Method for determining a spatially resolved charge distribution of electrical charges of particles in a gas stream and pneumatic conveying device

Description

[0001] The invention relates to a method for determining at least the spatially resolved charge distribution of electrical charges in particles in a gas stream. According to a second aspect, the invention relates to a pneumatic conveying device according to the preamble of claim 1.

[0002] When particles are transported in a gas stream, especially an air stream, they often become electrostatically charged. If this electrostatic charge becomes too high, a spark discharge can occur. This is particularly detrimental if an exothermic chemical reaction between the particles and the gas in the gas stream is possible. This can then lead to an explosion.

[0003] The generation of electrostatic charging of particles conveyed by a gas stream is the subject of intensive research but is still poorly understood.

[0004] To determine a charge characteristic in the form of the total electrical charge of particles in the gas stream, so-called Faraday cups are currently used. The particles induce charges at the Faraday-Becker, which are then measured. Such a setup is described in the paper by Susanti and Großhans "Measurement of the deposit formation during pneumatic transport of PMMA powder," in: Advanced powder technology, Vol. 31, 2020, pp. 3597-3609. - ISSN 0921-8831. The disadvantage of this measurement method is that only the sum of the charges is recorded, not the respective positive and negative charges.

[0005] Another disadvantage is that the electrostatic charge cannot be measured with spatial resolution.

[0006] WO 2014 / 106078 A2 discloses probes for monitoring electrostatic phenomena in challenging environments, such as fluidized-bed reactors. These probes include a coated or uncoated static probe for measuring the electric field and / or the charge state of the particles, an oscillating electric field probe for measuring the electric field, a split electric field probe for measuring the electric field, and a radio-frequency antenna probe for detecting electrostatic discharges. A modified surface ensures that particles impacting the electrode reduce the triboelectric charging of the probe.

[0007] US Pat. No. 6,049,382 describes an apparatus and method for characterizing sprays consisting of spherical particles. A laser source generates a collimated laser beam, which is guided through the spray to be characterized. The laser beam coincides with the x-axis of a Cartesian coordinate system located in a measurement plane perpendicular to the spray axis. 90° scattering generated by the spray material in a small probe volume formed at the intersection of the laser beam and the object volume of the scattering collection means is detected. Additional detection means detect the attenuation of the laser beam traversing the measurement plane. The optical systems are coupled to photodetectors and signal processing units capable of generating electrical signals proportional to the received light intensities.A transverse device moves the spray in the direction of the laser beam and perpendicular to it to sequentially obtain a tomographic recording of the scattering and attenuation activity at points in the spray that form a Cartesian grid within the measurement plane. Concentration measuring devices are coupled to the electrical output signals of the attenuation and scattering to obtain information about the number density of the spray at the nodes of the tomographic grid system.

[0008] US Pat. No. 8,470,151 B2 presents a microfluidic pumping approach that utilizes traveling-wave dielectrophoresis (tw-DEP) of microparticles. The flow is generated directly in the microfluidic objects by inducing electromechanical effects in the fluid through microelectrodes. The fluidic propulsion mechanisms due to particle-fluid and particle-particle interactions under traveling-wave dielectrophoresis are analyzed, and the induced flow field is obtained from numerical simulations.

[0009] EP 3 431 264 B1 describes a 3D printer having an optical determination device by means of which a parameter suitable for characterizing the flow properties of the gas flow through the process chamber is determined. The optical determination device has an optical measuring unit for optically measuring a measured value within the process chamber, wherein the measured value is related to the gas flow. An evaluation unit determines the at least one parameter from the measured value. The measured values ​​are measured at different locations within the process chamber. This allows the particle flow to be well controlled during printing.

[0010] In the paper by Großhans et al. "Exploring the mechanism of inter-particle charge diffusion. In: The European physical journal: applied physics, Vol. 82, 2018, No. 1, Art. 11101 (9 8.), ISSN 1286-0042 a numerical mathematical model for inter-particle charge transfer is presented.

