WO2012022843A1

WO2012022843A1 - Particle sensor

Info

Publication number: WO2012022843A1
Application number: PCT/FI2011/050730
Authority: WO
Inventors: Kauko Janka
Original assignee: Pegasor Oy
Priority date: 2010-08-20
Filing date: 2011-08-19
Publication date: 2012-02-23
Also published as: FI20110065A0; FIU20100360U0; FI20110066A0; FI20110067A0; WO2012022842A1; EP2606344A1; WO2012022844A1

Abstract

Apparatus and process for measuring particle concentration, comprising a sensing element (12) for particle concentration measurement, means (22) for switching or modulating a parameter, such as the electrical charge or the volumetric flow, which affects the output of the sensing element (12) and means (23) for determining the volumetric flow on the basis of the response of a transfer function which switching or modulation creates to the sensing element (12) output.

Description

Particle sensor

Field of invention

The present invention relates to a particle sensor for measuring particles according to the preamble of claim 1 and specifically a particle sensor for the control of particle emissions from combustion engines with integrated volumetric flow measurement or monitoring. The present invention further relates to a process for measuring particles according to the preamble of claim 11.

Background of the invention

There is a constant increase in the demand for real-time particle measurement. Such demand exists e.g. in the measurement of atmospheric aerosols, in particle filter development and within different high-temperature processes, such as combustion processes. Especially the real-time exhaust control of combustion engines, such as vehicles, requires reliable and non- expensive particle monitoring. Particle sensor should be able to provide at least a rough estimation on the particle size distribution of the aerosol passing through the sensor, as the health effects of the particles seem to be dependent on their particle size or the total surface area of the particles, fine particles dominating adverse health effects.

Various particle sensors have been developed for combustion engine particle emission monitoring. Patent application US 2006/0156791, 20.7.2006, Dekati Oy, describes a method and a sensor device for determining particle emissions from exhaust gases of a combustion engine substantially during the use in an exhaust pipe system or a corresponding exhaust gas duct, in which method emitted particles contained in the exhaust gases are charged and the particle emissions are determined by measuring the electric charge carried by the emitted particles in the exhaust gas duct. The emitted particles are charged by varying the way of charging or the charging power with respect to time in such a manner that as a result of the charging, emitted particles brought into at least two different electrical charge states are present, wherein the charge of the emitted particles is further determined as a difference value/values measured from the emitted particles brought into said at least two different electrical charge states. The application states that the relative lengths in time of the charging cycles generated by the charger C (charger on/off cycles) may vary freely as required by each application. The measurement method does not provide information on the volumetric flow through the sensor, which information would be valuable for e.g. determining the particle concentration or controlling the sensor operation, such as potential sensor blocking. The method also does not provide information on the particle size distribution of the aerosol passing through the particle sensor.

Particle measurements are frequently carried out by cascade impactors, where particulate matter is withdrawn (preferably isokinetically) from a source and segregated by size. Cascade impactors use the principle of inertial separation to size segregate particle samples from a particle laden gas stream. Conventional cascade impactors cannot be used in real-time particle size distribution (PSD) measurement and especially in measuring changes in PSD in real time. In the Electrical Low Pressure Impactor (ELPI™, Dekati Oy, Finland) The particles are first charged into a known charge level in the corona charger. After charging, the particles enter a cascade low pressure impactor with electrically insulated collection stages. The particles are collected in the different impactor stages according to their aerodynamic diameter, and the electric charge carried by particles into each impactor stage is measured in real time by sensitive multi-channel electrometers. This measured current signal is directly proportional to particle number concentration and size. The particle collection into each impactor stage is dependent on the aerodynamic size of the particles. Measured current signals are converted to (aerodynamic) size distribution using particle size dependent relations describing the properties of the charger and the impactor stages. The result is particle number concentration and size distribution in real-time. By using ELPF^M without operating corona charger, it can be used for particle charge distribution measurements. As the electrical impactor collects the particles, it is possible also to carry out post-sampling measurements, i.e. weight the samples collected on each impactor stage and/or analyze the composition of the particles collected on each stage. However, for essentially continuous monitoring of particle emissions from combustion engines and especially for on-board diagnostics of combustion engine particle emissions, impactor is not a useful apparatus.

