Apparatus and process for measuring characteristics of particle flow

Abstract

The present invention relates to an apparatus (1) and process for measuring characteristics of a particle flow. The measuring is done with two different cut-off diameters of a particle trap (13) of which one cut-off diameter is adjusted based on the measured particle characteristics.

Claims

1. Process for measuring characteristics of a particle flow, comprising steps: a) guiding a sample flow comprising particles through a passage; b) electrically charging the particles; c) trapping essentially all free ions and charged particles having a particle diameter smaller than a trap cut-off diameter, the cut-off diameter being a particle diameter above which penetration through the trapping means essentially deviates from zero; d) measuring an electrical current carried by said charged particles that are not trapped; e) adjusting the trapping means to adjust the cut-off diameter based on the measured electrical current carried by said charged particles that are not trapped; f) adjusting the trapping means to adjust the cut-off diameter to a reference cut-off diameter and to a measuring cut-off diameter resulting in a measured reference electrical current and a measured electrical current, respectively, carried by said charged particles that are not trapped; g) comparing the measured reference electric current to the measured electrical current; and h) dynamically adjusting the trapping means to adjust the measuring cut-off diameter based on the comparing of the measured reference electrical current to the measured electrical current.

2. Process of claim 1, comprising repeating steps a) to h) one or more times.

3. Process of claim 1, wherein step f) comprises adjusting a trap voltage of the trapping means to a reference voltage and to a measuring voltage resulting in the measured reference electrical current and the measured electrical current, respectively, carried by said charged particles that are not trapped; and step h) comprises dynamically adjusting the measuring voltage based on the comparing of the measured reference electrical current and the measured electrical current, wherein the trapping means comprises a voltage trap.

4. Process of claim 1, wherein step f) comprises adjusting a flow of the sample flow to a reference flow and to a measuring flow resulting in the measured reference electrical current and the measured electrical current, respectively, carried by said charged particles that are not trapped; and step h) comprises dynamically adjusting the sample flow based on the comparing of the measured reference electrical current to the measured electrical current, wherein the trapping means comprises a diffusion trap.

5. Process as in claim 4, wherein the adjusting of the sample flow comprises adjusting a purge flow of particle free gas prior to the diffusion trap.

6. Process as in claim 1, comprising adjusting the trapping means to adjust the cut-off diameter to occasionally deviate the measuring cut-off diameter from a level which results in a desired ratio of the measured reference electrical current and the measured electrical current, and estimating a particle size distribution based on derivative of said ratio.

7. Process as in claim 1, comprising dynamically adjusting the trapping means to adjust the measuring cut-off diameter for setting and maintaining the cut-off diameter on a desired position on a curve representing a particle size distribution.

8. Apparatus for measuring characteristics of a particle flow, the apparatus comprising: a passage with an inlet and an outlet for guiding a sample flow comprising particles through the apparatus; means for electrically charging the particles; trapping means for trapping essentially all free ions and charged particles having particle diameter smaller than a trap cut-off diameter, the cut-off diameter being a particle diameter above which penetration through the trapping means essentially deviates from zero; means for measuring an electrical current carried by said charged particles that are not trapped; and control means for adjusting the trapping means to adjust the cut-off diameter based on measured electrical current carried by said charged particles that are not trapped, wherein the control means for adjusting the trapping means comprises means for adjusting the cut-off diameter to a reference cut-off diameter and to a measuring cut-off diameter resulting in a measured reference electrical current and a measured electrical current, respectively, carried by said charged particles that are not trapped, and wherein the means for adjusting the cut-off diameter is arranged to compare the measured reference electric current to the measured electric current, and to dynamically adjust the measuring cut-off diameter based on the comparison of the measured reference electrical current to the measured electrical current.

9. Apparatus as in claim 8, wherein the trapping means comprises a voltage trap and the control means is further arranged to adjust a trap voltage of the voltage trap to a reference voltage and to a measuring voltage resulting in the measured reference electrical current and the measured electrical current, respectively, carried by said charged particles that are not trapped, wherein the control means is further arranged to dynamically adjust the measuring voltage based on the comparison of the measured reference electrical current to the measured electrical current.

