Patent Description:
Particles suspended in aerosols play a significant role in ambient and indoor air and in many technical processes. An important task lies in detecting the concentration of the particles by way of measurement technology. Particles in the size range of smaller than <NUM> micrometers diameter may be breathed in by humans and may have a detrimental effect on health. The most recent research results indicate that the usual protective functions of humans are no longer effective for nanoparticles <<NUM> in diameter. Nanoparticles arise mainly in combustion processes such as in motor vehicles, coal-fired power stations, wood heating installations, etc..

The increased awareness of the adverse health effect of the air pollutants has led to a growing need for up to date information on the air quality. This information is usually expressed as an Air Quality Index, which combines the measurement results of different pollutants such as for instance nitrogen dioxide (NO2), ozone (O3) and particulate matter (PM). The combination method may differfrom one country to another. Apart from the official air quality measurement sites, there is a growing interest towards sensor networks, which would provide supporting information on the air quality. In such networks the measured information, originated from many measurement nodes scattered over a large area, is collected and processed. As a result, spatially more accurate information is obtained, which could be used to follow the dispersion of pollutants from the emission sources and to support the localized air quality forecasts. <NPL> demonstrates a network of eight low-cost optical sensors for monitoring urban area particle concentrations.

The recent development of low-cost optical particle sensors has made it possible to construct affordable instruments based on the optical particle detection. <NPL> presents an extensive study on one such instrument in a field conditions. The detectors based on the optical detection rely on the light scattering from a cloud of aerosol particles. Such instruments can be calibrated to give the mass concentration of particles for an aerosol with a constant size distribution, particle density and particle refractive index. This is of course seldom the case. However, the instrument response is a relatively slow function of particle diameter for particles larger than the illumination wavelength, i.e. particles larger than approximately <NUM>.

Electrical aerosol detection methods based on the diffusion charging are a promising starting point, when designing apparatus for the detection of ultrafine particles. The output of the charging process is very repeatable and relatively independent on the particle material. While the charging efficiency drops for the small particles, the sensitivity for particles below <NUM> is still good and far better than in the direct optical methods.

<CIT>, describes a method and apparatus for measurement of the number concentration and of the average diameter of aerosol particles. Particles of an aerosol are firstly charged in a unipolar manner in a diffusion charger. They are subsequently led through a diffusional precipitator in which a part of the particles is precipitated. The diffusional precipitator current is measured and a value for the number concentration is evaluated from the current. A single diffusional precipitator may be used for this. According to preferred embodiment, a means for measuring the influence current and/or an aerosol electrometer are additionally present, by way of which one may measure a complete current. The latter additionally permits the evaluation of the average particle size.

The simplest approach for electrical instrument consists of the following components: a diffusion charger, an ion collecting trap, a particle collecting filter inside a Faraday gage, an electrometer to measure the electric current and a pump to provide the sample flow. While forming a perfectly working setup for the particle measurement, the prior art does however have a drawback when considering the application: the aerosol flow through apparatus needs to be controlled or continuously measured. In practice, the sample flow is usually kept constant which leads to an economically unacceptable solution for applications requiring sensors with low cost and low power consumption, e. g solar powered measurement stations.

Document <CIT> discloses an apparatus and process for measuring characteristics of a particle flow. Document<NPL> discloses a modification of electrical low pressure impactor (ELPI) for the particle effective density measurement. Document <NPL> discloses calibration results together with a comprehensive model for the response of the PPS-M sensor. Document <CIT> discloses a device for characterizing a size distribution of electrically-charged particles in an air flow. Document <CIT> discloses a particle count measurement device. Document <CIT> discloses an apparatus for and method of analysing a mixture comprising a fluid and a plurality of electrically charged particles contained therein. Document <CIT> discloses a miniaturized ultrafine particle sizer and monitor.

The aim of the current invention is to introduce a method which solves at least some of the problems of the prior art, in which solution the response of apparatus is essentially independent on the sample flow rate. The method of the invention is characterized by the features of the independent method claim <NUM>.

The problem is thus non-expensive and reliable measurement of particle concentration, especially the measurement of ultrafine (less than <NUM>,<NUM> in diameter) particle concentration, in a fluctuating flow. Flow fluctuation may be rising from poor flow generator (like a fan which is sensitive to various parameters such as power voltage and current, changes in front and back pressures, soiling of blades, air density variation, air pressure variation, etc..