A major problem in any particle measurement device where particles are charged is the accumulation of the charged particles on the inner surfaces of the measurement sensor. This is caused by the repulsive Coulombic force of the charged particles on each other. This repulsive force is especially effective in the space where the particles are charged by an electric discharge, because the strength of the electrostatic field affecting the particle accumulation is further increased by free ions and potentially also by the high-voltage electrode required for the electric discharge. Accumulation of the charged particles on the inner surfaces of the sensor may block the aerosol flow channel (pathway) or they may reduce the electrical insulation capacity of the electrical insulation

US patent publication US 4,837,440 A, Burtscher et al., 6.6.1989, describes a method for the characterization of particles in aerosol, comprising: using an aerosol, which has been brought to at least one predetermined temperature sufficient for evaporation or decomposition or preventing condensation of molecules on particles of said aerosol; exposing said aerosol to an electromagnetic radiation for activating particles contained in the aerosol to cause said aerosol particles to emit electrons and attain an electric charge correspondingly; and measuring the electric charge of said aerosol particles..

Patent application US 2005/0083633, Riebel et al., 21.4.2005, relates to a device for charging or adjusting the charge of gas-borne particles into a defined charge distribution under utilization of corona discharge in the aerosol space. In addition to an appropriate geometry of the charger and the electrodes, the voltage waveform and the voltage regulation are of great significance for the result. The application further relates to a method for operating the device.

Patent application WO 2010/049870, Koninkl Philips Electronics NV, 6.5.2010, presents a device that is capable of recording the evolution over time of the characteristics of a size distribution of electrically-charged airborne particles in airflow. The device comprises an air inlet, a particle charging unit, a concentration variation section, a particle sensing section and a data evaluation unit. Specifically, the particle sensing section of the device generates at least two serially obtained measurement signals II and 12 from which the data evaluation unit can infer values for both the average particle diameter d_p, and the number concentration N of the size distribution of electrically-charged airborne particles.

Patent specification GB 1 235 856, Maurice Sidney Beck., et al., 16.6.1971, describes a method of measuring the flow of a particulate material conveyed hydrodynamically by means of flowing fluid, comprising the operations of sensing the passage of random disturbances in the flow of the particulate material past points separated by a known distance along a path for flow of the particulate material, and cross-correlating the sensed disturbances to establish a transit time for the passage of the disturbances over the known distance. The described method requires at least first and second sensing elements, which makes the apparatus based on the invention too expensive for low-cost particulate sensors. Patent specification GB 1 485 750, Maurice Sidney Beck et al., 14.9.1977, also refers to similar flow measurement arrangement.

United States Patent application Publication US 2004/0080321 Al, Kingsley St. John Reavell, et al., 29.4.2004, describes modifications made to the design of electrostatic particle measurement instruments to compensate or eliminate the transient currents produced by the rate of change of charge near the sensing electrodes, and hence reduce the transient errors in measured particle concentrations. The publication also provides a hint for volumetric flow measurement where the travel time between two electrodes, the penultimate electrode and the final electrode could potentially be used. However, this measurement method suffers from the problem discussed earlier, i.e. the need for at least two sensing elements.

United States Patent US 5,214,386, Hermann Singer, et al., 25.5.1993, describes an apparatus and method for measuring particles in polydispersed systems and particle concentrations of monodispersed aerosols. The method includes the contactless measurement of the flow velocities in a pipe or the measurement of the volume flow in the pipe by annular sensors that do not completely surround the pipe. The average flow velocity can be

determined with knowledge of the sensor distance by measurement of the time lag of the individual signals or by correlation of at least two sensor signals in each case. Also this measurement requires at least two sensing electrodes.

The particle sensors of the prior art possess the technical problem of particle accumulation. There is need for a sensor which can measure or monitor particle concentration in real-time and with long measurement periods without frequent sensor cleaning or maintenance. Such need exists particularly with car combustion engine exhaust emission monitoring. One way of determining the potential sensor blocking by particle accumulation is to monitor the volumetric flow through the particle sensor. Advantageously such volumetric flow control should be realized without increasing the price of the particle sensor. And advantageously the volumetric flow measurement should be combined with other features for reducing particle accumulation.

The inventor of the present invention has found that the determination of the dynamic transfer function of a particle sensor provides an elegant way of determining also the volumetric flow through the sensor.

