techniques to Fluid Mechanics, Lisbon, Portugal, June 26-29, 2006 [0006]

Claims (13)

Method for determining the throughput of particles ( 8th ) through an area ( 34 ), wherein the particles move in a particle flow direction, with a plurality of time-shift measuring devices ( 1a . 1b ) each with a light source ( 2 ) for generating a light beam ( 4a . 4b ), a. where the number of particles ( 8th ) is determined in both a first and at least in a further, second time displacement measuring device ( 1a . 1b ) are detected, b. where the light rays ( 4a . 4b ) of the time-shift measuring devices ( 1a . 1b ) skewed lines form c. and where the area ( 34 ) as a cut surface ( 34 ) of the parallel projections with respect to the particle flow direction of the light beams ( 4a . 4b ) of time-shifting devices ( 1a . 1b ) determining the particles ( 8th ) have detected d. and then the throughput as a quotient of the number of particles ( 8th ) in a measured time interval and the sectional area ( 34 ) is determined. Method for determining the volume or mass flow of particles, whereby the particles ( 8th ) in a particle flow direction, with a plurality of time-shift measuring devices ( 1a . 1b ) each with a light source ( 2 ) for generating a light beam ( 4a . 4b ), a. where the number of particles ( 8th ) is determined in both a first and at least one further second time displacement measuring device ( 1a . 1b ) are detected, b. where the light rays ( 4a . 4b ) of the time-shift measuring devices ( 1a . 1b ) skewed lines form c. and wherein an area ( 34 ) as a cut surface ( 34 ) of the parallel projections with respect to the particle flow direction of the light beams ( 4a . 4b ) of time-shifting devices ( 1a . 1b ) determining the particles ( 8th ) have detected d. and wherein the time of flight t _{FZ of} the particles ( 8th ) from the light beam ( 4a ) of the first time-shifting device ( 1a ) to the light beam ( 4b ) of the second time-shifting device ( 1b ) is determined in order subsequently to determine from the time of flight t _FZ and the distance b of the light beams ( 4a . 4b ) of the first and second time-shifting devices ( 1a . 1b ) in the area of the cut surface ( 34 ) to determine the mean velocity of the particle, e. and wherein for each particle at least one of the time-shifting devices ( 1a . 1b ), which is the particle ( 8th ) has been detected, the time shift t _{ZV is} determined in order then to determine from the time shift t _ZV and the velocity the size of the particle ( 8th ), f. and where from the size of the particles ( 8th ) the respective volume or the mass is determined and thus the volume or mass flow as a quotient of the sum of the volumes or masses, the per area of time the area ( 34 ) and the surface ( 34 ) is determined. Method for determining the volume flow of particles ( 8th ) according to claim 2, characterized in that the first and / or second time-shifting device ( 1a . 1b ) at least two radiation detectors ( 6a . 6b ) which, under predetermined scattering angles Θ _{a, b,} each have a time-resolved intensity profile ( 36a . 36b . 36c . 36d ) of the particle ( 8th ) scattered light of the light source ( 2 ), whereby in the intensity curves ( 36a . 36b . 36c . 36d ) characteristic scattered light peaks ( 38a . 38b . 38c . 38d ) and the time shift t _ZV on the basis of the time difference between similar scattered light peaks ( 38a . 38b . 38c . 38d ) at least two intensity profiles ( 36a . 36b . 36c . 36d ) a time-shifting device ( 1a . 1b ) is determined. Method for determining the volume flow of particles ( 8th ) according to claim 2, characterized in that the first and / or second time-shifting device ( 1a . 1b ) at least one radiation detector ( 6a . 6b ), which at a predetermined scattering angle Θ _{a, b has} a time-resolved intensity profile ( 36a . 36b . 36c . 36d ) of the particle ( 8th ) scattered light of the light source ( 2 ), whereby in the intensity curve ( 36a . 36b . 36c . 36d ) characteristic scattered light peaks ( 38a . 38b . 38c . 38d ) and the time shift t _ZV on the basis of the time difference between two characteristic scattered light peaks ( 38a . 38b . 38c . 38d ) is determined. Method for determining the volume flow of particles ( 8th ) according to one of Claims 2 to 4, characterized in that the first and second time-shifting devices ( 1a . 1b ) a number of radiation detectors ( 6a . 6b ) which, under predetermined scattering angles Θ _{a, b,} each have a time-resolved intensity profile ( 36a . 36b . 36c . 36d ) of the particle ( 8th ) scattered light of the associated light source ( 2 ), whereby in the intensity curves ( 36a . 36b . 36c . 36d ) characteristic scattered light peaks ( 38a . 38b . 38c . 38d ) and t _FZ based on the time difference between similar scattered light peaks ( 38a . 38b . 38c . 38d ) of the intensity courses ( 36a . 36b . 36c . 36d ) of a radiation detector of the first and second time-shifting device ( 1a . 1b ) is determined. Method for determining the throughput of particles ( 8th ) through an area ( 34 ) or the volume flow of particles ( 8th ) according to one of the preceding claims, characterized in that an identification of the particles in the time-shift measuring devices ( 1a . 1b ) is made. Method for determining the throughput of particles ( 8th ) through an area ( 34 ) or the volume flow of particles ( 8th ) according to one of the preceding claims, characterized in that the parallel projections with respect to the particle flow direction of the light beams of the first and second time-shifting device ( 1a . 1b ) enclose an angle of 10 ° -30 °. Method for operating a paint shop, wherein the throughput of the paint droplets through an area ( 34 ) or the volume or mass flow of the paint droplets is determined according to one of claims 1 to 7 and is used as input for the adjustment of the paint shop. Method for operating a spray-drying plant, wherein the throughput of the food particles through an area ( 34 ) or the volume or mass flow of Speiseteilchen is determined according to one of claims 1 to 7 and is used as input for the adjustment of the spray-drying plant. Measuring device ( 32 ) for determining the flow rate or volume flow of particles ( 8th ) by a measuring surface ( 34 ), wherein the particles move in a particle flow direction, with a plurality of time-shift measuring devices ( 1a . 1b ) each with a light source ( 2 ) for generating a respective light beam ( 4a . 4b ), the light sources ( 2 ) of the time-shift measuring devices ( 1a . 1b ) are aligned such that the light beams ( 4a . 4b ) skewed straight lines and the measuring surface ( 34 ) through the cut surface ( 34 ) of the parallel projections with respect to the particle flow direction of the light beams ( 4a . 4b ) is defined. Measuring device ( 32 ) for determining the flow rate or volume flow of particles ( 8th ) by a measuring surface according to claim 10, characterized in that the parallel projections with respect to the particle flow direction of the light beams ( 4a . 4b ) of the first and second time-shifting devices ( 1a . 1b ) enclose an angle of φ = 10 ° -30 °. Measuring device ( 32 ) for determining the flow rate or volume flow of particles ( 8th ) by a measuring surface according to claim 10 or 11, characterized in that two time displacement measuring devices ( 1a . 1b ) are provided. Measuring device ( 32 ) for determining the flow rate or volume flow of particles ( 8th ) by a measuring surface according to one of claims 10 to 12, characterized in that each time displacement measuring device ( 1a . 1b ) a number of radiation detectors ( 6a . 6b ) at an angle of Θ _{a, b} = 150 ° -170 ° to the associated light beam ( 4a . 4b ) are arranged.

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