10. Apparatus as in claim 8, wherein the apparatus comprises first and second means for measuring the measured electrical current carried by said charged particles that are not trapped and first and second trapping means for trapping essentially all free ions and charged particles having a particle diameter smaller than a cut-off diameter, wherein the control means is further arranged to adjust the first trapping means to adjust a cut-off diameter of the first trapping means to the reference cut-off diameter resulting in the measured reference electrical current in the first means for measuring, and to adjust the second trapping means to adjust a cut-off diameter of the second trapping means to the measuring cut-off diameter resulting the measured electrical current in the second means for measuring, wherein said first trapping means and said first means for measuring are arranged in cascade or in parallel with said second trapping means and said second means for measuring.

11. Apparatus as in claim 8, wherein the trapping means comprise a diffusion trap, the control means is further arranged to adjust the sample flow to a diffusion trap to a reference flow and to a measuring flow resulting in the measured reference electrical current and the measured electrical current, respectively, carried by said charged particles that are not trapped, wherein the control means is further arranged to dynamically adjust the sample flow to a measuring flow based on the comparison of the measured reference electrical current to the measured electrical current.

12. Apparatus as in claim 11, wherein the control means is further arranged to adjust the sample flow and alternate the sample flow between a reference flow and at least one measuring flow resulting in the measured reference electrical current and at least one measured electrical current, respectively, carried by said charged particles that are not trapped.

13. Apparatus as in claim 8, wherein the control means is further arranged to dynamically adjust said measuring cut-off diameter based on a ratio of the measured reference electrical current and the measured electrical current in order to reach and maintain a desired level of said ratio.

14. Apparatus as in claim 13, wherein said desired level of said ratio is between 0.3 and 0.7.

15. Apparatus as in claim 13, wherein said control means is further arranged to occasionally deviate the measuring cut-off diameter from a level which results in said desired ratio, wherein the apparatus comprises means for estimating the particle size distribution based on derivative of said ratio with respect to the measuring cut-off diameter, or any transformation of that.

16. Apparatus as in claim 8, wherein the control means is further arranged to dynamically adjust the measuring cut-off diameter for setting and maintaining the cut-off diameter on a desired position on a curve representing a particle size distribution.

17. Apparatus as in claim 16, wherein said desired position of the cut-off diameter is at an inflexion point of a curve having the measured electrical current as a function of the particle size distribution.

18. Apparatus as in claim 16, wherein said desired position of the cut-off diameter is at an inflexion point of a curve having the measured electrical current as a function of the sample flow.

19. A process for measuring characteristics of a particle flow, comprising steps: a) guiding a sample flow comprising particles through a passage; b) electrically charging the particles; c) trapping essentially all free ions and charged particles having a particle diameter smaller than a trap cut-off diameter, the cut-off diameter being a particle diameter above which penetration through the trapping means essentially deviates from zero; d) measuring an electrical current carried by said charged particles that are not trapped; e) adjusting the trapping means to adjust the cut-off diameter based on the measured electrical current carried by said charged particles that are not trapped; f) adjusting the trapping means to adjust the cut-off diameter to a reference cut-off diameter and to a measuring cut-off diameter resulting in a measured reference electrical current and a measured electrical current, respectively, carried by said charged particles that are not trapped; g) comparing the measured reference electric current to the measured electrical current; h) dynamically adjusting the trapping means to adjust the measuring cut-off diameter based on the comparing of the measured reference electrical current to the measured electrical current; and i) adjusting the trapping means to adjust the cut-off diameter and alternate the cut-off diameter between a reference cut-off diameter and at least one measuring cut-off diameter to result in the measured reference electrical current and at least one measured electrical current, respectively, carried by said charged particles that are not trapped.