[0011] WO 2021 / 005396 A1 and JP 2015 102386 A disclose methods for determining the velocity of droplets or particles in water.

[0012] The invention is based on the object of reducing disadvantages in the prior art.

[0013] The invention solves the problem by a method for determining the spatially resolved charge distribution with the features of claim 1.

[0014] According to a second aspect, the invention solves the problem by a pneumatic conveying device having the features of claim 8.

[0015] An advantage of the invention is that it provides spatially resolved information about the charge distribution in the gas stream. Unlike prior art methods, it is possible to detect any areas of positive and negative charge. Methods using a Faraday cup only determine the total charge. Electrostatic fields that may be formed between regions of the gas stream and / or between particles cannot be detected using prior art methods, but can be detected using the method according to the invention.

[0016] It is advantageous that the risk of explosion can generally be detected much better, since the conditions for ignition sparks, namely an excessively large electrostatic field, which is present independently of any electrical measuring field present, can be determined with greater certainty.

[0017] Another advantage is that the charge characteristic can be measured without contact. Therefore, the measurement does not significantly influence the gas flow and / or the particles. In particular, it is preferable not to decouple any part of the gas flow to measure the charge characteristic.

[0018] It is therefore advantageous if the charge characteristic is measured without particle loss. In In other words, it is not necessary to remove particles from the gas stream, at least temporarily, in order to determine the charge characteristic.

[0019] In the context of the present description, the electrical measuring field is preferably understood to be an at least substantially homogeneous measuring field. An at least substantially homogeneous measuring field is understood in particular to mean that the measuring field may be inhomogeneous, but that this inhomogeneity is so weak that it increases the measurement uncertainty in determining the charge characteristic by a maximum of 5%. However, it is also possible to use an inhomogeneous measuring field.

[0020] The spatially resolved determination of the particle velocity is understood in particular to mean that the particle velocity is determined as a function of at least one spatial coordinate. In particular, this spatial coordinate runs perpendicular to a flow direction of the gas stream.

[0021] The particle velocities are preferably determined with respect to at least two spatial coordinates. In particular, determining the particle velocities includes a particle velocity component transverse to the flow direction and / or along the flow direction. It is possible, but not necessary, for the particle velocity to be known in all three spatial coordinates. It is also possible, but not necessary, for the particle velocities to be known as a function of two or three spatial coordinates; however, it is generally sufficient if the particle velocities are known as a function of one spatial coordinate.

[0022] The feature that the measuring field is applied transversely to the flow direction is understood in particular to mean that it is possible, but not necessary, for the electrical measuring field to be applied perpendicular to the flow direction in the mathematical sense. The flow direction is understood to be the macroscopic flow direction of the gas stream. If the fluid line in the measuring area is prismatic, as provided in a preferred embodiment, i.e., if it is a non-curved pipe, for example, the flow direction corresponds to the longitudinal direction of the fluid line.

[0023] The feature that at least one charge characteristic is determined is understood in particular to mean that it is possible, but not necessary, for exactly one charge characteristic to be determined. This charge characteristic can, for example, be a maximum value of the charge density. Alternatively or additionally, it can be the magnitude of the maximum gradient.

[0024] Determining particle velocity refers, in particular, to determining temporal averages. The temporal averages preferably refer to an averaging time between one second and one minute.

[0025] The term “diameter” refers to the Sauter diameter according to DIN ISO 9276.

[0026] However, it is particularly advantageous if the determination of at least one charge characteristic involves the determination of a plurality of charge characteristic values. In particular, the charge characteristic values ​​are function values ​​of a spatially resolved charge distribution. The charge distribution assigns the charge density to at least one spatial coordinate perpendicular to the flow direction, or a quantity from which the charge density can be determined. The charge density is specified as charge per unit volume or charge per unit area.

[0027] Alternatively or additionally, determining the at least one charge characteristic involves determining at least one charge gradient. The charge gradient is obtained by deriving the charge distribution. The local electric field, which arises due to the electrostatic charging of the particles, can be determined from the charge gradient.

