Apparatus and methods for charge conditioning

Abstract

An apparatus and method for charge conditioning aerosol particles in an aerosol particle containing gas to produce a controlled charge distribution.

Description

[0001] APPARATUS AND METHODS FOR CHARGE CONDITIONING

[0002] Field of the invention

[0003] The present invention relates to the field of charge conditioning, typically of aerosol particles. to the invention

[0004] Aerosols consist of solid and / or liquid particles (aerosol particles) suspended in a gaseous medium, for example air. Aerosols can adversely affect the climate, environment and human health, and are commonly emitted as a by-product of combustion or industrial processes. Thus, it is advantageous to quantify aerosol particles and it is known to electrically charge aerosols in a charge conditioner for analysis. Existing sensors range from high-resolution devices for characterising particle size or surface area for regulatory monitoring or experimental purposes to low- resolution, low-cost devices for monitoring averaged particle properties whether indoors or outdoors, sometimes as part of a network of sensors. Such devices aim to provide measurements with high spatial and time resolution.

[0005] Aerosol sensors which seek to provide information on particle size, surface area, mass and / or number concentration, commonly comprise a charging region which applies a discrete and often known electrical charge level (e.g. -1, 0, +1 , +2 charges etc.) to aerosols, typically with a probability distribution. Charge conditioning may be accomplished by mixing, typically flowing, aerosol particles, with ions. Ions may, for example, be generated in a gas by ionizing radiation from a radioactive (alpha or beta) source.

[0006] A classification region typically selects specific particles by a characteristic parameter, involving a balance of ferees acting on individual particles in continuous flow, typically involving electrostatic versus drag or inertial forces. After classification, particles are typically counted with a particle counter. In compact, low-cost sensors for environmental or personal exposure monitoring, detection and classification may be combined, using an ion trap which capture highly mobile, excess ions, downstream of the charging region using an externally applied electrical field, and the remaining particles are sensed via electrical currents, using sensitive electrometers. These simplified devices typically yield averaged size and concentration measurements rather than resolved size distributions.

[0007] Aerosol particles may also be engineered for the production of materials with unique properties and aerosol charge conditioners may be used to characterise, manufacture or use such materials. For example, charge neutralisers are used to neutralise the charge on aerosols used in spray painting or prior to sampling / collecting of the particles using electrostatic methods.

[0008] There are a number of known approaches to the charge conditioning of aerosol particles. In bipolar charging, a mixture of positive and negative ions mixed with the aerosol particles, condition the charge of the aerosol particles by virtue of collisions arising from typically diffusion and electrostatic forces. The fraction of total particle concentration at each discrete charge level tends to typically follow an asymmetric Boltzmann distribution, centred near neutral.

[0009] It is understood in the field that bipolar charge conditioners, are typically used in configurations where the particle charging process reaches a steady-state. Otherwise, the charge distribution of charge conditioned aerosols is sensitive to parameters such as: particle concentration; ion concentration (and so source strength, where ions for charging are generated using an ionizing radiation source, such as a radioactive isotope, x-ray or corona discharge);

[0010] - flow rate / particle residence time in the charging region; and

[0011] - the initial charge state of received aerosol particles.

[0012] Thus, the performance of the charge conditioners using bipolar charging typically limits the sensitivity, capacity or response time of sensors and other apparatus for working with aerosol particles. Some embodiments of the invention seek to address one or more of these limitations. Some embodiments of the invention seek to improve the cost, sensitivity, reliability, capacity or footprint of charge conditioners using bipolar charging. Some embodiments of the invention seek to facilitate the use of smaller or lower-activity ionizing sources (such as radioactive isotopes below the IAEA exemption limit) than are in common use.

[0013] In the case of unipolar charge conditioners, ions of one polarity (i.e. either positive or negative) are generated, for example by corona discharge, and these transfer charge to aerosol particles by collision. Unipolar charging does not reach a steady-state and so the process typically requires calibration due to sensitivity to the rate of ion generation and the efficiency of charge transfer to the aerosol particles. Some embodiments of the invention seek to expand the sensitivity, reliability, performance or footprint of unipolar charging by using a more stable ion generator (e g. radioactive isotope), while accounting for the effect of ions of both polarities initially being generated. Some embodiments of the invention intend to operate in a regime between traditional bipolar and unipolar charging, resulting in a highly-asymmetric bipolar charge distribution that is advantageous for many applications (e.g. particle manufacture or sensing), while recognizing the benefits of reaching a steady-state charge distribution previously mentioned.