20. Apparatus for measuring characteristics of a particle flow comprising: a passage with an inlet and an outlet for guiding a sample flow comprising particles through the apparatus; means for electrically charging the particles; trapping means for trapping essentially all free ions and charged particles having a particle diameter smaller than a trap cut-off diameter, the cut-off diameter being the particle diameter above which penetration through the trapping means essentially deviates from zero; means for measuring an electrical current carried by said charged particles that are not trapped; and control means for adjusting the trapping means to adjust the cut-off diameter based on the measured electrical current carried by said charged particles that are not trapped, wherein the control means comprises means for adjusting the cut-off diameter to a reference cut-off diameter and to a measuring cut-off diameter resulting in a measured reference electrical current and a measured electrical current, respectively, carried by said charged particles that are not trapped, wherein the control means is further arranged to compare the measured reference electric current to the measured electric current, and to dynamically adjust the measuring cut-off diameter based on the comparison of the measured reference electrical current to the measured electrical current; wherein the trapping means comprise a voltage trap and the control means is further arranged to adjust a trap voltage of the voltage trap to a reference voltage and to a measuring voltage resulting in the measured reference electrical current and the measured electrical current, respectively, carried by said charged particles that are not trapped, wherein the control means is further arranged to dynamically adjust the measuring voltage based on the measured reference electrical current and the measured electrical current; wherein the control means is further arranged to adjust the trap voltage and alternate the trap voltage between a reference voltage and at least one measuring voltage resulting in the measured reference electrical current and at least one measured electrical current carried by said charged particles that are not trapped.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) In the following the invention will be described in greater detail, in connection with preferred embodiments, with reference to the attached drawings, in which

(2) FIG. 1 shows trap penetration, number distribution and mass distribution of a typical single-source aerosol where the particle concentration obeys lognormal distribution;

(3) FIG. 2 is a schematic view of one embodiment of the invented apparatus; and

(4) FIG. 3 shows two different trap distribution curves and two penetration curves at different trap voltages;

(5) FIG. 4 shows the effect of continuously modulated trap voltage on measured current; and

(6) FIG. 5 shows trap voltage and trap current for a typical measurement response to a changing particle size and particle number size distribution as well as trap penetration functions for different trap voltages.

DETAILED DESCRIPTION OF THE INVENTION

(7) FIG. 1 shows a typical example of a lognormal particle size distribution. Such particle size distribution with count median diameter (CMD), i.e. the particle diameter where the cumulative number distribution hits 0.5 being around 50 nm and geometric standard deviation (GSD) around 1.7 could well represent particle size distribution from diesel engine exhaust. As seen from the figure, the mass median diameter is almost twice as high as CMD. In FIG. 1 the trap voltage is set to a value which corresponds to cut-off diameter of 50 nm. Penetration through the trap is essentially zero for particles smaller than the cut-off diameter and above it the trap penetration smoothly increases.

(8) FIG. 2 shows one embodiment of the invented apparatus 1 for particle mass concentration measurement. Apparatus 1 comprises passage 2 with inlet 3 and outlet 4 for guiding sample flow Q comprising particles P, P* with a certain particle size distribution through apparatus 1. The flow Q through passage 2 can be realized in various ways such as by using a pump, by using chimney effect or by using ion wind. In the embodiment of FIG. 2, fan 5 drives air into inner passage 2* through filter 6. This air is passed next to the means 7, 8 for ionizing the air. In one embodiment of the present invention, the means 7, 8 for ionizing the air are realized by a corona discharge unit 7, powered by a high voltage source 8, which is electrically isolated from mains with an isolation transformer 9. The ionized air forms the motive fluid flow of ejector 10 placed inside passage 2. The ejector generates an underpressure, i.e. pressure lower than ambient pressure, which drives sample flow Q with different size particles P, P* into apparatus 1 vial inlet 3. The ionized air and particles are effectively mixed in the mixing zone 11 and thus particles P,P* are charged 12, 12*. Free ions, which as described previously may also be very fine charged particles, are removed by trapping means 13. The trapping means can comprise for example an electrostatic precipitator, i.e. a voltage trap, removing small charged particles due to their high electrical mobility in an electrical field or a coarse filter, i.e. a diffusion trap. The necessary trap voltage for the voltage trap is generated by a power source 14, which is controlled by control means 15 for controlling the trap voltage. In another embodiment of the present invention the trapping means 13 may be connected to the control means 15 which control the distance of the trapping electrodes and the electrical field strength is adjusted by adjusting the electrode separation without necessarily adjusting the electrical voltage across the trap electrodes. In an embodiment a flow prior to the diffusion trap is generated by an air pump which is controlled by means for controlling the flow. The electrical current carried by particles escaping passage 2 via outlet 4 is measured using means 16, 17 for measuring electrical current carried by charged particles. Although the preferable way to measure the current is to use means 16 for measuring the escaping current, i.e. the electrical current escaping from apparatus 1 with the particles, other current measurement techniques, such as electrodes or a particle filter collecting at least a fraction of the charged particles may be used as well. The measured current is converted to mass concentration value using suitable means 18, which may be situated in apparatus 1 or the conversion may be carried out elsewhere, e.g. by recording the current values and providing the conversion afterwards.