[0028] The fluid is preferably a gas, in particular air. Preferably, the particle-containing gas stream is ignitable by an electric spark. This particularly means that an ignition spark can trigger a chemical reaction between the particles and at least one component of the gas stream or between components of the gas stream. In this case, it is particularly important to ensure that the electrostatic charge does not become excessive.

[0029] According to a preferred embodiment, the determination of the fieldless particle velocity is carried out using particle image velocimetry. Alternatively or additionally, the determination of the fieldless particle velocity is carried out using laser Doppler anemometry. Alternatively or additionally, the spatially resolved determination of the co-field particle velocity is preferably carried out using particle image velocimetry and / or laser Doppler anemometry. These measurement methods achieve comparatively high measurement speeds, spatial resolutions, and low measurement uncertainties.

[0030] The spatially resolved determination comprises the following steps: (a) of the field-less particle velocity: (i) recording a first field-less image of a plurality of particles at a first point in time, (ii) recording a second field-less image of the particles at a second point in time which is later by a time offset, (iii) determining the field-less particle velocity from the field-less images and (b) of the co-field particle velocity: (i) recording a first co-field image of a plurality of particles at a third point in time, (ii) recording a second image of the particles at a fourth point in time which is later by a time offset, in particular the same time offset, and (iii) determining the co-field particle velocity from the co-field images.

[0031] When capturing images, the particles in the gas stream are preferably irradiated with a light sheet, preferably generated by a laser. This determines the positions of the particles captured by the light sheet. Images are preferably captured with a camera whose optical axis extends transversely to the flow direction. Thus, the optical axis preferably runs at least substantially perpendicular to the light sheet.

[0032] Preferably, the angle β H between the light sheet and a horizontal line is at most 22°, in particular at most 15°. In this case, the gravitational force is negligible.

[0033] It is advantageous if the time offset is no more than three seconds, in particular no more than one second, preferably no more than 0.5 seconds. Alternatively or additionally, it is advantageous if the time offset is selected such that the particles have moved by no more than 30 millimeters, in particular no more than 15 millimeters, between the first and second points in time. These are average values.

[0034] It is possible that the number of particles used to determine the co-field particle velocity differs from the number of particles used to determine the co-field particle velocity and the zero-field particle velocity by a maximum of 30%. However, the zero-field particle velocity and the co-field particle velocity are usually determined at least predominantly using different particles.

[0035] It is advantageous if the spatially resolved determination of the measurement-timeless particle velocity comprises the following steps: (i) in a first period of time, recording a measurement-fieldless image of a plurality of particles which are irradiated by a light sheet, wherein the light sheet extends along the flow direction of the gas flow, (ii) in the first period of time, changing a light property, in particular a brightness or a color, of the light sheet and (iii) determining the measurement-fieldless particle velocity from the measurement-fieldless image.Alternatively or additionally, the spatially resolved determination of the co-field particle velocity comprises the following steps: (i) in a second period of time, recording a second co-field image of a plurality of particles irradiated by the light sheet, (ii) in the second period of time, changing a light property, in particular a brightness or a color, of the light sheet and (iii) determining the co-field particle velocity from the co-field image.

[0036] In order to distinguish the influence of the application of the electric measuring field from the influence of turbulence on the particles, the method preferably comprises the steps of: (a) adding tracer particles to the gas flow, (b) determining an air velocity of the gas flow using particle image velocimetry, and (c) determining the charge characteristic from the spatially resolved air velocity, the fieldless particle velocity, and the co-field particle velocity (with and without an electric field). These steps are preferably performed at least when the Reynolds number has changed by more than 10%, in particular at least 20%.

[0037] The method is preferably carried out on particles whose diameter is at least 10 micrometers. Preferably, their diameter is at most 500 micrometers. The diameter of the tracer particles is preferably smaller than the diameter of the particles, in particular, it is preferably at most one-fifth of the diameter of the particles. It is advantageous if tracer particles have a diameter of at least 1 micrometer and / or at most 10 micrometers.