[0014] Some embodiments of the invention are also relevant to static neutralisers which operate on gases with minimal aerosol particle content.

[0015] It is in this context that the present inventions have been devised. of the invention

[0016] The invention extends in a first aspect to apparatus comprising: a body having an inlet and an outlet and a conditioning chamber therebetween, the body defining a flow-through path for a gas from the body inlet and through the conditioning chamber to the body outlet; the conditioning chamber having an inlet and an outlet, the apparatus comprising an ion generator, the conditioning chamber defining a charging region receiving generated ions; the conditioning chamber comprising a flow expansion region between the inlet of the conditioning chamber and a maximum cross sectional area region, the flow expansion region configured to increase the cross-sectional area of gas flowing through the conditioning chamber, wherein the charging region is within or overlaps the maximum cross sectional area region and the mean position of generated ions in the charging region is closer to the outlet of the conditioning chamber than to the inlet of the conditioning chamber.

[0017] In a second aspect, the invention extends to a method of charge conditioning a gas (typically a gas comprising aerosol particles), the method comprising: providing a body comprising a conditioning chamber, the conditioning chamber having an inlet and an outlet, receiving a flowing gas (typically a gas comprising aerosol particles) into the conditioning chamber through the inlet of the conditioning chamber, expanding the cross-sectional area of the flowing gas within the conditioning chamber, generating ions mixed with the expanded flowing gas, the ions colliding with aerosol particles within the gas and thereby charge conditioning the charge distribution of the aerosol particles within the gas, the charging process progressing towards a steady-state, wherein the charging process is ongoing when either (i) the gas flows out of the conditioning chamber through the outlet of the conditioning chamber, or (ii) ions are removed from the flow gas in a quenching region defined between at least two electrodes.

[0018] The apparatus may be a particle charge conditioner. The gas may comprise aerosol particles. Thus, the gas may be an aerosol. The apparatus may be for charge conditioning a sample of aerosol particles in an aerosol to have a controlled charge distribution.

[0019] The apparatus may comprise a processor programmed to determine a particle size, surface area and / or concentration parameter (of aerosol particles in an aerosol) taking into account the flow rate of gas through the apparatus. The method may comprise determine a particle size, surface area and / or concentration parameter (of aerosol particles in an aerosol) taking into account the flow rate of gas through the apparatus. The gas may be a sample gas. The gas may be an aerosol sample.

[0020] However, the apparatus may be a static eliminator. The gas may be air.

[0021] It may be that the flow expansion region is at least 3.25 times longer axially than the maximum cross sectional area region.

[0022] It may be that the flow expansion region has an internal volume at least 1.25 times the volume of the maximum cross sectional area region.

[0023] The generated ions may be received from the ion generator. The ion generator may generate the ions within the charging region (for example, the ion generator may comprise a radioactive source which emits radiation which ionizes gas molecules in the charging region). Ions may be mixed with the flowing gas by being generated in the flowing gas. For example, ions may be generated in the charging region by irradiating or ionizing the charging region. The apparatus may comprise a radioactive source, which may be in the conditioning chamber.

[0024] However, it is also possible to generate ions in gas outside of the charging region and to mix the ion-containing gas thereby generated with the (flowing) gas in or upstream of the charging region to form a gas containing ions. The collisions between ions and the aerosol particles in the gas typically change the charge distribution of the aerosol particles.

[0025] It may be that the charging region is the volume within which a density of generated ions is at least 1%, or at least 5%, of the highest density of generated ions

[0026] It may be that the charging region extends along at least 87% of the axial length of between the mean position of the generated ions within the conditioning chamber and the outlet.

[0027] It may be that the charging region includes at least 99% of the internal volume of the conditioning chamber which is between the mean position of the generated ions within the conditioning chamber and the outlet.