(9) In the apparatus 1 the trapping means 13 for trapping essentially all free ions 11 and charged particles 12 having particle diameter smaller than trap cut-off diameter D.sub.c-o, the cut-off diameter D.sub.c-o being the particle diameter above which penetration through the trapping means 13 essentially deviates from zero, are connected to the control means 15 for adjusting the trapping means 13 to adjust the trap cut-off diameter D.sub.c-o. It is essential to the present invention that the trapping means 13 are adjusted to cut-off diameter D.sub.trap, which is significantly higher than the diameter of the essentially free ions, i.e. trapping means 13 remove a significant amount of charged particles from particle flow Q and that the trapping means 13 are adjusted to cut-off diameter D.sub.ref, which is about the same as the diameter of the essentially free ions. By analyzing the electric currents carried by the charged particles that penetrate the trap and/or are captured by the trap the higher cut-off diameter D.sub.trap can be adjusted to a value which facilitates measuring of particle characteristics of interest. For example, when measuring mass concentration of particles, the cut-off diameter D.sub.trap can be adjusted to essentially match the count median diameter of the particles in the particle flow Q. Preferably apparatus 1 comprises means 18 for converting the electrical current signal to particle mass concentration value.

(10) FIG. 3 illustrates particle distribution curves with count median diameter of 50 nm and 200 nm. FIG. 3 also illustrates penetration through the particle with cut-off diameters corresponding to the CMD's of distribution curves. Note that the x-axis representing particle size is logarithmic. When the particle distribution is 50 nm CMD and corresponding cut-off diameter is set at 50 nm, the current measured after the trap is about 50% of the current with cut-off diameter at about 4 nm. By looking at the 50 nm distribution and the 200 nm penetration curves, it is evident that the portion of the particle flow penetrating the trap is very small and thus represents poorly the particle flow and thus leads to significant unreliability when it is used as basis for calculating characteristics of the particle flow. Correspondingly, with 200 nm distribution curve and D.sub.trap at 50 nm cut-off diameter, the amount of trapped particles is almost the same as with reference cut-off diameter D.sub.ref at about 4 nm. Thus it is important to change the cut-off diameter as the CMD of the particle flow changes. As said, the cut-off diameter can be changed e.g. by adjusting trap voltage of the voltage trap or by adjusting flow prior to the diffusion trap.