Kaarle Hameri's, "Instrumentation for aerosol physical properties", About

Atmospheric Aerosols, Summer Workshop, 19.-20.6.2006, describes the idealized transfer function of an impactor, i.e. the deposition efficiency as function of particle size, can be described by means of a step-function at particle size dso (particle size with 50% collection efficiency on the collection stage). However, real impactors exhibit a deposition

characteristic, i.e. their transfer function deviates from the idealized transfer function. The impactor transfer functions are often plotted as function of the square root of the Stokes number for 50% collection efficiency, Stkso. A theoretical calculation of real impactor transfer functions, i.e. deposition efficiencies, is possible using numerical methods. The particle size distribution can be then reconstructed if the transfer functions of the different stages are known. It should be noted that Hameri describes a transfer function which is used to determine the PSD. This is not the same as a dynamic transfer function which can be used to describe the time response of the collection stage.

Thus there exists a need for a non-expensive way for the determination of volumetric flow through a particle sensor. There also exists a need for reducing the possibility for sensor blocking especially with long measurement periods, e.g. with combustion engine particle emission control.

Brief description of the invention

The inventor has surprisingly found a method which will solve the prior art problems described above, especially the problem of low-cost flow measurement or monitoring. The invention comprises a process where an essential parameter of the particle sensor is switched, preferably on and off, or modulated, over time and the essential parameters of the sensor transfer function are determined from the switching/modulation response. What is essential for the invention is that due to the modulation of an essential parameter, i.e. a parameter which affects the measurement result of the particle sensor, the sensor time constant can be determined without the use of several sensing electrodes or equivalent, which has a significant effect on the particle sensor cost. In addition, the switching/modulation provides additional benefits, such as reduced sensor soiling and possibility for more accurate determination of particle size distribution.

In a preferred embodiment of the invention, the process is used with a particle measurement apparatus described in applicant's patent application WO/2009/109688, Pegasor Oy, 11.9.2009, describing a process for measuring particle concentrations in a gas using an ejector for producing an essentially constant sample flow and for efficient mixing of the particle-containing sample and an essentially clean, ionized gas. The invention also relates to an apparatus implementing such process. The process and the apparatus can be utilized for example in measuring particle concentrations in an exhaust system of a combustion engine.

In the process described in WO/2009/109688, the main flow of the ejector consists of essentially clean ionized gas flow. The phrase 'essentially clean' means that the particle concentration in the ionized gas is so low that it does not adversely affect the monitoring process. The main flow causes suction to the side flow channel and thus a sample flow from the particle-containing gas is sucked to the monitoring apparatus. The ionized clean gas forms the main flow and the sample flow forms the side flow. When an ejector is used, two different phenomena, efficient transfer of momentum and effective particle charging happen in a single process step which is advantageous in shortening the process time and thus reducing the ion losses. The efficient mixing makes it possible to design small measurement apparatuses with fast response time, which is a great advantage when measuring vehicle emissions.

WO/2009/109688 states that when the particle concentration of the gas is monitored, it is advantageous to produce an essentially constant gas flow through the measurement apparatus. Typically the mass flow in e.g. the exhaust duct of a combustion engine is anything but constant, typically depending on the rotation speed of the engine. Using an ejector for sucking the sample flow from the exhaust duct results an essentially constant side flow, the flow being typically pulse-free, i.e. constant. Such a flow can then be modulated or switched in a controlled way so that the measurement apparatus works in AC mode, which provides more reliable particle concentration results that using the apparatus in DC mode. WO/2009/109688 does not mention volumetric flow measurement using

switching/modulation.

With the measurement process described in WO/2009/109688, at least (1) the particle charging, (2) free ion and small particle collection, or (3) flow through the sensor are essential parameters which affect the sensor measurement result. They can be switched, preferably on and off, or modulated, over time, and the essential parameters of the transfer function are determined from the switching/modulation response. The essential parameters of the transfer function are favorable the delay time t^and time constant rof the sensor or at least part of the sensor.

Although it is obvious for a person skilled in the art, it should be notified that the process of modulating/switching an essential parameter affecting the particle sensor measurement result is not limited to the particle measurement process described in

WO/2009/109688, but may be used with various other particle measurement processes as well.

In the preferred embodiment of the present invention, the invented sensor comprises an electrical discharging unit which can be switched or modulated between different charging modes. In one embodiment of the present invention the electrical discharging unit is a corona charger which is switched periodically between the ON-mode and OFF-mode, i.e. the corona voltage is periodically switched ON and OFF. The term "periodical switching" means that the switching is continuous for at least the time which is required to determine the essential parameters of the transfer function. Switching or modulation may have a fixed frequency or the frequency may vary and also the length of the ON and OFF modes may vary. The length of the ON mode may be less than 100 seconds, less than 10 seconds or even less than a second. The duty cycle may vary between 1 and 99%, preferably between 5 and 50% and more preferably between 5 and 20%.