[0028] It may be that the charging region extends from the conditioning chamber through the outlet of the conditioning chamber, preferably wherein the external radiation exposure at the conditioning chamber outlet is approximately 0 mSv / h.

[0029] It may be that the charging region is within the conditioning chamber and extends to the outlet from the conditioning chamber, but does not extend to the inlet to the body.

[0030] It may be that the charging region is within the conditioning chamber and extends to the outlet from the conditioning chamber, but does not extend to the inlet to the conditioning chamber. It may be that conditioning of the charge distribution begins only after the gas has entered the conditioning chamber and continues until the gas reaches the exit from the conditioning chamber. It may be that the method comprises expanding the cross-sectional area of the flow of the gas upstream of the charging region. It may be that the method comprises expanding the cross-sectional area of the flow of the gas within the charging region. It may be that the method comprises expanding the cross-sectional area of the flow of the gas in a region overlapping the charging region.

[0031] It may be that the method then comprises contracting the cross-sectional area of the flow of gas downstream of the charging region, typically prior to at least partial quenching, whereupon the contraction takes place over a shorter distance than the expansion. It may be that the method then comprises contracting the cross-sectional area of the flow of gas within the charging region, typically prior to at least partial quenching, whereupon the contracting takes place over a shorter distance than the expansion. It may be that the method then comprises contracting the cross-sectional area of the flow of gas in a region overlapping the charging region, typically prior to at least partial quenching, whereupon the contracting takes place over a shorter distance than the expansion.

[0032] The conditioning chamber may comprise a flow expansion region between the inlet of the conditioning chamber and a region of maximum cross-sectional area, the flow expansion region configured to increase the cross-sectional area of gas flowing through the conditioning chamber prior to charge conditioning in the charging region. By cross sectional area we refer to cross-sectional area through which gas (e g. aerosol) flows along the flow through path, perpendicular to the flow through path.

[0033] It may be that the shorter distance over which contracting takes place is less than half, less than 25%, less than 20% or less than 15% of the distance over which expansion takes place. The flow contracting region may have a length of less than half, less than 25%, less than 20% or less than 15% of the length of the flow expanding region.

[0034] Typically, the charging region comprises at least part of the region of maximum cross- sectional area.

[0035] The region of maximum cross-sectional area is typically a tubular region of the conditioning chamber. Nevertheless, the region of maximum cross-sectional area may have minimal axial extent. It may, for example, be a transition between a flow expansion region and a flow contraction region.

[0036] It may be that the majority of the generation of ions, by an ion generator (typically an ionizing radiation source), in the flow of the gas, takes place after the cross-sectional area of the gas has expanded to a maximum cross-sectional area.

[0037] It may be that the method comprises expanding the cross-sectional area of the flow of the gas upstream of the charging region and then compressing the cross-sectional area of the flow of gas downstream of or overlapping the charging region whereupon the compression takes place over a shorter distance than the expansion.

[0038] It may be that the flow expansion region extends along at least 60% of the length of the body and / or has a volume which is at least 50% of the internal volume of the body (comprising the conditioning chamber, the conduit between the body inlet and the conditioning chamber inlet, and, where present, a conduit between the conditioning chamber outlet and the body outlet).

[0039] It may be that the flow expansion region extends along at least 60% of the length of the conditioning chamber and / or has a volume which is at least 50% of the volume of the conditioning chamber.

[0040] The flow expansion region may taper outwards with an angle of less than 14°. It may be that the flow expansion region tapers outwards with a variable inclusion angle, for example it may comprise regions with two or more different inclusion angles.

[0041] The flow contraction region may taper inwards with an inclusion angle of 30° to 60°. The flow contraction region may taper inwards with a variable inclusion angle, for example it may comprise regions with two or more different inclusion angles.

[0042] By an inclusion angle we refer to the angle at which the wall of the conditioning chamber slopes inwards, relative to a longitudinal axis of the conditioning chamber.

[0043] The flow expansion region may comprise a perforated member, which may extend across the cross section of the flow expansion region, whereby the angle (relative to a central axis) and / or cross section (or diameter) of holes through the perforated member varies with radial position (relative to the central axis).

