GB2374671A - Methods to improve electrostatic particle measurement - Google Patents

Methods to improve electrostatic particle measurement Download PDF

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Publication number
GB2374671A
GB2374671A GB0109506A GB0109506A GB2374671A GB 2374671 A GB2374671 A GB 2374671A GB 0109506 A GB0109506 A GB 0109506A GB 0109506 A GB0109506 A GB 0109506A GB 2374671 A GB2374671 A GB 2374671A
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sense electrodes
electrodes
electrode
means according
sense
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GB0109506D0 (en
GB2374671B (en
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Kingsley Stjohn Reavell
Nicholas Collings
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Cambustion Ltd
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Cambustion Ltd
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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N15/00Investigating characteristics of particles; Investigating permeability, pore-volume, or surface-area of porous materials
    • G01N15/02Investigating particle size or size distribution
    • G01N15/0266Investigating particle size or size distribution with electrical classification

Abstract

An electrostatic instrument for measuring particle concentrations and possibly sizes in aerosols, such as an Electrostatic Low Pressure Impactor or Differential Mobility Analyser suffers from errors which limit the useful response bandwidth of the device. The invention minimises or eliminates these transient errors which are caused by changing particle concentrations in the aerosol. A system may be added to an otherwise conventional instrument to compensate for the transient effects based on a model of the charge production mechanism. Alternatively, a screening electrode 8 placed over the sense electrodes 7 in the instrument, and held at a controlled electrical potential difference, is added to the instrument to eliminate the effect. A third embodiment adds compensating electrodes which provide a direct measurement of the transient effect which can be subtracted from the signal.

Description

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Methods To Improve Electrostatic Particle Measurement.

This invention relates to methods to improve electrostatic particle measurement.

Particle concentrations in aerosols (a suspension of particles in a gas) are often measured by electrostatic techniques based on the principle of charging the particles in a sample of the aerosol and collecting them on one or several electrodes or filters. The current flowing to these electrode or filters, here referred to as'sense electrodes', is measured and indicates the quantity of particles collected and hence their concentration in the aerosol.

The particles may be charged by one of a number of methods, such as ultraviolet irradiation or corona discharge; or natural charging (often associated with a combustion process) may be relied upon. Frequently, differences in mobility (the readiness of particles to diffuse or drift through the gas) are used to separate different sizes of particle before collecting them on the electrode. Some devices alternatively use differences in momentum for this discrimination.

Such devices are used to make measurements of the number of particles and sometimes the spectrum of particle sizes in aerosols, but are limited to resolving accurately only relatively slow changes in the particle concentration. This is because faster changes lead to transient discrepancies between the actual particle concentration and the measured current which are caused by the rate of change of the concentration of charged particles in the aerosol near the detectors.

According to the present invention, modifications are made to the design of electrostatic particle measurement instruments to compensate for 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.

Three embodiments of this invention will now be described.

Figure I is a cutaway diagram of a conventional Differential Mobility Analyser (DMA) to which the invention can be applied.

Figure 2 is a block diagram showing the operation of the first embodiment of the invention, a compensation system for the transient current errors in a conventional DMA.

Figure 3 is a cutaway diagram of a DMA modified with the addition of a screening electrode according to the second embodiment of the invention.

Figure 4 is a cutaway diagram of a DMA modified with the addition of compensating electrodes according to the third embodiment of the invention.

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The first embodiment of this invention is a system added to or incorporated in an otherwise conventional particle measuring instrument which corrects for the effects of the rate of change of charged particle concentration, which is estimated from the variation over time of the currents measured on the sense electrode or electrodes. This embodiment is applicable to instruments where a sample of the aerosol to be measured, which may optionally be diluted and/or passed through a charging device, flows past or through the sense electrode or electrodes which are connected to a current measuring electrical circuit.

This embodiment is preferably applied to instruments where all or a fixed proportion of the total number of charged particles in the aerosol are collected on the sense electrodes.

These instruments often also include other electrodes mounted near the sense electrodes which create an electrical field for the purpose of evaluating the mobility of the particles, from which the current is not measured.

The correction for the effects of the rate of change of charged particle concentration is based on the following model for the mechanism of current generation: A charged particle near an electrode held at a fixed potential attracts a charge of the opposite sign onto the electrode from the connected circuit. The quantity of charge attracted is proportional to the charge on the particle but is also a function of its distance from the electrode and the geometry of other conductors nearby. As the particle approaches the electrode, the charge attracted to the electrode increases and therefore there is a current flow in the connected circuit. When the particle finally reaches the electrode, the total attracted charge due to that particle is equal and opposite to its charge.