The sensor may also comprise a neutralizer which neutralizes the electrical charge of the particles. The neutralizer may be a separate unit or the neutralization may be carried out by the electrical discharging unit, e.g. by such a way that the corona charger comprises two charging units with opposite electrical potential or by using a single corona charger in AC (alternating current) mode which produces ions with opposite charges. In yet another embodiments of the present invention, the electrical discharging unit is switched between two opposite voltages, ON⁺ and ON^" or between three different modes: ON-, OFF-, and NE-modes, where the NE mode describes a mode where either a separate neutralizer or the electrical discharging unit is used to neutralize the particles entering the sensor.

In another embodiment of the present invention the sensor comprises an ion particle trap which removes free ions or extremely small particles (typically having a diameter of few nanometers or less than 10 nanometers) from the sample flow. Free ion or particle removal is dependent on the strength of the electrical field across the ion trap and the modulating the strength of the electrical field a rough estimation of the particle size distribution of the aerosol passing through the sensor can be determined. It is also possible to connect various particle traps working with different electrical field strengths into series and thus receive a better estimation on the particle size distribution.

In all embodiments described above, the duty cycle or the length of the ON⁺, ON^", OFF and NE-modes can be varied during the measurement and thus optimize the operation of the measurement apparatus. The duty cycle control may be based on an internal signal of the measurement apparatus e.g. the particle concentration or on the time wise derivative of the particle concentration. The duty cycle control may also be based on an external signal, e.g. when measuring particle exhaust from a combustion engine; the external signal may include a change in the momentum of the engine or in the revolution speed of the engine. An

advantageous feature is that in high-concentration conditions, where the soiling rate is high, extremely low duty cycles can be used without practical degrading the noise properties of the signal. The reason for this is the fact that during even extremely short ON times the high signal levels can yield enough noise-free measurement data.

In yet another embodiment of the current invention, the modulated/switched parameter which affects the sensor measurement result is the volumetric flow Q through the sensor. Volumetric flow can be switched e.g. by switching a valve shutting the sample flow to the sensor inlet or from the sensor outlet or by switching or modulating the pump which creates the volumetric flow through the sensor. The term "pump" is here understood as any means of creating the volumetric flow through the sensor and thus pump can be e.g. an ejector pump where either the motive fluid flow or the side fluid flow may be switched or modulated. Such switching or modulating can be advantageously achieved by using a pulsating pump which at itself creates a modulated flow. Such pump may be e.g. a diaphragm pump.

In the preferred embodiment of the present invention the response from the switched or modulated mode of the electrical discharge unit is determined by synchronic detection. Synchronic detection can be realized by using either analog electronics or digitally. The digital realization can obviously be carried out in a separate computing unit or it may be integrated to a common controller or computing unit, where other control functions of the electrical impactor are carried out as well.

The main parameters can be determined even continuously when required e.g. due to rapidly changing aerosol composition. If the changes in the measurement environment are not remarkable on a short time interval, and when the maximum time response of the measurement is required, the determination of the main parameters may be carried out with longer intervals.

A surprising benefit of using a modulated signal instead of one sensor signal is that, in spite of simpler and cheaper practical solution, it can yield better performance. The reason of this feature is that one of the signals to be compared/correlated has practically no noise or other disturbances, whereas in prior-art solutions two noisy signals are compared correlated.

Brief description of the drawings

In the following, the invention will be described in more detail with reference to the appended schematic drawings, where

Fig. 1 shows a schematic drawing of an embodiment of the invented sensor;

Fig. 2 describes the time behavior of the ion/particle traps;

Fig 3 describes the determination of the parameters used in the time correction of the ion/particle traps; and

Fig 4 describes the time wise correction of the measurement signal. For the sake of clarity, the figure only shows the details necessary for understanding the invention. The structures and details which are not necessary for understanding the invention and which are obvious for a person skilled in the art have been omitted from the figures in order to emphasize the characteristics of the invention.

Detailed description of preferred embodiments

The present invention relates to a process for measuring particle concentration, comprising measuring a signal, which is a function of the particle concentration, with a sensing element, switching or modulating a parameter which affects the output of the sensing element and determining the volumetric flow on the basis of the time response which switching or modulation creates to the sensing element output. Thus the present invention solves the technical problem of prior art with a process requiring only one sensing element.