(11) In an embodiment wherein the apparatus comprises a voltage trap it has been found advantageous to be able to adjust the trap voltage during normal operation of apparatus 1. Thus in one embodiment of the present invention apparatus 1 comprises means 19 for controlling the control means 15 for adjusting the trapping means 13 on the basis of the output of the means 16,17 for measuring the electrical current carried by said charged particles 12, 12*. In such embodiment of the present invention the trap voltage is adjusted during the measurement. First the control means 15 for adjusting the trapping means 13 set the trap voltage to a value which ensures the removal of essentially all free ions 11, e.g. the trap voltage is set to reference voltage value V.sub.ref which corresponds to particle cut-off diameter D.sub.ref of around 4 nm or less. The current I.sub.ref carried by charged particles 12, 12* is measured using current measurement means 16, 17. Then the means 19 for controlling the control means 15 for adjusting trapping means 13 increase trap voltage to a higher level V.sub.trap and the current I.sub.trap measured by means for current measurement 16, 17 is smaller than I.sub.ref because the higher trap voltage V.sub.trap traps not only free ions but also particles from the sample flow Q and thus lowers the measured current. This trap voltage V.sub.trap is the measuring voltage which is used in actual mass concentration measurements. V.sub.trap can be controlled so that the particle cut-off diameter D.sub.trap is about the same as count median diameter. With such trap voltage the sensitivity of the current/mass conversion to changes of median particle diameter is significantly reduced. One or more V.sub.trap values can be used. For example alternating V.sub.trap to lower and higher than said voltage resulting D.sub.trap near the CMD produces also information about the distribution of particle diameter in the particle flow Q. For example the particle diameter distribution is broad if said lower and higher V.sub.trap voltages result only a minor change in electrical current I.sub.trap compared to a situation where the same change in voltage V.sub.trap results a major change in I.sub.trap.

(12) Similarly, in an embodiment wherein the apparatus comprises a diffusion trap it has been found advantageous to be able to adjust the flow during normal operation of apparatus 1. Thus in one embodiment of the present invention apparatus 1 comprises means 19 for controlling the means 15 for adjusting the trapping means 13 on the basis of the output of the means 16,17 for measuring the electrical current carried by said charged particles 12, 12*. In such embodiment of the present invention the flow is adjusted during the measurement. First the control means 15 for adjusting the trapping means 13 set the flow to a value which ensures the removal of essentially all free ions 11, e.g. the flow is set to reference flow value Q.sub.ref which corresponds to particle cut-off diameter D.sub.ref of around 4 nm or less. The current I.sub.ref carried by charged particles 12, 12* is measured using current measurement means 16, 17. Then the means 19 for controlling the control means 15 for adjusting trapping means 13 increase flow to a higher level Q.sub.trap and the current I.sub.trap measured by means for current measurement 16, 17 is smaller than I.sub.ref because the higher flow Q.sub.trap traps not only free ions but also particles from the sample flow Q and thus lowers the measured current. This flow Q.sub.trap is the measuring flow which is used in actual mass concentration measurements. Q.sub.trap can be controlled so that the particle cut-off diameter D.sub.trap is about the same as count median diameter. With such flow the sensitivity of the current/mass conversion to changes of median particle diameter is significantly reduced. One or more Q.sub.trap values can be used. For example alternating Q.sub.trap to lower and higher than said flow resulting D.sub.trap near the CMD produces also information about the distribution of particle diameter in the particle flow Q. For example the particle diameter distribution is broad if said lower and higher Q.sub.trap values result only a minor change in electrical current I.sub.trap compared to a situation where the same change in flow Q.sub.trap results a major change in I.sub.trap. An ion trap for capturing free ions and smallest particles can be used between the means 7, 8 for electrically charging particles P, P* and the diffusion trap. Said ion trap can be a voltage trap.

(13) Similarly, the principles of other embodiments comprising a voltage trap can be implemented with a diffusion trap by replacing voltages V.sub.ref, V.sub.trap of a voltage trap by flows Q.sub.ref, Q.sub.trap introduced into the passage 2 prior to the diffusion trap. The voltage adjustment and the flow adjustment both have a similar effect of changing the cut-off diameter of the particle trap. Therefore in the following embodiments concerning use of a voltage trap, the voltage trap and voltages can be substituted with a diffusion trap and flows with minor or no changes in operation principle of the apparatus. Thus to avoid unnecessary repetition and for the sake of consistency, the embodiments have been described concerning the voltage trap.