The fluctuating flow may also arise from other fluctuating flows, like flow generated by wind or generated by propeller (like in drone, helicopter or airplane), ventilator, extractor, vacuum cleaner, chimney, exhaust tube, intake flow to combustion engine, other combustion system or ventilation channel or similar not originally meant for generating flow through a particle sensor but for some other purpose.

The fluctuating flow may also be generated by a moving object, like balloon, airplane, helicopter, drone, sounding rocket, train, car, motorcycle, bicycle, roller board, boat, ship, horse or some other animal, or any other moving object.

Information on the fluctuating flow received from an outside source, like weather station or weather or air quality model (like Enfuser™ from Finnish Meteorological Institute, FMI) may be used to verify that the flow through the sensor is in operation range and/or to target the sensor to right direction.

The main functional blocks of an apparatus are shown in <FIG>.

The basic idea of an embodiment is to use the so called mobility analyzer (in this case a simple version called <NUM>th mobility analyzer by <NPL>). As opposite to normal use of this kind of mobility analyzers, the idea is to measure the collected current and use it alone as the measurement result indicating the particle concentration. It's important to find out that only a part of the current carried by charged particles are collected and measured, which is in fact against normal practice related to electrical particle monitors.

The particle size dependent response function (Rs) of apparatus, based on particle charging and measurement, by a mobility analyzer, of the electrical current carried by electrically charged particles, gives the measured current for a given particle number concentration, in units of Am<NUM> or, as below fAcm<NUM>. The response is the product of the charging efficiency Ech (fAcm<NUM>) and the collection efficiency of the mobility analyzer ηma (dimensionless) as shown in Equation <NUM>.

The response function Rs may also be called as response function of the mobility analyzer Rma.

Both factors of the response function in equation <NUM> are experimentally determined as a function of the particle diameter. In the present design, particles of no diameter range are collected with an efficiency of <NUM>%. In this special case of collecting only a fraction of the particles, the response function can be written e.g. for annular geometry (see e.g. <NPL>) Equation (<NUM>) can be written as <MAT> where.

As seen in equation <NUM>, the direct effect of the sample flow rate cancels out from the instrument response. This makes the response relatively independent of the flow rate and this does not happen if for any particle size all the particles are collected.

Although the direct flow rate effect is canceled, the product Pchn<NUM> is still in principle dependent on the flow rate. The charged particle losses in the charger decrease with decreasing residence time in the electric field. Therefore, the penetration Pch through the charger is increased with increasing sample flow rate Qs. The penetration as a function of flow rate is usually not a well-defined quantity, but it can be approximated by modeling the charger as a turbulent electrostatic precipitator (ESP). This would lead to flow dependence of the form of <MAT>.

Where Q<NUM> is a constant, dependent on the charger design. The value of this function starts from zero at zero flow rate and approaches unity at high flow rates.

On the other hand, the number of elementary charges per particle (n) depends on the product of the residence time and ion concentration in the charger (Nit-product, see e.g. <NPL>). The Nit-product of the charger decreases with increasing sample flow rate, which leads to decreased particle charging. The dependence of n on Nit is not completely understood in particle-size range in interest. However, the basic understanding of the nature of the charging mechanism predicts that it's close to logarithmic behavior. Neglecting the changes in the ion concentration one can approximate: <MAT>.

Where A and B are constants dependent on the charged design. This is a decreasing function as a function of flow rate. The product Pchn<NUM> behaves as a function of flow rate as follows: starting from zero flow rate, the product Pchn<NUM> increases from low values, goes through a maximum and then decreases with increasing flow rate. At the flow rate value producing the maximum, the derivative of the product with respect to flow rate is zero. Within a range of flow rate values close to this value, the product and the response of the whole instrument become independent on the sample flow rate. Compared to the state of the art where Pchn is maximized to optimize sensitivity of an analyzer, optimization for response independence to sample flow is thus surprisingly achieved by maximizing Pchn<NUM>.

As the method is good for essentially accurate monitoring of particle concentration with varying sample flows used in the method, the flow may be generated by movement of means where apparatus using the method is attached. Such moving means may be any moving vehicle, e.g. car, bus, airplane, train, ship, balloon or unmanned aerial vehicle. The apparatus based on the method can also be used in applications where apparatus is essentially stationary but air moves around it such as inside a ventilation channel or in any flow generated by chimney effect. The method may also be used in situations where both mechanisms are used for flow generation, such as in cases when air propeller enhances the flow generated by moving means.

Below the method and the apparatus are described in some embodiments. It has to be understood that such embodiments can only describe a brief detail of the possible uses of the method and the flow range may vary from flows in miniature sensors (less than <NUM> litres per minute) to flows in very large ventilation channels (more than <NUM><NUM> litres per minute).