Therefore, when a large number of particles steadily flowing to the electrode is considered, the current measured is as if it were just produced at the time the particles were collected.

If the concentration of charged particles in the aerosol near the electrode changes, however, this will lead to a change in the charge induced on the electrode and therefore an extra component of current in the connected circuit before the particles at the new concentration reach the electrode. If the change in concentration is a reduction, the magnitude of charge on the electrode will reduce producing a component of current in the opposite direction from that produced by collected particles.

Using this model, we can see that any current measured to the sense electrode is due to the sum of the effects of new charged particles entering the vicinity of the electrode and those charged particles already near it approaching, less the effect of any particles which leave the vicinity of the electrode. This embodiment of the current invention uses this to estimate accurately the concentration of particles in the aerosol at any instant without

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errors which come from ignoring the effects of changing charge concentration near the electrode and will be explained by reference to a specific example.

The invention may be applied to a Differential Mobility Analyser (DMA) shown in Figure 1. In this instrument, the sample aerosol flow 1 is passed through a charging device 4 which applies preferably a single positive charge to the particles and then flows along the centre of an annular channel surrounded by a sheath flow of clean gas 2 and 3 at the same flow velocity. The inner wall of the annular channel is formed by an electrode 5 held at a high positive potential. Coaxial with this, set into the outer wall 6 of the channel along its length, are rings 7 which act as the sense electrodes. The sense electrodes are held at electrical earth potential and the current flow from them is measured: for clarity, this connection is shown schematically for just one of the electrodes in figure 1. The sense and centre electrodes set up an electrical field in the channel which causes the positively charged particles to drift from the sample aerosol, through the sheath flow, towards the sense electrodes. As the charging is arranged so that most particles receive the same charge and therefore experience the same electrostatic force, the smaller particles, with less aerodynamic resistance, drift faster, reaching the outside of the channel sooner and being collected on an electrode further upstream than the larger particles. For this embodiment of the current invention to be applied to the DMA, either the voltages, flow rates and geometry of the instrument should be designed such that almost all the particles in the aerosol are collected before the flow exits the channel, or additional mesh, gauze or functionally equivalent, electrodes should be mounted at the exit end to collect the remaining particles, the current flow from these electrodes also being measured.

This type of instrument suffers particularly from transient errors due to the rate of change of particle concentration in the aerosol because generally more particles pass through an upstream sense electrode ring than are collected on it, so changes in the concentration of these larger particles which pass through can dominate the current from the particles actually collected on that ring.

The invention is embodied as the correction system shown in the block diagram in Figure 2. The Sense Electrodes El to E4 in this diagram are the four most downstream of the Sense Electrodes, 7, in Figure 1. El is the sense electrode furthest downstream. As very few charged particles leave the channel without being measured, the current on sense electrode EI indicates the number of particles per unit time ('particle flux') in the aerosol that passed through the previous electrode E2 uncollected. To correct for the transient errors due to the change in particle concentration in the vicinity of E2, therefore, the rate of change of the current on EI (shown by the d/dt box) is multiplied by a constant Al, l and subtracted from the current measured on E2. Preferably, this correction is applied to

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the value of the current measured on the penultimate electrode E2 a short period earlier and stored, as indicated by the delay blocks in figure 2; this delay period being equal to the time taken for the aerosol and sheath flow to travel from the average position of the penultimate electrode to that of the final electrode. Practically, the differentiation operation d/dt may be approximated, for example by a finite difference. The constant of proportionality A, between the rate of change of particle flux and the correction applied to the penultimate electrode current is approximately equal to the time spent by the particle in the vicinity of the penultimate electrode multiplied by the influence coefficient (defined below) of a charged particle at an average radial position in the vicinity of the penultimate electrode for particles collected on the final electrode. Preferably, the constant of proportionality is the integral over time of the influence factor along the path followed by the charged particle. The constant of proportionality may, alternatively, be derived from measurements of the particular instrument.

The influence coefficient of a charged particle near an electrode is the ratio of the magnitude of charge attracted onto the electrode by the charged particle divided by the charge on the particle. It is a function of the distance of the particle from the electrode and the geometry of the electrode and other conductive bodies (such as the centre electrode in the DMA) nearby. This can be calculated for the particular geometry of the instrument by conventional techniques such as Gauss's law and the principle of overall charge neutrality or widely available computer programmes. For the annular geometry of the DMA, which can be approximated as a two-dimensional coaxial field between a centre circle (the centre electrode) of radius rl and an outer circle of radius r2, the influence coefficient is approximately that in eqn 1 : <img class="EMIRef" id="024176559-00040001" />

in xi influence coefficient = c, = 1 ±- eqn 1 In AJ for a particle at a radial position rx.