(14) In an embodiment V.sub.trap is controlled by targeting ratio S to a desired reference value S.sub.ref, wherein S=I.sub.trap/I.sub.ref and S.sub.ref is between 0.3 to 0.7 and preferably 0.5 which results the particle cut-off diameter D.sub.trap to be about the same as median particle diameter. The measuring voltage V.sub.trap is adjusted until S.sub.ref is reached. The initial value of V.sub.trap can be a fixed value, a guess or same as V.sub.ref. The next value can be calculated for example from equation
V.sub.trap+1=V.sub.trapk*(SS.sub.ref)*dt/tau,
where V.sub.trap+1 is the next value, V.sub.trap is the current trap voltage, k is a steepness coefficient, S is the current ratio, dt is a sampling period and tau is a time constant. When V.sub.trap reaches a value where S=S.sub.ref the count median diameter can be approximated with CMD=f(V.sub.trap)=a*V.sub.trap+b, where a and b and calibration factors.

(15) In an embodiment reference electrical current I.sub.ref results from reference voltage V.sub.ref of a first voltage trap. Measuring electrical current I.sub.trap results from measuring voltage V.sub.trap of a second voltage trap. The measuring voltage V.sub.trap is adjusted based on ratio S of said electrical currents which are measured with measuring means after each voltage trap. The first and second voltage traps voltage traps can be arranged in cascade so that the particle flow Q travels first through the voltage trap having reference voltage and the through the voltage trap having measuring voltage. The first and second voltage traps voltage traps can also be arranged in parallel so that the particle flow Q is divided into the two voltage traps. The cascade and parallel arrangements allow for continuous monitoring of the current ratio and thus a continuous adjusting is possible. In an embodiment also a single voltage trap and measurement arrangement can be used if the trap voltage is altered between V.sub.ref and V.sub.trap in cycles. Therefore the adjustment is carried out in iterating manner and/or the adjusting is repeated one or more times.

(16) Shape or width of the particle size distribution is the second most important factor after the count median diameter when calculating the particle mass concentration value. In an embodiment the particle size distribution is approximated by calculating derivative dS/dV.sub.trap in the vicinity of S=0.5. The trap voltage V.sub.trap can be adjusted e.g. in order to reach values 0.3; 0.4; 0.5; 0.6 and 0.7 for S. Also the measurements which were made before the ratio S reached 0.5 can be used. The width of the particle size distribution can be approximated from the results and calibration factors a and b can be determined for CMD=a*V.sub.trap+b.

(17) In an embodiment the trap voltage V.sub.trap can be dynamically adjusted so that it sets and maintains the trap cut-off diameter D.sub.trap on a desired position on a curve representing the particle size distribution or the electrical current response of the particle size distribution curve. Advantageously the trap cut-off diameter D.sub.trap is adjusted to inflexion point at the curve representing I.sub.trap as a function of V.sub.trap, where the trap cut-off diameter D.sub.trap essentially matches the count median diameter. The inflexion point can be found for example by analyzing distortion of a continuously modulated signal.

(18) FIG. 4 shows the fundamentals of calculating the inflexion point. The trap voltage V.sub.trap can be for example sine wave or square wave or some other waveform. As the response curve is not linear in used voltage range of V.sub.trap far from the inflexion point, the resulting I.sub.trap (near 150 fA range) is distorted and comprises even harmonics, especially second order harmonics. The even harmonics of the distortion go to zero near the inflexion point where the response curve is linear in the used voltage range of V.sub.trap and this can be detected for example by monitoring second order harmonics of I.sub.ref and correlating it with I.sub.trap. Another way is to square the voltage signal V.sub.trap and find a zero point of correlation between current signal I.sub.trap and squared voltage signal. Squaring the voltage signal introduces second order harmonics to the squared signal. Second order harmonics do not exist in current signal when operating near the inflexion point and thus the correlation would be zero with squared voltage signal. This method allows the acquisition of both the CMD and concentration signals continuously, independent of each other, with higher frequency due to no need for settling time between different voltage levels. Another advantage is that the deviation between trap voltages and resulting electrical currents can be used in approximating the width of the particle size distribution, for example by comparing amplitude of I.sub.trap and amplitude of V.sub.trap. Known signal processing methods or transformations, such as logarithm conversion or squaring, are preferably used to convert I.sub.trap and V.sub.trap signals during or before comparison or other analyses.