The method can be used to monitor particle concentrations in different applications, such as monitoring indoor air quality, ambient air quality and particle emissions.

In the following, the invention will be described in more detail with reference to the appended principle drawing, in which.

For the sake of clarity, the figures 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 figure to emphasize the characteristics of the invention.

The invented method comprises measuring or monitoring the content of particulate matter in a flowing gas stream. In the method, electrical particle charging is used to charge at least some of the particles in sample flow taken into the measurement apparatus. The electrical current carried by at least some of the charged particles is measured, and thus that is the response of the method. Typical feature of the method is that it can measure the content of particulate matter within +/-<NUM>% accuracy when the volumetric flow through the measuring or monitoring apparatus which is measuring or monitoring particulate content in the flowing gas stream has a dynamic range of <NUM>, i.e. with nominal flow, Qsample or Qs, is <NUM>, the flow range is <NUM>-<NUM>. The term "accuracy" has here the meaning that when the particle concentration is measured with a certain volumetric flow inside the mentioned dynamic range, the same concentration in the sample flow is measured within +/-<NUM>% value from the first measurement with another volumetric flow within the same dynamic range. Nominal flow can change widely depending on the sensor design. The tests were mainly carried out by nominal flow of <NUM> litres per minutes, but the nominal flow can be e.g. <NUM>, <NUM>, <NUM> or <NUM> litres per minute as well. Such conditions are achieved e.g. by designing the essential operational parameters, like nominal sample flow through the filter and/or ion production in the electrical charger and constructing the apparatus (e.g. mobility analyzer dimensions) in such a way that the flow dependence of the charging efficiency Ech(Q) and the flow dependence of the particle collector collection efficiency η(Q) essentially cancel each other, making the response R of the method and apparatus essentially independent of the flow rate through the apparatus, Q.

<FIG> shows as an example a schematic drawing of one embodiment. Apparatus <NUM> is used for measuring or monitoring particles <NUM> in sample flow <NUM>. Apparatus <NUM> comprises means <NUM> for driving flow <NUM> into apparatus <NUM>. In other words, these means <NUM> can be called sample intake which may be a conventional tube or specially designed intake nozzle, various nozzle types being familiar to a person skilled in the art. Means <NUM> may also comprise functional elements to treat the sample flow, such as heating it (especially with ambient air measurements), removing volatile elements, such as water from the flow, neutralizing or charging the particles in the flow, etc. For the sake of clarity it is mentioned that means <NUM> may comprise means for active flow generation such as a blower but if active flow generation is required it is typically realized by means <NUM> placed downstream of electrical charger <NUM> and particle analyzer.

Apparatus <NUM> further comprises means <NUM> for electrically charging particles <NUM> to become electrically charged particles <NUM> by ions <NUM> produced by charger <NUM>. As understood from above, the flow dependency of charging efficiency Ech(Q) is a parameter important for the operation of the invented method and apparatus. Thus the method and apparatus may comprise method and means for controlling the charging efficiency Ech, although this is not shown in <FIG>.

Apparatus <NUM> further comprises means <NUM> for removing the free ions <NUM> which are not attached to particles <NUM> before the electrical charge carried by at least a fraction of charged particles <NUM> is measured. Typically the free ion removing means <NUM> is an electrical precipitator comprising voltage source 117A and ion trap created by electrical field between electrodes 117B and 117C. Ion trap can, however, be based on other mechanisms collecting free ions <NUM>, e.g. due to their higher diffusion coefficient as compared to particles <NUM> and charged particles <NUM>. If required, the electrical current generated to the collecting surface by free ions attaching on it may also be measured.

Apparatus <NUM> further comprises means <NUM> for collecting a fraction of charged particles <NUM>. The electrical current generated by the collected electrical charge on the fraction of collected electrically charged particles <NUM> is measured by means <NUM>, typically by an electrometer. <FIG> shows the collection method for a fraction of charged particles to be an electrostatic precipitator but it may as well be any device based on particle properties such as a diffusion-based collector combined to electrical charge monitoring means <NUM>.

Apparatus <NUM> further comprises means <NUM> for measuring electrical current/charge carried by essentially all charged particles <NUM>. Such measurement may be based on collecting essentially all charged particles <NUM> on a conductive filter <NUM> and measuring the charge collected on filter <NUM> by means <NUM> (on other words measuring the electrical current generated by collected charge with an electrometer). The total electrical charge/current measurement may also be based e.g. on measuring the escaping current as described in e.g. <CIT>. The same patent describes a way to generate the intake flow by using an ejector which may be applied with the current invention.