The radial location of a charged particle at the penultimate ring can be estimated from the forces applied to it as it passes through along the DMA channel. The most important forces are aerodynamic and the electrostatic forces due to the applied electric field and charge image attraction to surfaces. For the DMA, the applied electric field is large and therefore dominates the charge image effects in the bulk of the column. The radial entry location is approximately known, as the aerosol all enters near the centre of the channel, and one other point on the path is known to be the location of the final electrode. The

<Desc/Clms Page number 5>

electric field can be calculated from Gauss's law: for the concentric geometry of the <img class="EMIRef" id="024176559-00050001" />

DMA channel, the result is standard (eqn 2). <img class="EMIRef" id="024176559-00050002" />

electric field = E = (X) eqn 2 r In 2 r <img class="EMIRef" id="024176559-00050003" />

Along with Stokes'theorem (strictly modified by a Cunningham slip correction factor for these small particles) which states that the drift velocity of a particle is proportional to the force on it, this allows the path the particle follows through the channel to be predicted and thus the influence coefficient of any collected particle on any upstream ring to be evaluated. In figure 2, the coefficient Am, n is the influence coefficient of particles collected at sense electrode Em on the charge on sense electrode En, multiplied by the time they spend in the vicinity of electrode En.

When this correction is applied to the current from the penultimate electrode E2, this then gives an accurate reflection of the particle flow collected at that electrode. Therefore, in the same way, the current measurement on the next electrode E3 can be corrected for the change in particle concentration of particles collected at both downstream rings E I and E2 (separately, as the radial location and hence coefficient Am, n of the two sizes of particles will be different at the axial position of the upstream electrode). The same process is then applied successively to all upstream electrodes: each of which is corrected for the currents collected on all the downstream electrodes.

The compensation system is also applicable to other types of electrostatic particle measuring instruments such as the Electrostatic Low Pressure Impactor (henceforth ELPI). This instrument charges the particles in an aerosol to be measured, in a similar way to the DMA, and then passes the aerosol through a column of impactors, consisting of perforated plates followed by collection plates covered with grease. When the aerosol passes through these perforations, relatively massive particles are forced by their momentum to hit the collection plate where they stick to the grease, while the lighter particles are carried by the gas flow to the subsequent stages. The size of the perforations and plates is varied throughout the column such that the largest particles are collected on the earliest collection plates, and successively smaller particles on the later collection plates. Measurement of the electrical current flowing to these collection plates, which act as the sense electrodes in this instrument, indicates the number of particles collected by each and hence the concentration of each size class of particles.

The ELPI is thus also susceptible to transient errors in the particle concentration measurement caused by changes in the concentration of smaller charged particles near the early collection plates which measure the larger particles. The scheme described above

<Desc/Clms Page number 6>

and in Figure 2 would correct the signal on the upstream sense electrodes (collection plates) for the numbers of charged particles collected on the downstream sense electrodes.

The second embodiment of this invention is a modification to a standard electrostatic particle measuring consisting of an additional electrode which screens the sensing electrode or electrodes from the effect of changing charged particle concentrations in the aerosol nearby.

The existing sense electrodes in the instrument are largely covered by an additional screening electrode which is constructed so that the majority of particles can still pass through it. This might be achieved by making the screening electrode of conductive mesh, gauze, perforated sheet, an array of wires, or similar. The electrical potential of the screening electrode is controlled (possibly to electrical ground), probably by connecting it to a voltage supply. The screening electrode should be mounted close to the sense electrodes because it eliminates the effect on the sense electrode current of changes in the charged particle density in the aerosol beyond the screening electrode but not in the volume between the screening electrode and the sense electrodes. For practicality, one screening electrode may screen one or more sense electrodes or several screening electrodes may be used. The screening electrode should be designed to impede the flow of charged particles through it as little as possible, so for instance, the open fraction of the gauze should be maximised.

Figure 3 shows the invention applied to a Differential Mobility Analyser. The basic instrument is as in Figure I described above. This example uses a single screening electrode 8 to cover all the ring shaped sense electrodes set into the outer wall of the channel. The screening electrode takes the form of a gauze tube which is supported regularly on electrically insulating rings. The screening electrode is connected to a voltage supply: the voltage on the screening electrode should ideally be such that there is still a significant electrostatic field to attract particles from the vicinity of the screening electrode to the sense electrodes, which must overcome the charge image force that will attract particles to the screening electrode.