Preferably the process comprises electrically charging at least a fraction of the particles entering the measurement apparatus, measuring the electrical current carried by the charged particles and switching or modulating the electrical discharge unit at least between OFF-mode where the electrical discharge unit essentially does not charge the particles and ON-mode where the electrical discharge unit essentially does charge the particles.

Alternatively switching or modulating the electrical discharge unit can be carried out at least between NE-mode where the electrical discharge unit essentially electrically neutralizes the particles and ON-mode where the electrical discharge unit essentially electrically charges the particles. In the preferred process the measurement process described in WO/2009/109688 is used, including measuring the current escaping from the measurement apparatus with the charged particles and removing ions, charged ultrafme particles or charged fine particles from the aerosol passing through the measurement apparatus with the aim of at least one electrical field. According to one embodiment of the present invention the ion/particle trap is switched at least between OFF-mode where the ion/particle trap essentially removes free ions and ON- mode where ion/particle trap essentially removes particles having a diameter smaller than d_p . The electrical field strength of the ion/particle trap in the ON-mode can be adjusted which further adjusts maximum trapped particle diameter d_p.

One embodiment of the present invention comprises switching or modulating the sample flow through the measurement apparatus between at least two essentially different values. In the preferred process the measurement process described in WO/2009/109688 is used, including pumping sample flow through the measurement apparatus with an ejector pump, ionizing the motive fluid of ejector and. switching/modulating the motive fluid flow of ejector.

The essential parameters of the transfer function of at least a part of the measurement apparatus can be determined e.g. by providing a computational reference signal, comparing the sensing element output to the reference signal, adjusting the reference signal for maximum correlation between the sensing element output and the reference signal, determining the transfer function of at least a part of the measurement apparatus from the reference signal with maximum correlation and determining the volumetric flow through the measurement apparatus using at least some parameters of the computed transfer function.. In one embodiment of the present invention the transfer function is determined by providing a computational reference signal following a first-order low-pass filter, determining the delay time t^and time constant rof the first-order low-pass filter and determining the volumetric flow through the measurement apparatus using the inverse of the sum of td+ T. In most cases only the inverse of ¾or Tor the inverse of some other combination than the sum of f_rf+ rcan also be used for the volumetric flow determination.

In the invented process the switching/modulation frequency of a parameter affecting the sensing element output is preferably adjusted between 0,01 Hz and 10 Hz and the duty cycle between 1% and 50%.

The present invention also relates to apparatus 1 for measuring particle

concentration. Apparatus 1 is preferably a particle sensor, which comprises a sensing element 12 for particle concentration measurement, means 22 for switching or modulating a parameter which affects the output of the sensing element 12 and means 23 for determining the volumetric flow on the basis of the response which switching or modulation creates to the sensing element 12 output.

Figure 1 shows one embodiment of the present invention. This embodiment differs slightly from the preferred embodiment which follows the construction of the particle sensor described in WO/2009/109688. Apparatus (1) shown in Figure 1 comprises an electrical discharge unit 20 for electrically charging at least a fraction of the particles entering apparatus 1 through connection 14 a. Apparatus 1 further comprises means 12 for measuring the electrical current carried by the charged particles and means 22 for switching or modulating the electrical discharge unit 20 at least between OFF-mode where the electrical discharge unit 20 essentially does not ionize the essentially clean air entering apparatus 1 through connection 14 b and ON-mode where the electrical discharge unit 20 essentially does ionize the essentially clean air entering apparatus 1 through connection 14 b, the ionized air then charging, essentially by unipolar diffusion charging, the particles in the mixing part 14 of apparatus 1.

Apparatus 1 may further comprise means 12 for measuring the electrical current carried by the charged particles and means 22 for switching or modulating the electrical discharge unit 20 at least between NE-mode where the electrical discharge unit 20 essentially ionizes, by bipolar ionization, the essentially clean air entering apparatus 1 through connection 14b, which unipolar ionized air then further electrically neutralizes the particles in the mixing part 14 of apparatus 1 and ON-mode where the electrical discharge unit 20 essentially ionizes the essentially clean air entering apparatus 1 through connection 14 b, the ionized air then charging, essentially by unipolar diffusion charging, the particles in the mixing part 14 of apparatus 1.