Apparatus <NUM> further comprises means <NUM> for comparing the electrical charge <NUM> detected in the means <NUM> collecting a fraction of charged particles <NUM> to electrical charge <NUM> generated by essentially all charged particles <NUM> and means <NUM> which are connected to the comparison means <NUM> and which are used to control collecting means <NUM> so that collecting means <NUM> collects less than a certain fraction of electrically charged particles <NUM>.

If collecting means <NUM> is an electrostatic precipitator, collection efficiency can be controlled by adjusting the electrical field strength generated by voltage source 119A and electrodes 119B and 119C, which generate the electrical field between them, of the electrical precipitator <NUM>. If collections means are e.g. a diffusion-based collector, the collection efficiency can be adjusted by adjusting volumetric flow through the collector (note that excess clean air flows can be used in addition to the sample flow) or by adjusting the physical parameters of the diffusion-based collector such as the surface area or the length of the collector.

The description above describes the method and the apparatus when it is used in DC mode. It can as well be used in AC mode by modulating a suitable parameter such as the electrical field strength of means <NUM> (when it is an electrostatic parameter), modulating the volumetric flow or the physical parameters of the diffusion-based collection unit <NUM> or modulating the electrical charger <NUM>. In AC mode problems, typical in DC-mode measurement can be avoided which is obvious for a person skilled in the art.

Apparatus <NUM> may also comprise means <NUM> for separating charger mechanically from the particle-polluted sample flow <NUM> flowing inside the outer wall <NUM> of apparatus <NUM>. Grid <NUM> ensures in this case ion <NUM> flow from charger to particles <NUM>.

The atmosphere inside separating means <NUM> may be different from atmosphere inside apparatus <NUM>. This may be actively generated by e.g. directing essentially pure air or other suitable gas inside separating means <NUM> and generating positive pressure inside means.

Free ion remover <NUM> is in one embodiment an electrostatic precipitator comprising preferably adjustable voltage source 115A and electrodes 115B and 115C between which electrical field is formed.

In one embodiment, free ion remover <NUM> and charged particle remover <NUM> are connected to the same central axis <NUM> of apparatus <NUM>. Means <NUM> removing fraction of charged particles <NUM> comprises an electrostatic precipitator comprising preferably adjustable voltage source 119A and electrodes 119B and 119C between which electrical field is formed.

Apparatus <NUM> can be constructed to be lightweight as there is no absolute need for means of generating flow <NUM>. Apparatus <NUM> can be installed to a moving object/means such as car, train, ship, airplane or equivalent. shows apparatus <NUM> being installed by fixing means <NUM>, <NUM> and <NUM> to a balloon <NUM> and the essentially vertical movement of balloon <NUM> (either up or down) generates flow through apparatus <NUM>.

Flow through apparatus <NUM> can also be realized without means of generating flow <NUM> even if apparatus <NUM> is stationary. <FIG> shows an embodiment where apparatus <NUM> is placed inside ventilation channel <NUM>. With ventilation channel there are basically three different measurement points: inlet (outdoor air), after the filters (purified air) and from the room (outlet, recirculation channel). Movement of intake air <NUM>, generated by blower <NUM> and preferably filtered, heated or cooled by treatment unit <NUM> generates the necessary flow through apparatus <NUM> fixed into channel <NUM> by fixing means <NUM> and <NUM>. The air from ventilation channel <NUM> is distributed through output terminals <NUM>.

One essential goal of the invention is to produce apparatus <NUM> which is low-cost and can thus be used in sensor networks. Such embodiment is shown in principle in <FIG> where urban area <NUM>, comprising e.g. residential building <NUM>, streets <NUM>, highways <NUM>, parks <NUM> and factories <NUM> are measured by apparatus <NUM> distributed widely into the urban area <NUM>. The measurement data from apparatuses <NUM> is preferably sent wirelessly into a cloud system and the data is analyzed and combined to provide a spatially accurate information in the air quality of the urban area. Such function can be carried out e.g. by Enfuser™ software developed by Finnish Meteorological Institute, FMI. Sensor network in urban area <NUM> may comprise various different pollutant sensors in addition to apparatus <NUM>, such as sensors for pollutant gases (e.g. O<NUM>, SOx, NOx), weather-related sensors (rainfall, wind direction, wind speed), noise sensors, traffic density sensors, etc..