[0044] The flow expansion region and / or flow contraction region may comprise two or more nested tapered surfaces, where the change in the inclusion angle between each nested taper surface is 14 degrees or less for flow expansion, and between 30 to 60 degrees for flow contraction.

[0045] It may be that the mean position of the generated ions within the chamber is within a maximum cross sectional area region of the chamber, and is closer to the outlet of the chamber than to the inlet of the chamber. The mean position of the generated ions may be determined by calculating the mean of the x, y and z coordinates of the generated ions within the chamber.

[0046] The apparatus may further comprise a flow contracting region between the maximum cross sectional area region and the outlet of the chamber, the flow contracting region configured to decrease the cross-sectional area of flowing gas in the conditioning chamber.

[0047] Typically, the contracting takes place over a shorter distance than the expansion.

[0048] It may be that the flow expansion region has a frusto-conical cross-section and the flow contracting region has a frusto-conical cross section. It may be that the chamber has a cylindrical cross section therebetween.

[0049] It may be that the charging region extends into at least 75% of the flow contraction region’s axial length and / or at least 95% into the flow contraction region’s internal volume.

[0050] The apparatus may comprise an ionizing radiation source, located downstream of the flow expansion region and upstream of the flow contraction region.

[0051] It may be that the flow expansion region and the flow contracting region each have frusto-conical cross sections, wherein the decrease in radius of the flow contracting region per unit of axial length is at least 5 times the increase in radius of the flow expansion region per unit of axial length.

[0052] The apparatus may be configured to avoid flow recirculation.

[0053] It may be that the ion generator comprises an ionizing radiation source which generates radiation that causes gas particles to become ionized.

[0054] It may be that the ionizing radiation source is located closer to the flow contracting region than the flow expansion region.

[0055] It may be that the apparatus further comprises at least two electrodes which define a quenching region overlapping or downstream of the charging region, the apparatus comprising a circuit to generate a potential difference between the at least two electrodes and thereby remove ions from the gas in the quenching region.

[0056] It may be that the removal of ions by the quenching region preserves the asymmetry of the charge distribution of the aerosol particles in the gas (whether or not a steadystate charge distribution is reached).

[0057] The apparatus may comprise an electrometer to measure the charge of aerosol particles captured by the electrometer or remaining in the flow gas within a measurement region. The electrometer may comprise at least two electrodes defining the measurement region. The apparatus may comprise a circuit to generate a potential difference between the at least two electrodes of the electrometer to remove charged aerosol particles (i.e. some or all of the charged aerosol particles) from flowing gas and measure the resulting current of aerosol particles captured by the electrodes or remaining in the flowing gas. The electrometer may comprise a Faraday cage (in which case one electrode may function as the Faraday cage and another electrode as a reference, and there is no need to generate a potential difference between the electrodes).

[0058] Typically, the electrometer is downstream of the charging region. Typically, the electrometer is downstream of the quenching region (where present). The apparatus may comprise a processor to calculate one or more parameters of the size and concentration of the aerosol particles, which calculation comprises processing both the measured current and the known flow rate.

[0059] The method may comprise the step of measuring the current of charged aerosol particles downstream of the charging region, and typically also downstream of the quenching region.

[0060] The method may comprise the steps of measuring the charge of a portion of the charge-conditioned aerosol particles with an electrometer which generates a current and calculating one or more parameters of the size and concentration of the aerosol particles comprises processing both the measured current and the known flow rate.

[0061] It may be that the method comprises measuring a current at at least one electrode of the electrometer and a current at at least one electrode which defines the quenching region, and processing both of the measured currents to determine one or more parameters. The determined parameters may comprise one or more of ion mobility (Zj), mean ion concentration n, or interaction time t, and the nt product.

[0062] The electrometer may comprise a Faraday cage. The Faraday cage may comprise a filter to capture aerosol particles. The electrometer may comprise a Faraday cup.

[0063] The electrometer may be operated in a pulsed mode to improve the signal to noise ratio, for example, it may be that the electrometer alternately receives only positively charged aerosol particles and then only negatively charged aerosol particles. The apparatus may comprise a particle diverter or trap to remove or divert along a path select positively or negatively charged aerosol particles at any time.