[0038] It is advantageous if at least one charge characteristic is a spatially resolved charge distribution.

[0039] Preferably, the method comprises the step of issuing a warning message if the at least one charge characteristic lies outside a target charge characteristic interval. For example, the target charge characteristic interval is an interval in which a maximum of the charge distribution lies. If at least one of the charge characteristic values ​​lies outside the target charge characteristic interval, this indicates that the electrostatic charge of the particles in the gas stream has become excessive.

[0040] Preferably, the at least one charge characteristic is determined from the spatially resolved particle velocities, which are calculated using the formula Q y z = πρC d r 2 2 E ⋅ cosβ u ¯ β − v ¯ β , E u ¯ β − v ¯ β , E − u ¯ β − v ¯ β u ¯ β − v ¯ β occurs, with ρ is the air density, β is the angle between the electric measuring field lines and the light sheet, C d is the flow resistance coefficient, r is the Sauter diameter according to DIN ISO 9276 of the particles, u β is the velocity component of the gas flow in the light sheet perpendicular to the flow direction, v β is the velocity component of the particles in the light sheet perpendicular to the flow direction without an electric measuring field and v β,E is the velocity component of the particles in the light sheet perpendicular to the flow direction with an applied electric measuring field.

[0041] The derivation of this formula is given below. The feature that at least one charge characteristic is determined using the specified formula is understood, in particular, to mean that a calculation is performed whose result deviates by a maximum of 10%, in particular a maximum of 5%, from the result that would have been achieved using the specified formula. InIn other words, it is irrelevant whether the formula is implemented, i.e., used, directly in the calculation. What matters is that the measurement results are calculated in such a way that they at least essentially correspond to the result obtained using the specified formula.

[0042] Alternatively or additionally, the determination of the at least one charge characteristic value from the spatially resolved particle velocities is carried out using the formula Q y z = πρC d r 2 u ¯ β − v ¯ β + v ¯ β , E u ¯ β − v ¯ β β + v ¯ β , E 2 E ⋅ cosβ carried out.

[0043] The method is preferably performed on a turbulent gas flow. To date, no meaningful measurement of a charge characteristic has been possible for turbulent fluid flows.

[0044] It is advantageous if the gas flow is at least substantially vertical. This means, in particular, that the angle between the flow velocity vector and the vertical is at most 25°, in particular at most 15°.

[0045] The method preferably comprises the steps of (a) changing a position of the light sheet relative to the fluid line and (b) determining the at least one charge characteristic value from the spatially resolved particle velocities. It is advantageous if the at least one charge characteristic value is recorded for a plurality of different positions of the light sheet.

[0046] A gas flow charge measuring device according to the invention is preferably designed to automatically emit a warning signal when the charge characteristic lies outside the target charge characteristic interval. The emission of a warning signal can, for example, be the emission of an acoustic, optical, electrical, or electromagnetic signal. The warning signal is preferably an electronic signal that encodes the message that the charge characteristic lies outside the target charge characteristic interval. It is advantageous if the gas flow charge measuring device is designed to emit this warning message via a bus system.

[0047] The invention also provides a pneumatic conveying device for transporting particles by means of a gas stream, comprising (a) a gas stream generator, in particular a blower or a compressor, for generating the gas stream, in particular a gas stream, and (b) a gas stream charge measuring device according to the invention. The pneumatic conveying device preferably has a particle feed for feeding particles to the gas stream. In such a pneumatic conveying device, the gas stream charge measuring device according to the invention significantly reduces damage caused by excessive electrostatic charges on the particles.

[0048] The invention is explained in more detail below with reference to the accompanying drawings. Figure 1 in part 1a a schematic side view of a pneumatic conveying device according to the invention with a gas flow charging device according to the invention for carrying out a method according to the invention, in part 1b a cross section AA according to Figure 1 and in part 1c an enlargement of the area B according to Figure 1a .

[0049] Figure 1a shows a pneumatic conveying device 10 according to the invention with a gas flow generator 12 in the form of a blower for generating a gas flow, a particle feed 14 for feeding schematically drawn particles 16.i and a gas flow charge measuring device 18 according to the invention.