In urban networks and in other ambient measurements apparatus <NUM> can be fixed e.g. to measurement pod show in <FIG>, <NUM>. Bar <NUM> is fixed to ground <NUM> and apparatus <NUM> is fixed to bar <NUM> via fixing means <NUM> and <NUM>. Flow intake means <NUM> of apparatus <NUM> may comprise heater <NUM> to heat sample flow <NUM> to <NUM>-<NUM>°C higher than the ambient temperature. The measured signal is sent to a wireless unit <NUM> via cable <NUM> and sent wirelessly e.g. to cloud server.

Apparatus <NUM> can further be used in indoor air measurement <NUM>, shown in <FIG>. Apparatus <NUM> is fixed to indoor area which is surrounded e.g. by wall <NUM>, door <NUM> and window <NUM>. Apparatus <NUM> is connected to display unit <NUM> either by cable <NUM> or wirelessly. Apparatus <NUM> and display <NUM> may be installed away from each other. The measurement signal can also be sent to a cloud system from which the raw or analyzed/synthesized data can be sent to a separate display unit, like tablet, mobile phone or equivalent.

The sensitivity of the response function R to flow rate Q was tested with polydisperse laboratory test aerosol using different sample flow rates in the range of <NUM> - <NUM> liters per minute (lpm). The used measurement setup is shown in <FIG>. As the aerosol generator <NUM> using pressurized air entering via line <NUM>, an evaporation condensation generator described by <NPL>, was used, using dioctyl sebacate (DOS) as particle material. Like in the monodisperse test measurements, the aerosol was generated to a mixing chamber <NUM> connected to aerosol generator via line <NUM> and valve <NUM>. Sample was diluted in mixing chamber <NUM> with filtered pressurized air (not shown in picture). The dilution air feed was kept constant and it exceeds the maximum flow rate needed for the prototype and the reference instruments, which made it possible to vary the prototype apparatus flow rate without affecting the particle size or the concentration. The excess flow <NUM> from the mixing chamber was led to the ventilation. During the measurements, the test aerosol size distribution median size (dm) was varied between <NUM> - <NUM>, while the geometric standard deviation (GSD) varied between <NUM> and <NUM>. The size distribution was measured using Scanning Mobility Particle Sizer Spectrometer (SMPS) <NUM>, consisting of a model <NUM> Differential Mobility Analyzer (DMA, TSI Inc. ) operated at closed loop setup described by <NPL>, with a flow circulating unit (FCU) <NUM> connected to SMPS via lines <NUM> and a model <NUM> Condensation Particle Counter (CPC, TSI Inc. ) <NUM> connected to SMPS via line <NUM>. A Water Condensation Particle Counter (WCPC) <NUM> model <NUM> (TSI Inc. ) connected to line <NUM> exiting from mixing chamber <NUM> was used to measure the total number concentration to improve the concentration measurement accuracy. In order to dilute the test aerosol concentration to the reference instruments two ejector dilutors <NUM> were used, one in front of the SMPS <NUM> and two in front of the WCPC <NUM>. The corresponding dilution ratios were <NUM> for the SMPS and <NUM> for the WCPC. The sample flow through apparatus <NUM> was controlled in the manner as in the monodisperse response measurements. Apparatus <NUM> which was connected to mixing chamber <NUM> via line <NUM> was further connected to vacuum line <NUM> via filter <NUM>, valve <NUM> and mass flow controller <NUM>.

<FIG> shows the measured and simulated test values. On the left side of <FIG>, apparatus response in polydisperse test measurements is shown (both measured and fitted responses shown). The measured values are plotted as the function of the particle size representing the mean response over the number size distribution. On right, the correlation plot between the simulated and measured apparatus outputs is shown. As seen from <FIG>, the response of apparatus <NUM> remains nearly independent on the sample flow rate in the range of <NUM> - <NUM> lpm.

The flow rate independence of the measured response is further demonstrated in <FIG>. On the left hand side, the normalized response of apparatus <NUM> is plotted against the sample flow rate for different particle size distributions. As seen on the left in <FIG>, the apparatus response remains nearly constant for the sample flow rate range from <NUM> lpm to <NUM> lpm. On the right hand side, in <FIG>, is plotted the apparatus output and the measured number concentrations during a ramp in the sample flow rate. During the flow ramp, the aerosol generation was kept constant, however the median size decreasing steadily from <NUM> in the beginning to <NUM> at the end. This is seen as a slight change in the apparatus <NUM> signal when comparing the signal level in the beginning and the end of the flow ramp. Despite this, the apparatus <NUM> signal is staying within <NUM> % of the average level during the experiment.