[0064] In a third aspect of the invention there is provided an apparatus for charge conditioning a sample of aerosol particles to have a controlled charge distribution, the apparatus comprising: a body having an inlet and an outlet and a conditioning chamber therebetween, the body defining a flow-through path for a gas, comprising aerosol particles, from the body inlet and through the conditioning chamber to the body outlet; wherein the conditioning chamber comprises first and second electrodes, a controller configured to apply a potential difference (which may be constant or variable) between the first and second electrodes, the first electrode having an ion generator (typically an ionizing radiation source) adjacent or integrated therewith, the conditioning chamber and ion generator (typically an ionizing radiation source) defining a charging region within the conditioning chamber, the conditioning chamber configured to define a flow path for aerosol particles therethrough from the inlet to the outlet, through the charging region, the controller being configured to select the potential difference between the first and second electrodes to thereby control the distribution of positively and negatively charged ions and thereby control the balance between unipolar and bipolar charging.

[0065] The invention also extends in a fourth aspect to a method of operating the apparatus of the third aspect, the method comprising causing a gas, comprising aerosol particles, to flow through the body, along the flow-through path and controlling the potential difference across the first and second electrodes to thereby select the balance between unipolar and bipolar charge conditioning of aerosol particles in the gas. The method may comprise changing the charge conditioning from bipolar to unipolar or vice versa. The method may comprise changing the charge conditioning along a range which extends at least part of the way from bipolar to unipolar charge conditioning.

[0066] By applying a potential difference between the first and second electrodes, positive ions are subject to a force in one direction and negative ions are subject to a force in the opposite direction. Thus, a controllable gradient of mean ions charge is created. The gradient can be controlled such as to vary the ratio between positive and negative ions, and so the balance between unipolar and bipolar charging experienced by aerosol particles on the flow through path from the inlet to the outlet.

[0067] It may be that the conditioning chamber is configured such that aerosol particles at the outlet will on average have been closer to the second electrode than the first. The configuration such that aerosol particles will have on average been closer to the second electrode than the first could cause them to pass through more ions of one polarity than the other, depending on potential difference between the first and second electrodes.

[0068] Where the apparatus is a bipolar charger, or operated as a bipolar charger, typically the charge distribution of aerosol particles progresses towards a steady-state as they flow along the flow-through path. Where the apparatus is a bipolar charger, or is operated as a bipolar charger, it may be that the charge distribution of aerosol particles does reach the steady-state or it may be that the charge distribution of aerosol particles does not reach the steady-state. (In the case of a unipolar charger or device operated a unipolar charger, such as a variable charger, this reasoning does not apply).

[0069] The apparatus may further comprise at least two electrodes which define a quenching region, the quenching region overlapping or downstream of the charging region, the apparatus comprising a circuit to generate a potential difference between the at least two electrodes and thereby remove ions from the gas in the quenching region.

[0070] Typically, the removal of ions by the quenching region preserves the asymmetry of the charge distribution of the aerosol particles in the gas whether not a steady-state charge distribution is reached.

[0071] It may be that the at least two electrodes are configured such that the potential gradient generated between them in use crosses the direction of flow of gas along the flow- through path.

[0072] It may be that the at least two electrodes are configured such that the potential gradient generated between them in use is within 10% of perpendicular, or is perpendicular to the direction of flow of gas along the flow-through path.

[0073] It may be that the at least two electrodes are configured such that the potential gradient generated between them in use is within 10% of perpendicular, or is perpendicular to a longitudinal axis of the charging chamber.

[0074] The quenching region may overlap with the outlet of the chamber. The apparatus may be configured such that the quenching region removes ions from the gas after charge conditioning in the charge conditioning zone.

[0075] It may be that the quenching region is located at the outlet of the conditioning chamber and / or overlaps with the charging region.

[0076] It may be that the mean position of generated ions within the region of maximum crosssection of the conditioning chamber and is closer to the outlet of the conditioning chamber than to the inlet of the conditioning chamber.