[0050] In this case, the gas flow generator 12 is a blower for generating a gas flow in the form of a compressed air flow with a pressure of, for example, p = 300 kPa. The particles 16.i can be, for example, food particles, such as tea, coffee, or flour.

[0051] The gas stream flows through a fluid line 20. It is advantageous if the fluid line 20 has a turbulence-forming section 22 whose length L 22 is preferably at least 2 meters, in particular at least 3 meters. Preferably, the length L 22 is at most 100 meters. In the turbulence-forming section 22, a turbulent flow of the gas stream forms, which no longer changes in terms of its turbulence level in the rear section in the flow direction S. It is advantageous if the length L 22 is at least ten times the diameter of the fluid line 20.

[0052] The gas flow charge measuring device 18 is arranged downstream of the turbulence-forming section 22 in the flow direction S. A particle sink 24 is schematically arranged downstream of the gas flow charge measuring device 18 in the flow direction S. The particle sink 24 can, for example, be a storage facility for the particles 16.i. Alternatively, the particle sink can also be a machine for further processing the particles, for example, for packaging, forming, pressing, or the like.

[0053] It is advantageous and, regardless of the features otherwise mentioned for the embodiment, represents a preferred embodiment that the fluid line 20 has identical cross-sections in the turbulence-forming section 22 and a measuring section 26. This is an equality in the technical sense, meaning that it is possible for the cross-sections and / or the cross-sectional shapes of the fluid line 20 to change, but that this change is so small that it leads to a measurement uncertainty of at most 10% when measuring a charge characteristic value K to be measured.

[0054] Figure 1b shows a cross section A - A according Figure 1aIt can be seen that the gas flow charge measuring device 18 has a measuring field generator 28. The measuring field generator 28 comprises a voltage source 30 and electrodes 32.1, 32.2. An electric field E is formed between the electrodes 32.1, 32.2, which can be considered homogeneous to a good approximation. The measured value generator 28 is configured to generate a field of at least E = 1 kV / m. Preferably, the electric field is smaller than E = 2 MV / m.

[0055] The electric field is applied in a measuring range M. The gas flow charge measuring device 18 also comprises a particle velocity meter 34, which in the present case is formed by a particle image velocimetry measuring unit. The particle image velocimetry measuring unit 34 comprises a laser 36 (see Figure 1b) for generating a light sheet 38 and a camera 40. Images taken by the camera 40 are analyzed by an evaluation unit 40 and a particle velocity distribution v(y,z) is determined therefrom. By averaging over an averaging time τ M of, for example, τ M = 60 seconds, an averaged particle velocity distribution v (y,z).

[0056] A method according to the invention is carried out by first determining the particle velocity spatially resolved, so that the particle velocity distribution v(y,z) is obtained. For this purpose, for example, two images are recorded, which are called zero-field images because no electric field E is applied. The two zero-field images are recorded successively at a first time t 1 and t 2 . The two times are offset by a time τ v = t 2 - t 1 separated from each other in time.

[0057] After a waiting period τ WAfter the second time t 2 , the evaluation unit 42 controls the voltage source 30 so that a voltage U 38 is applied between the electrodes 32.1, 32.2. The waiting time τ W is as small as possible and ideally amounts to no more than 500 milliseconds.

[0058] Two co-field images are then recorded by the camera 40 at a third time t 3 and a fourth time t 4 , respectively. A co-field particle velocity v E is determined from these images. It should be noted that it is irrelevant whether the co-field particle velocity v E is determined first and then the non-field particle velocity v or vice versa.

[0059] Alternatively, the particle velocity distribution v(y,z) can also be determined using laser Doppler anemometry. Alternatively, the particle velocity distribution v(y,z) can be determined using single-image particle image velocimetry. In this method, a light property, such as the brightness or color, of the light sheet 38 is changed during the acquisition of the respective image. Trajectories of the particles 16.i can then be seen in the respective image, i.e., the zero-field image and the with-field image, with the change in the light property encoding the change in time.