The screening electrode in this example also reduces the sensitivity of the instrument to changes in the dielectric constant of the gas being measured and to noise in the high voltage supply. The sense electrodes in the DMA carry a constant charge due to the capacitance between them and the centre electrode 5. In the case of the conventional DMA in Figure 1, changes either in the electrostatic permittivity (due to, for example, water vapour concentration changes) of the aerosol or the voltage on the centre electrode will lead to a change in this charge which will be observed as an erroneous current. The screening electrode will prevent changes in the permittivity of gas except in the gap

<Desc/Clms Page number 7>

between the screening electrode and sense electrodes from affecting the charge on the sense electrodes, and in the DMA the sample flow remains near the centre of the channel, away from this region. With the screening electrode fitted, the potential difference which determines the charge on the sense electrodes is now that between the screening electrode and the sense electrodes, and as this potential difference is smaller, it is may be easier to minimise the electrical noise on this voltage.

A third embodiment of this invention is a modification to an electrostatic particle measuring instrument by which additional compensating electrodes are mounted near the sensing electrodes but held at a different potential from them such that they do not collect significant numbers of charged particles. The compensating electrodes may be the same size or a different size to the sense electrodes, and should be mounted as close to them as possible. The current flowing to these electrodes is then due only to the rate of change of charged particle density in the aerosol near the electrodes. With appropriate gain to take account of the different sizes of the electrodes, the current measured to the compensating electrodes is subtracted from the current measured on the sense electrodes and thus the effect of changing charged particle concentrations is eliminated.

Figure 4 shows such modification made to a Differential Mobility Analyser. The compensating electrodes 9 are mounted between successive sense electrodes 7 in the channel. The compensating electrodes must be controlled to a voltage such that there is little electric field attracting the charged particles towards them. In figure 4, one such electrical connection is shown for clarity. For the DMA, the compensating electrode voltage should preferably be approximately equal to the voltage of the centre electrode. In this example, the correction used for each sense electrode is preferably equal to the average of the currents measured flowing to the electrodes on either side, multiplied by approximately the ratio of the sense electrode length to the compensating electrode length. Preferably, this multiplication factor can be modified during calibration of the instrument.

Claims (14)

  1. Claims 1. In an instrument measuring current flow to an electrode or electrodes, hereinafter known as the sense electrodes, to indicate the quantity of particles in an aerosol, means to eliminate or compensate for the part of the current flowing to said sense electrodes which is caused by the rate of change of charge density in said aerosol near said sense electrode.
  2. 2. Means according to claim 1 where the rates of change of the currents measured on said sense electrodes are used to compensate for the components of the currents measured which are due to the rates of change of charge density in said aerosol near said sense electrodes.
  3. 3. Means according to claim 2 where the currents measured on said sense electrodes at some time are compensated according to the rates of change of currents measured at a later time.
  4. 4. Means according to claim 2 where said sense electrodes are mounted along a channel through which said aerosol flows and the current measured on each of said sense electrodes is compensated according to the rate of change of current measured on those of said sense electrodes which are downstream.
  5. 5. Means according to claim 4 where the current measured on each of said sense electrodes at some time is compensated according to the rate of change of current measured at a later time on those of said sense electrodes which are downstream.
  6. 6. Means according to claim 5 where the current measured on each of said sense electrodes at some time is compensated according to the rate of change of current measured on those of said sense electrodes which are downstream after these have themselves been compensated by consideration of those of said sense electrodes which are further downstream.
  7. 7. Means according to claims 4,5 or 6 where one or more of said sense electrodes take the form of meshes or conductive filters located across all or part of the downstream end of said channel.
  8. 8. Means according to claims 4, 5,6 or 7 where the compensation applied to the current measured on each of said sense electrodes takes account of a predicted charged particle trajectory along said channel due to diffusion or drift effects.
  9. 9. Means according to claims 4,5, 6,7 or 8 where the flow down said channel includes both said aerosol and an additional flow of gas of no or known particle content.
    <Desc/Clms Page number 9>
  10. 10. Means according to claim 1 where an additional electrode, hereinafter known as the screening electrode, is mounted near to said sense electrodes such that the flow of charged particles from the aerosol towards said sense electrodes must pass through said screening electrode which is therefore constructed to allow the passage of a proportion of said charged particles, said screening electrode being controlled to an electrical potential relative to said sense electrode.
  11. 11. Means according to claim 10 where more than one of said screening electrodes is mounted near said sense electrodes.
  12. 12. Means according to claim 10 where said sense electrodes are in the form of rings around a channel containing said aerosol and said screening electrode is in the form of a gauze or similar tube mounted inside said sense electrodes.
  13. 13. Means according to claim 1 where one or more additional electrodes, hereinafter known as the compensating electrodes, are mounted near said sense electrodes but controlled to an electrical potential relative to said sense electrodes such that said compensating electrodes collect relatively few or no charged particles and the current flowing to said compensating electrodes, caused substantially by the rate of change of charge near the electrodes, is used to compensate the current measured on said sense electrodes either electronically or mathematically either at the time of measurement or later thereby to improve the accuracy of the indication of the quantity of particles.
  14. 14. Means according to claim 13 where a number of said sense electrodes in the form of rings is mounted coaxially around a chamber containing said aerosol, and said compensating electrodes are in the form of rings mounted coaxially between some or all of said sense electrodes.
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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
ES2206019A1 (en) * 2002-04-11 2004-05-01 Consejo Sup. Investig. Cientificas Method for improving resolution of electrical differential mobility analyzer (DMA), involves exchanging input streams of transporting air and aerosol so that aerosol flows beside inner electrode while air flows beside outer electrode
ES2212745A1 (en) * 2003-01-13 2004-07-16 Centro De Investigaciones Energeticas, Medioambientales Y Tecnologicas (C.I.E.M.A.T.) High sensitivity and high resolution device for measuring particle concentrations in cascade impactors.
WO2010061327A1 (en) * 2008-11-25 2010-06-03 Koninklijke Philips Electronics N.V. Sensor for sensing airborne particles
US8181505B2 (en) 2008-02-06 2012-05-22 Basf Se Measurement system for the multidimensional aerosol characterization
WO2014033040A1 (en) 2012-08-30 2014-03-06 Naneos Particle Solutions Gmbh Aerosol measuring device and method
US9222856B2 (en) 2010-08-27 2015-12-29 Regents Of The University Of Minnesota Measurement of particle morphology using filtration
US10502710B2 (en) 2016-06-06 2019-12-10 Alphasense Limited Particulate matter measurement apparatus and method