[0077] It may be that the charging region comprises a variable potential gradient region defined by the first and second electrodes and wherein the volume of the conditioning chamber between the variable potential gradient region and the quenching region is less than the volume of the variable potential gradient region.

[0078] It may be that the ionizing source comprises a radioactive material coated on the first or second electrode.

[0079] As mentioned above, it may be that the apparatus comprises at least two electrodes which define a quenching region overlapping or downstream of the charging region, the apparatus comprising a circuit to generate a potential difference between the at least two electrodes and thereby remove ions from the gas in the quenching region and it may be that the removal of ions by the quenching region preserves the asymmetry of the charge distribution of the aerosol particles in the gas whether not a steady-state charge distribution is reached. This is useful with other configurations of conditioning chamber and so, in accordance with a fifth aspect of the present invention, there is provided a method of charge conditioning a sample of aerosol particles, in a gas, to have a controlled charge distribution, the method comprising: providing a body having an inlet and an outlet and a conditioning chamber therebetween, the body defining a flow-through path for a gas from the body inlet and through the conditioning chamber to the body outlet; causing a gas comprising aerosol particles (e.g. an aerosol) to flow along the flow-through path, through the conditioning chamber at a known flow rate; causing the gas to contain ions of both positive and negative polarity in a charging region of the conditioning chamber; the ions colliding with the aerosol particles and thereby charge conditioning the charge distribution of the aerosol particles in the gas; the charge distribution of the aerosol particles progressing towards a steadystate as they flow along the flow-through path.

[0080] It may be that the charge distribution of the aerosol particles does not reach a steady state. It may be that the method comprises the step of removing some or all of the remaining free ions from the gas within or downstream of the conditioning chamber to at least partially quench the charging conditioning process without the charge distribution of the aerosol particles reaching the steady state.

[0081] Thus, the invention extends in a sixth aspect to an apparatus for charge conditioning a sample of aerosol particles, in a gas, to have a controlled charge distribution (a charge conditioner), the apparatus comprising: a body having an inlet and an outlet and a conditioning chamber therebetween, the body defining a flow-through path for a gas from the body inlet and through the conditioning chamber to the body outlet; an ion generator to cause the gas to contain ions of both positive and negative polarity in a charging region of the conditioning chamber; either or both (i) an actuator to cause a gas to flow along the flow-through path, through the conditioning chamber at a known flow rate and (ii) a sensor to measure the flow rate of gas flowing along the flow-through path; the apparatus configured such that in use the ions collide with the aerosol particles and thereby charge conditioning the charge distribution of the aerosol particles in the gas, the charge distribution of the aerosol particles progressing towards but not reaching a steady-state as they flow along the flow-through path.

[0082] It may be that charge conditioning due to the collision of ions with the aerosol particles stops while > o.oi, or while : > o.O2 or while 0.05, or while

[0083] : > 0.1, or while ^qs_ > o.2, where is mean charge of aerosol particles at a given location in the flow path, is the charge of aerosol particles at the inlet to the charging chamber and is the steady state charge which would be reached if the aerosol particles remained aerosolized in the charging region indefinitely.

[0084] Charge conditioning may stop before the steady state due to the provision of a restricted density of ions (for a given flow rate, body geometry).

[0085] However, it may be that the method comprises the step of at least partially quenching the charge conditioning due to the collision of ions with the aerosol particles by removal of the ions from the flowing gas before the charge distribution of the aerosol particles reaches the steady state.

[0086] Once the charge conditioning has been at least partially quenched by removal of the ions from the flowing gas, the charge distribution will remain relatively constant for a longer period of time than would be the case were ions still present. Thus, the charge conditioned aerosol particles may be transported further or maintained for longer before they are measured or used for an end application, than would be the case without the quenching step.

[0087] It may be that the at least partial quenching takes place within or at the outlet from the conditioning chamber.

[0088] The quenching may take place in a quenching region.