The electrical discharge unit 20 is preferably a corona charger where the high voltage source 20a produces the required high voltage to the corona needle 20b which is electrically isolated from the body of apparatus lby the electrical isolator 8 . In unipolar ionization a single corona needle 20b with a suitable high electrical potential is required. In bipolar ionization either two corona needles 20b working with opposite electrical potentials or a single corona needle 20b switched between opposite electrical potentials is required.

In the embodiment of Figure 1, apparatus 1 further comprises a sensing element 12 which measures the current escaping from apparatus 1 with the charged particles. Such escaping current sensing element 12 may also be arranged as described in WO/2009/109688.

Apparatus 1 further comprises an ion or particle trap 4 for removing ions, charged ultrafine particles, charged fine particles, or charged particles with any size from the aerosol passing through the apparatus 1. Apparatus 1 may also comprise more than one ion trap Means 22 switches or modulates the ion/particle trap 4 at least between OFF-mode where the ion/particle trap4 essentially removes free ions (but not particles) and ON-mode where ion/particle trap 4 essentially removes particles having a diameter smaller than d_p. Diameter dp may be adjusted by adjusting the electrical field strength of the ion/particle trap 4. The electrical field strength is adjusted with the power source 4a which creates an electrical potential between electrode 4b and apparatus body 10. Electrode 4b is electrically isolated from the apparatus body 10 with an electrical isolator 8.

Means 22 may also switch or modulate the sample flow through apparatus 1 between at least two essentially different values. Such switching or modulation obviously requires a flow restriction component adjusted by means 22, and the flow restriction component may be a valve, throttle, or equivalent, obvious for a person skilled in the art. in the apparatus described in WO/2009/109688 apparatus (1) comprises an ejector for pumping the sample flow through apparatus 1 and means (20) for ionizing the motive fluid of ejector and the flow through apparatus 1 is switched or modulated by switching/modulating the motive fluid flow of the ejector.

Apparatus 1 of Figure 1 further comprises means 27 for determining the essential parameters of the transfer function of apparatus 1. This is preferably realized by providing a computational reference signal, connected to the means 22 for switching or modulating a parameter essentially affecting the sensing element output and means for comparing the sensing element output to the reference signal as well as means for adjusting the reference signal for maximum correlation between the sensing element output and the reference signal and means for computing the transfer function of apparatus 1 from the reference signal with maximum correlation and means 23 for determining the volumetric flow through apparatus 1 using at least some parameters of the computed transfer function.

In the preferred embodiment of the present invention, the computational reference signal follows at least a first-order low-pass filter and means 27 for determines the delay time td and time constant Tof the first-order low-pass filter, in which case the volumetric flow through apparatus 1 is determined by using the inverse of the sum of td+ T.

Apparatus 1 is preferably used so that means 22 switches or modulates a parameter affecting the sensing element 12 output essentially continuously and the switching or modulating frequency is preferably between 0,01 Hz and 10 Hz, depending e.g. on particle concentration. Switching or modulating frequency does not need to be constant but it may be varied based on internal reasons (such as particle concentration in apparatus 1) or external reasons (such as change in e.g. combustion engine's torque).

The duty cycle, i.e. ratio——— is preferably as small as possible, because soiling

tON + tOFF

of apparatus 1 is then minimized. In an embodiment of the present invention, means 22 adjust the switching duty cycle between 1% and 50%.

In a preferred embodiment of the present invention the determination of the transfer function parameters is realized by describing the time-wise behavior of at least the sensor so, that it comprises a delay component t_s and a time constant rwhich describes the behavior of a first-order low-pass filter. The volumetric flow passing the sensor is inversely proportional to the sum of the time delay t^and time constant τ. Thus the determination of the time constants and Tprovides also a tool to monitor or measure the volumetric flow.

The time-wise behavior of the first-order low-pass filter is similar to the one of a full-mixed reactor. The first-order low-pass filter can be described in Laplace notation as (_{5) =} £ i£2 ₌ tf_L ₍₁₎

J Si(s) l+TS ' where So(s) is the output signal, Si(s) is the input signal, K is the filter passband gain (which in this case can be set to unity), ris the time constant and s is the Laplace transform variable.

The derivative of function F(s) in time domain is dSo(t) _ Si t)-So(t) ...

dt ~ T ^W

In the embodiment of the present invention modulation initiates a measurement signal receives signals at a certain frequency and with a certain duty cycle, the frequency being preferably higher than 0,01 Hz, more preferably higher than 0,1 Hz and even more preferably higher than 1 Hz and the duty cycle being preferably between 5-20%. The differential equation (2) can be approximated by difference equation (3): It is also possible to use ion particle traps in series, in which case the response of the whole system can be described as a serial connection of first-order low-pass filters and delay times, as shown in Figure 2.