(19) An aspect of the invention is a process for particle mass concentration measurement. The method comprises guiding sample flow Q comprising particles P, P*, with a count median diameter of CMD and a certain particle size distribution, through a passage 2 and electrically charging particles P, P*. The process also comprises measuring the electrical current carried by said charged particles 12, 12*, trapping essentially all free ions 11 and charged particles 12 having particle diameter smaller than trap cut-off diameter D.sub.c-o, the cut-off diameter D.sub.c-o being the particle diameter above which penetration through the trapping means 13 essentially deviates from zero and adjusting the trapping means 13 to adjust the trap cut-off diameter D.sub.c-o based on measured electrical current carried by said charged particles 12, 12*. The process further comprises adjusting a trap voltage of the trapping means 13 to a reference cut-off diameter D.sub.ref and to a measuring cut-off diameter D.sub.trap resulting a reference electrical current I.sub.ref and a measuring electrical current I.sub.trap carried by said charged particles 12, 12*, and dynamically adjusting the measuring cut-off diameter based on the measurements of the reference electrical current I.sub.ref and the measuring electrical current I.sub.trap.

(20) In an embodiment the process further comprises adjusting of the cut-off diameter and alternating the cut-off diameter between a reference cut-off diameter D.sub.ref and at least one measuring cut-off diameter D.sub.trap resulting a reference electrical current I.sub.ref and at least one measuring electrical current I.sub.trap carried by said charged particles 12, 12*.

(21) In an embodiment the process further comprises adjusting a trap voltage of the trapping means 13 to a reference voltage V.sub.ref and to a measuring voltage V.sub.trap resulting a reference electrical current I.sub.ref and a measuring electrical current I.sub.trap carried by said charged particles 12, 12*, and dynamically adjusting the measuring voltage based on the measurements of the reference electrical current and the measuring electrical current, wherein the trapping means comprises a voltage trap.

(22) In an embodiment the process further comprises adjusting a flow of the trapping means 13 to a reference flow Q.sub.ref and to a measuring flow Q.sub.trap resulting a reference electrical current I.sub.ref and a measuring electrical current I.sub.trap carried by said charged particles 12, 12*, and dynamically adjusting the measuring flow based on the measurements of the reference electrical current and the measuring electrical current, wherein the trapping means comprises a diffusion trap.

(23) In an embodiment the process further comprises adjusting cut-off diameter to occasionally deviate the measuring cut-off diameter from the level which results said desired ratio, and estimating the particle size distribution based on derivative of said ratio with respect to the measuring cut-off diameter.

(24) In an embodiment the process further comprises dynamically adjusting the measuring cut-off diameter for setting and maintaining the trap cut-off diameter D.sub.trap on a desired position on a curve representing the particle size distribution.

(25) In an embodiment the adjusting of the flow of the trapping means 13 comprises adjusting a purge flow of particle free gas prior to the diffusion trap for adjusting the flow. The purge flow is introduced to the particle flow Q for increasing the speed of the particles in the flow which then changes the cut-off diameter of the diffusion trap.

(26) FIG. 5 shows trap voltage and trap current in the left panel for a typical measurement response to a changing particle size. The right panel shows the particle number size distribution as well as trap penetration functions for different trap voltages corresponding to the trap voltages in the left panel. The shaded areas of the size distribution correspond to measured particles at different trap voltages. When suitable V_trap is found, current is half of current I.sub.trap at reference trap voltage V.sub.Ref.

(27) It is apparent to a person skilled in the art that as technology advances, the basic idea of the invention can be implemented in various ways. The invention and its embodiments are therefore not restricted to the above examples, but they may vary within the scope of the claims.