[0089] The at least partial quenching may completely quench the charge conditioning. Typically, the at least partial quenching removes at least 50% or at least 75% of the ions which flow into a quenching zone. The apparatus may further comprise at least two electrodes which define a quenching region, the quenching region overlapping or downstream of the charging region, the apparatus comprising a circuit to generate a potential difference between the at least two electrodes and thereby remove ions from the gas in the quenching region. The ions which are removed may be some (for example at least 50%, at least 75%, at least 90%, at least 95%, or at least 99%) or all of the ions present in the gas at the quenching region.

[0090] The apparatus may comprise a current sensor to measure a current arising from quenching.

[0091] The quenching region may be located at the outlet of the conditioning chamber. The quenching region may overlap with the outlet of the chamber.

[0092] The quenching region may overlap with the charging region. The quenching region may be adjacent to the charging region. The quenching region may define the downstream end of the charging region.

[0093] The apparatus may be configured such that the quenching region removes ions from the gas after charge conditioning in the charge conditioning zone but before the charge distribution of the aerosol particles reaches a steady-state.

[0094] It may be that the charge conditioning is at least partially quenched by removal of the ions from the flowing gas before the charge distribution of the aerosol particles, q, has proceeded more than 99%, more than 98%, more than 95%, more than 90%, more than 80% or more than 75% of the way to the steady state, (typically from initial state q^~) (but does not reach steady-state).

[0095] Typically, the charge transfer is at least partially quenched while: > 0.01. It may be that charge transfer is at least partially quenched while: > o.O2. It may be that charge transfer is at least partially quenched while: > °-05- ItmaY be that charge transfer is at least partially quenched while: > 0.1. It may be that charge transfer is at least partially quenched while: > o.2. It may be that charge transfer is at

[0096] Qin Qss least partially quenched while: > 0.25.

[0097] Qin Qss

[0098] It is surprising that it is beneficial to at least partially quench the charging process before the steady-state is reached. This is because, in contrast to a charger in which a steady-state is reached, the distribution of charge levels will not be independent of: particle concentration ion concentration (and so source strength)

[0099] - flow rate / particle residence time in the charging region

[0100] - the initial charge state of received aerosol particles

[0101] However, surprisingly, we have found a number of technical benefits to operating in this regime, in particular this enables: higher gas flow rates, enabling higher currents at a detector

[0102] - smaller conditioning chamber volumes

[0103] - the use of lower activity radioactivity sources to generate ionising radiation, or, more generally, lower ion concentrations are required, usually resulting in smaller and more economical ion generators

[0104] - a reduced aerosol particle residence time in the charging region

[0105] By the flow rate we refer to the volumetric flow rate through the apparatus. This determines the flow velocity at each location within the apparatus at a fixed geometry and so the aerosol particle residence time.

[0106] Thus, because the steady state is not reached, the charge distribution of the aerosol particles depends on the residence time of aerosol particles in the charging region and so is a function of the flow rate. Thus, it may be that the charge distribution of the aerosol particles is a function of the flow rate.

[0107] The method may comprise the step of measuring the flow rate. The apparatus may comprise an air flow rate sensor.

[0108] It may be that the flow rate is controlled. The apparatus may comprise a flow rate regulator, which may comprise a flow actuator, for example a fan, to cause the gas (e.g. an aerosol) to flow along the flow path at a controlled flow rate. Thus, the flow rate may be known because it is measured and / or because it is controlled.

[0109] Accordingly, although there is a requirement to know the flow rate, the technical benefits listed above can be obtained.

[0110] It may be that the body comprises a radioactive source configured to irradiate the charging region, wherein the radioactive source has an activity which is exempted from notification under “Radiation Protection, R. Safety of Radiation Sources: International Basic Safety Standards. IAEA Safety Standards Series No. GSR Part 3. Vienna: IAEA: (2014)”. It may be that the ions are generated by a radioactive source which has an activity which is exempted from notification under “Radiation Protection, R. Safety of Radiation Sources: International Basic Safety Standards. IAEA Safety Standards Series No. GSR Part 3. Vienna: IAEA: (2014)”.