Delay time tdi and time constant n of each trap 4; can be determined by setting them to such values that the model describing the time-wise behavior of each trap has a maximum correlation to the actual measured signal. In one embodiment of the present invention, the required impulse for the response is generated by modulating the electrical discharge unit 20 between ON and OFF modes. In another embodiment of the present invention, the required impulse for the response is generated by modulating the electrical discharge unit 20 between ON and NE modes. In yet another embodiment of the present invention, the required impulse for the response is generated by modulating the electrical discharge unit 20 between ON⁺ and ON^" modes.

Figure 3 shows a block diagram of an embodiment where the time- wise parameters are determined. Correlator X compares the measurement signal Soi from trap / to the calculated signal ,- and the time parameters t<# and Tj are set to the values where the correlator X provides a maximum signal C„ i.e. the correlation between the model and the measured signal is set to maximum.

When the parameters t and ¾ have been determined, the time response of the measured signal can be compensated or correlated by modifying the signal with the inverse function. With the transfer function following the Laplace notation this means multiplying the signal with the inverse of the transfer function F(s).

Compensating the time delay is merely applying a time shift. Compensating the first- order low-pass filter is achieved by the difference equation

Si = So + r ^ (4)

An embodiment with the compensation algorithm based on time delay and first-order low-pass filter is shown as a block diagram in Figure 4. The corrected output signals Set from each stage i are calculated by modifying the stage output signals So, with the inverse transfer functions It is obvious for a person skilled in the art that modeling the time-wise behavior with a first-order low-pass filter is only given here as an example and any suitable model which describes the behavior of ion/particle trap and which can be presented in analog or digital form can be used for modeling. The best model depends on the construction of the multitrap particle sensor.

Claims

Apparatus (1 ) for measuring particle concentration, c o mpri s i n g :

a. sensing element (12), the output of which is a function of the particle

concentration;

b. means (22) for switching or modulating a parameter which affects the output of the sensing element (12); and

c. means (23) for determining the volumetric flow on the basis of the time response which switching or modulation creates to the sensing element (12) output.

Apparatus (1) of claim 1, c o mp ri s i n g :

a. an electrical discharge unit (20) for electrically charging at least a fraction of the particles entering apparatus (1);

b. means (12) for measuring the electrical current carried by the charged particles; and

c. means (22) for switching or modulating the electrical discharge unit (20) at least between OFF-mode where the electrical discharge unit (20) essentially does not charge the particles and ON-mode where the electrical discharge unit (20) essentially does charge the particles.

Apparatus (1) of claim 1, c o mpri s in g :

c. means (22) for switching or modulating the electrical discharge unit (20) at least between NE-mode where the electrical discharge unit (20) essentially electrically neutralizes the particles and ON-mode where the electrical discharge unit (20) essentially electrically charges the particles.

Apparatus (1) as in any of the previous claims, c o mp ri s i n g sensing element (12) which measures the current escaping from apparatus (1) with the charged particles.

5. Apparatus (1) as in any of the previous claims, compri sing an ion/particle trap (4) for removing ions, charged ultrafine particles or charged fine particles from the aerosol passing through the apparatus (1).

6. Apparatus (1) as in any of the previous claims, comprising more than one ion/particle trap (4).

7. Apparatus (l)ofclaim5 or 6, comprising:

a. means (22) for switching or modulating the ion/particle trap (4) at least between OFF-mode where the ion/particle trap(4) essentially removes free ions and ON- mode where ion/particle trap(4) essentially removes particles having a diameter smaller than d_p and

b. means for adjusting the electrical field strength of the ion/particle trap (4) in the ON-mode and thus adjusting d_p.

8. Apparatus (1) as in any of the previous claims, comprising means (22)for switching or modulating the sample flow through apparatus (1) between at least two essentially different values.

9. Apparatus of claim 8, comprising a diaphragm pump.

10. Apparatus ( 1 ) as in any of the previous claims, comprising:

a. an ejector for pumping the sample flow through apparatus (1);

b. means for ionizing the motive fluid of ejector;

c. means for switching/modulating the motive fluid flow of ejector.