[0060] Figure 1c schematically shows particle 16.1, which is at position x 1 at time t 1. The x-coordinate is measured in the flow direction S, the y- and z-coordinates perpendicular to it, resulting in a right-hand system.

[0061] At time t 2 , the position x 2 results when there is no electric field, that is, when E = 0. If the electric field is present, the position x 2,E results.

[0062] To characterize the flow conditions of the gas stream, in a preliminary test, exclusively tracer particles are introduced into the gas stream, for example, via particle feed 14. The tracer particles are significantly smaller than the particles 16.i, whose electrostatic charge is to be determined using a method according to the invention. It is assumed that the tracer particles are so light that a tracer particle velocity distribution u(y,z) corresponds to a good approximation of the velocity distribution of the fluid volumes of the gas stream.

[0063] It should be noted that the velocities of the tracer particles are denoted by u, but the velocities of the particles 16.i are denoted by v.

[0064] From a plurality of measurements with the tracer particles, respective hit particle velocity distributions uj(y,z) are obtained. By averaging an averaging time T, an average spatially resolved tracer particle velocity u (y , z). The bar indicates the time average.

[0065] Without an electric field, the aerodynamic force acts on the particles 16.i: F d = 1 2 ρ C o π r 2 u ¯ β − v ¯ β u ¯ β − v ¯ β

[0066] If the electric field is applied, the result is: F d , E = 1 2 ρ C D π r 2 u ¯ β − v ¯ β , E u ¯ β − v ¯ β , E

[0067] The electrostatic force acts on the particles: F e = QE cos β

[0068] In this case, the angle β H between the light sheet 38 and a horizontal line H is β H = 22°. The gravitational force is therefore negligible.

[0069] By forming the difference between equations (1) and (2) we get Q y z = πρC d r 2 2 E ⋅ cosβ u ¯ β − v ¯ β , E u ¯ β − v ¯ β , E − u ¯ β − v ¯ β u ¯ β − v ¯ β

[0070] The straight brackets denote the absolute value. This yields the charge distribution Q(y,z). This charge distribution has a maximum value Q max , which represents a charge characteristic K. The charge distribution Q(y,z) also has a gradient field VQ(y,z). This gradient field has a maximum gradient, which is the gradient of maximum magnitude. This also represents a charge characteristic K. All individual function values ​​of the charge distribution Q(y,z) also represent charge characteristics K.

[0071] The project leading to this patent application received funding from the European Research Council under grant agreement No. 947606 as part of the European Union's Horizon 2020 research and innovation program. List of reference symbols 10 Pneumatic conveyor device S Flow direction 12 Gas power generator T Averaging time 14 Particle feed t Time 16 particles U 38 Tension 18 Gas flow meter u(y,z) Fluid velocity distribution 20 Fluid line u(y,z) time-averaged fluid velocity distribution 22 Turbulence training section 24 Particle sink v(y,z) Particle velocity distribution 26 measuring section v(y,z) time-averaged particle velocity distribution 28 Measuring field generator v Fieldless particle velocity 30 Voltage source 32 electrode v E Particle speed 34 Particle velocity meter 36 Laser 38 Light sheet 40 camera 42 Evaluation unit τ W Waiting time τ v Time offset VQ(y,z) Gradient field E electric field H Horizontal i Running index K Charge characteristic L 22 length M Measuring range p Fluid pressure Q(y,z) Load distribution Qmax Maximum value of the charge distribution

Claims

1. A method for determining a spatially-resolved charge distribution (Q(y,z)) of electrical charges of particles (16) in a gas stream of a pneumatic conveyor device (10), wherein the particles (16) in the gas stream can be electrostatically charged, comprising the steps: (a) directing the gas stream, which contains particles (16), through a fluid line (20), (b) spatially-resolved determination of a measuring field-less particle velocity (v) in a measurement area without an electrical measuring field, (c) applying an electrical measuring field transverse to the flow direction (S) in the measurement area, (d) spatially-resolved determination of a midfield particle velocity (vE) in the measurement area and (e) determining the a spatially-resolved charge distribution (Q(y,z)) from the spatially-resolved particle velocities.