<FIG> Shows the calculated response function for one embodiment of the invention. It shows , with logarithmic scaling, that the accuracy of the equipment <NUM> is within +/-<NUM>% when the dynamic range of the equipment is <NUM>, i.e. flow is <NUM> - <NUM> when the nominal flow is <NUM>. The result of the invented method and apparatus is shown as response function curve of the equipment, Rma. Th other curve, Rf, shows the calculated response curve for a prior-art construction with particle charging and filter capture. The difference on the sensitivity of the response curve to flow rate Qsample has an outstanding difference between the prior-art and invented method.

A particular embodiment had a nominal flow of <NUM> litres per minute and thus the flow range where the accuracy of equipment <NUM> was +/- <NUM>% was <NUM> - <NUM> litres per minute.

The design procedure relating to the charger and the analyzer are explained with reference to the <FIG>. In <FIG> the following terms and symbols are used:.

The given starting values for the sensor are the sample flow rate Qs, desired minimum particle size of interest Dp min, charger produced ion concentration starting value, Ni and residence time inside the charger, t, and weakly bound mobility analyzer voltage Vma. In this example, the volume flow is given, but in different applications, the available pressure difference or gas velocity may be given. In these cases the resulting volume flow may be iteratively computed or experimented. The ion exposure Nit is a best estimate for a corona charger in the middle of normal operating range. Based on the residence time, the diameter of the cylindrical charger DC is computed according to: <MAT>.

The length of the charger, hc is preferably the same. This sets the residence time in the desired value.

Next, a measurement of the collection efficiency P and median charge number n is done. This is done according to <FIG>, where particles of known diameter are produced (as disclosed by<NPL>, for example). The particles are neutralized e. g with a radioactive bipolar neutralizer, after which particles are sampled (in Pn-mode) by a particle counter, e. g condensation particle counter (CPC). Rest of the sample is directed through the charger and the charge carried by the penetrated particles are measured with a Faraday cup electrometer. The ratio of the acquired charge per air volume divided by the particles counted by the particle counter gives the product of penetration and mean charge, Pn. Next the system is set to n-mode, where everything else being equal, the sampling point for the particle counter is changed to be parallel to the Faraday cup electrometer. Computing again the ratio of concentrations gives the mean charge per particle, n. Computing the product of these two gives the Pn<NUM>. This procedure is repeated in the expected flow rate range of the charger and the maximum point of Pn<NUM> is determined relative to Qs through the charger.

If the maximum of product Pn<NUM> is below the set nominal flow rate Qs (less than <NUM>,<NUM> of the nominal flow rate), the charger efficiency can be adjusted by reducing the charger voltage (decreasing losses) or by increasing the residence time by increasing the size of the charger. In case the maximum is above the set point (more than <NUM> of the nominal flow rate), opposite changes are effected to adjust the peak of the Pn<NUM> product to the nominal sample flow. The precision of the adjustment needs to be within <NUM> to <NUM> of the nominal sample flow to still fulfill the flow independence in flow range in similar flow range relative to nominal (<FIG>). In case the charger voltage and diameter adjustments are not sufficient to reduce the Pch to appropriate level it is also possible to add an additional electrostatic ion trap, independently adjustable relative to the charger voltage. The adjustment of the voltage in relation to the peak of the Pn<NUM> product is the same as described for the charger voltage.

After this adjustment, the analyzer diameters are set, preferably such that the cross section of the sample passage is increased or kept the same. According to an embodiment, for a cylindrical analyzer the inner diameter of the analyzer is two times the charger diameter and outer diameter of the analyzer four times the charger diameter.

After this the final dimension is the analyzer maximum length, which can be calculated according to: <MAT>.

This provides necessary conditions of both the lossy charger and incomplete collection by the analyzer section, which provide the optimal flow independence.

After this procedure, the design is complete.

Claim 1:
Method for measuring or monitoring the content of particulate matter suspended in carrier gas, sampled with sample volume flow rate (Qs), the method comprising electrical particle charging of at least some of the particles in the same sample flow, collecting at least some of the charged particles using electrostatic force in the same sample flow and measuring the electrical current carried by at least some of the charged particles, characterized in that the charging and collecting are dimensioned by maximizing Pchn<NUM>, where Pch = particle penetration through charger and n = number of elementary charges on a particle, such that the current carried by the particles collected is measured and the said current is only part of total current carried by the charged particles.