11. Apparatus ( 1 ) as in any of the previous claims, comprising means (27) for

determining the essential parameters of the transfer function of apparatus (1).

12. Apparatus (1) as in claim 11, comprising:

a. means for providing a computational reference signal, connected to the means (22) for switching or modulating a parameter essentially affecting the sensing element output;

b. means for comparing the sensing element output to the reference signal;

c. means for adjusting the reference signal for maximum correlation between the sensing element output and the reference signal;

d. means for computing the transfer function of apparatus (1) from the reference signal with maximum correlation; and e. means (23) for determining the volumetric flow through apparatus (1) using at least some parameters of the computed transfer function.

13. Apparatus (1) as in claim 12, c ompri s i n g :

a. means for providing a computational reference signal following at least a first- order low-pass filter; and

b. means (27) for determining the delay time t^and time constant Tof the first-order low-pass filter; and

c. means (23) for determining the volumetric flow through apparatus (1) using the inverse of t<j, TOT the sum thereof, td+ τ.

14. Apparatus (1) as in any of the previous claims, c o mpri s ing means (22) for adjusting the switching/modulation frequency of a parameter affecting the sensing element output, between 0,01 Hz and 10 Hz.

15. Apparatus ( 1 ) as in any of the previous claims, c o mpri sing means (22) for adjusting the switching duty cycle between 1% and 50%.

16. Process for measuring particle concentration, co mpri s in g :

a. measuring a signal being a function of the particle concentration with a sensing element;

b. switching or modulating a parameter which affects the output of the sensing element; and

c. determining the volumetric flow on the basis of the time response which

switching or modulation creates to the sensing element output.

17. Process of claim 16, c o mp ri s in g :

a. electrically charging at least a fraction of the particles entering the measurement apparatus;

b. measuring the electrical current carried by the charged particles; and

c. switching or modulating the electrical discharge unit at least between OFF-mode where the electrical discharge unit essentially does not charge the particles and ON-mode where the electrical discharge unit essentially does charge the particles.

18. Process of claim 16, c o mpri s in g :

b. measuring the electrical current carried by the charged particles; and c. switching or modulating the electrical discharge unit at least between NE-mode where the electrical discharge unit essentially electrically neutralizes the particles and ON-mode where the electrical discharge unit essentially electrically charges the particles.

19. Process as in any of the claims 16-18, c o mp ri s i n g measuring the current escaping from the measurement apparatus with the charged particles.

20. Process as in any of the claims 16-19, c o mp ri s i n g removing ions, charged ultrafine particles or charged fine particles from the aerosol passing through the measurement apparatus with the aim of at least one electrical field.

21. Process of claim 20, c o mp ri s i n g :

a. switching or modulating the ion/particle trap at least between OFF-mode where the ion/particle trap essentially removes free ions and ON-mode where ion/particle trap essentially removes particles having a diameter smaller than d_p; and

b. adjusting the electrical field strength of the ion/particle trap in the ON-mode and thus adjusting d_p.

22. Process as in any of the claims 16-21, c o mp ri s i n g switching or modulating the

sample flow through the measurement apparatus between at least two essentially different values.

23. Process as in any of the claims 16-22, c o mp ri s i n g :

a. pumping sample flow through the measurement apparatus with an ejector pump ; b. ionizing the motive fluid of ejector;

c. switching/modulating the motive fluid flow of ejector.

24. Process as in any of the claims 16-23, c o mp ri s i n g determining the essential

parameters of the transfer function of at least a part of the measurement apparatus.

25. Process of claim 24, c o mp ri s i n g :

a. providing a computational reference signal;

b. comparing the sensing element output to the reference signal;

c. adjusting the reference signal for maximum correlation between the sensing element output and the reference signal;

d. determining the transfer function of at least a part of the measurement apparatus from the reference signal with maximum correlation; and e. determining the volumetric flow through the measurement apparatus using at least some parameters of the computed transfer function.

26. Process of claim 25, comprising:

a. providing a computational reference signal following at least a first-order low- pass filter;

b. determining the delay time t^and time constant 2" of the first-order low-pass filter; and

c. determining the volumetric flow through the measurement apparatus using the inverse of tj, TOT the sum thereof, td+ T.

27. Process as in any of the claims 16-26, comprising adjusting the switching/modulation frequency of a parameter affecting the sensing element output, between 0,01 Hz and 10 Hz.

28. Process as in any of the claims 16-27, comprising adjusting the switching duty cycle between 1% and 50%.