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4574004A (en) * 1980-10-28 1986-03-04 Schmidt Ott Andreas Method for charging particles suspended in gases
WO1999041585A2 (en) * 1998-02-13 1999-08-19 Tsi Incorporated Instrument for measuring and classifying nanometer aerosols

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4574004A (en) * 1980-10-28 1986-03-04 Schmidt Ott Andreas Method for charging particles suspended in gases
WO1999041585A2 (en) * 1998-02-13 1999-08-19 Tsi Incorporated Instrument for measuring and classifying nanometer aerosols

Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
ES2206019A1 (en) * 2002-04-11 2004-05-01 Consejo Sup. Investig. Cientificas Method for improving resolution of electrical differential mobility analyzer (DMA), involves exchanging input streams of transporting air and aerosol so that aerosol flows beside inner electrode while air flows beside outer electrode
ES2212745A1 (en) * 2003-01-13 2004-07-16 Centro De Investigaciones Energeticas, Medioambientales Y Tecnologicas (C.I.E.M.A.T.) High sensitivity and high resolution device for measuring particle concentrations in cascade impactors.
WO2004063677A1 (en) * 2003-01-13 2004-07-29 Centro De Investigaciones Energeticas Medioambientales Y Tecnologicas (C.I.E.M.A.T.) High-sensitivity and high-resolution device for measuring concentrations of particles in cascade impactors
US8181505B2 (en) 2008-02-06 2012-05-22 Basf Se Measurement system for the multidimensional aerosol characterization
WO2010061327A1 (en) * 2008-11-25 2010-06-03 Koninklijke Philips Electronics N.V. Sensor for sensing airborne particles
CN102224406A (en) * 2008-11-25 2011-10-19 皇家飞利浦电子股份有限公司 Sensor for sensing airborne particles
US8607616B2 (en) 2008-11-25 2013-12-17 Koninklijke Philips N.V. Sensor for sensing airborne particles
CN102224406B (en) * 2008-11-25 2016-12-21 皇家飞利浦电子股份有限公司 Sensor for sensing airborne particles
US9222856B2 (en) 2010-08-27 2015-12-29 Regents Of The University Of Minnesota Measurement of particle morphology using filtration
WO2014033040A1 (en) 2012-08-30 2014-03-06 Naneos Particle Solutions Gmbh Aerosol measuring device and method
US10502710B2 (en) 2016-06-06 2019-12-10 Alphasense Limited Particulate matter measurement apparatus and method

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