2. The method according to claim 1, characterised in that the spatially-resolved determination of the measuring field-less particle velocity (v) and / or the midfield particle velocity (vE) occurs by means of particle image velocimetry.

3. The method according to one of the preceding claims, characterised in that the spatially-resolved determination of the measuring field-less particle velocity (v) and the midfield particle velocity (vE) occurs via laser Doppler anemometry.

4. The method according to one of the preceding claims, characterised in that (a) the spatially-resolved determination of the measuring field-less particle velocity comprises the following steps: (i) recording a first measuring field-less image of a plurality of particles (16) at a first point in time (t1), (ii) recording a second measuring field-less image of the particles at a second point in time (t2) that is later by a time offset (τv), (iii) determining the measuring field-less particle velocity from the measuring field-less images, (b) the spatially-resolved determination of the midfield particle velocity (vE), which comprises the following steps: (i) recording a first midfield image of a plurality of particles (16) at a third point in time (t3), (ii) recording a second image of the particles (16) at a fourth point in time that is later by a time offset (τv), in particular the same time offset, and (iii) determining the midfield particle velocity (vE) from the midfield images.

5. The method according to one of the preceding claims, characterised in that (a) the spatially-resolved determination of the measuring field-less particle velocity comprises the following steps: (i) in a first time period, recording a measuring field-less image of a plurality of particles (16) irradiated by a light sheet (38), the light sheet extending along the flow direction (S) of the gas stream, (ii) in the first time period, altering a property of light, particularly a brightness or colour, of the light sheet (38) and (iii) determining the measuring field-less particle velocity from the measuring field-less image and / or (b) the spatially-resolved determination of the midfield particle velocity (vE) comprises the following steps: (i) in a second time period, recording a second midfield image of a plurality of particles (16) irradiated by a light sheet (38), (ii) in the second time period, altering a property of light, particularly a brightness or colour, of the light sheet (38) and (iii) determining the midfield particle velocity (vE) from the midfield image.

6. The method according to one of the preceding claims, characterised by the steps: (a) adding tracer particles to the gas stream, (b) determining an air velocity of the gas stream by means of particle image velocimetry and (c) determining the spatially-resolved charge distribution (Q(y,z)) from the spatially-resolved air velocity, the measuring field-less particle velocity and the midfield particle velocity (vE).

7. The method according to one of the preceding claims, characterised by the steps: emitting a warning when the spatially-resolved charge distribution (Q(y,z)) lies outside of a target charge characteristic interval.

8. A pneumatic conveyor (10) for transporting particles (16) in a gas stream, wherein the particles (16) in the gas stream can be electrostatically charged, with a gas stream generator, particularly a fan or a compressor, for generating the gas stream, and a particle feed for feeding particles (16) to the gas stream, characterised by a gas stream charge measuring device (18) with (a) a gas line (20), (b) a measuring field generator (28) for generating an electrical measuring field in a measurement area of the gas line (20), (c) a particle velocity measure that is configured to automatically determine a spatially-resolved particle velocity distribution by means of particle image velocimetry and (d) an evaluation unit (42) that is configured to automatically carry out a method comprising the steps: (i) measuring a spatially-resolved measuring field-less particle velocity by means of the particle image velocimetry measurement unit without the measuring field generator (28) generating an electrical measuring field, (ii) generating the electrical measuring field by means of the measuring field generator (28) and (iii) measuring a spatially-resolved midfield particle velocity (vE) by means of the particle image velocimetry measurement unit (iv) determining at least one spatially-resolved charge distribution (Q(y,z)) from the spatially-resolved particle velocities.

9. The pneumatic conveyor (10) according to claim 8, characterised that that the gas stream charge measuring device (18) is designed to automatically emit a warning signal when the spatially-resolved charge distribution (Q(y,z)) lies outside of a target charge characteristic interval.