[0111] (Full name EUROPEAN COMMISSION, FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS, INTERNATIONAL ATOMIC ENERGY AGENCY, INTERNATIONAL LABOUR ORGANIZATION, OECD NUCLEAR ENERGY AGENCY, PAN AMERICAN HEALTH ORGANIZATION, UNITED NATIONS ENVIRONMENT PROGRAMME, WORLD HEALTH ORGANIZATION, Radiation Protection and Safety of Radiation Sources: International Basic Safety Standards, IAEA Safety Standards Series No. GSR Part 3, IAEA, Vienna (2014)).

[0112] For example, it may be that the radioactive sources comprises not more than 10 kBq of Am-241 or not more than 100 M Bq of Ni-63 or not more than 10 kBq of Kr-85 or not more than 1 GBq of H-3 or not more than 18.5 MBq of Po-210.

[0113] Due to a preconception in the field that a steady-state must be reached, it has not been common to use radioactive sources below the exemption level (set out in the said Directive), requiring additional regulatory compliance.

[0114] It may be that the body comprises a non-radioactive ion generator configured to generate ions in the charging region. The ion generator may comprise a nonradioactive ion generator. The non-radioactive ion generator may, for example, be an x-ray source, a corona discharge source (e.g. a dual-polarity or alternating polarity corona discharge source) or an plasma discharge source. The method may comprise generating ions in the charging region using a non-radioactive ion generator.

[0115] As well as charge conditioning an aerosol, the charging chamber can be useful for applications such as a static eliminator which charge conditions a gas which need not be an aerosol.

[0116] According to a seventh aspect of the invention there is provided a method of designing an apparatus for conditioning a sample of aerosol particles, in a gas, with a controlled charge distribution (a particle charge conditioner), the apparatus comprising: a body having an inlet and an outlet and a conditioning chamber there between, the body defining a flow-through path for a gas from the body inlet and through the conditioning chamber to the body outlet, the method comprising modelling, simulating or experimentally determining the charge distribution of aerosol particles, in the gas, passing through a charging region of the conditioning chamber due to ions in the charging region colliding with the aerosol particles and then passing through a quenching region within or downstream of the conditioning chamber in which ions are removed, and varying one or more parameters of the apparatus, the parameters comprising one or more of: the shape of the conditioning chamber, the rate of flow of gas sample along the flow-through path, and the concentration of ions in the charging region, selecting one or more parameters of the apparatus, whereby according to the selected one or more parameters the charge distribution of the aerosol particles remains such that providing an apparatus having the selected parameters.

[0117] The invention extends in an eighth aspect to an apparatus (a particle charge conditioner) designed by the method of the seventh aspect. Optional features disclosed in respect of any aspect of the invention are optional features of each aspect of the invention. of the Drawings

[0118] An example embodiment of the present invention will now be illustrated with reference to the following Figures in which:

[0119] Figure 1 (a) is an illustration of bipolar charging of aerosol particles, showing a positively-charged particle, a negatively-charged particle and an uncharged particle surrounding and interacting with positive and negative ions. Figure 1(b) shows the distribution of charge on samples of bipolar charged aerosol particles.

[0120] Figure 2 shows the mean charge per particle on aerosol particles as they flow progressively through a charging regime, a steady-state regime, and a discharging regime. Each of the charging regime and the steady-state regime are within the charging region of the charger.

[0121] Figure 3 shows (a) experimental and (b) numerical results of mean charge per particle as a function of convective time in tubing between charging region and ion trap. Data were gathered for a wide range of tube lengths and flow rates for the same Kr-85 charger and 217 nm diameter particles.

[0122] Figure 4 shows a first embodiment of a bipolar charger apparatus.

[0123] Figure 5 shows a bipolar charger apparatus utilised within a scanning mobility particle size spectrometer (SMPS).

[0124] Figure 6 shows a second embodiment of a bipolar charger apparatus.

[0125] Figure 7 shows a modified version of the second embodiment of a bipolar charger apparatus. Figure 8 shows a bipolar charger device with ion trap electrodes upstream of the flow contracting region.

[0126] Figure 9 shows a static eliminator embodiment.

[0127] Figure 10 shows a third embodiment of a charger apparatus.

[0128] Figure 11 shows a method of designing an apparatus.

[0129] Detailed Description of Example Embodiments

[0130] A bipolar charge conditioner, otherwise known as a neutralizer, is commonly operated using an alpha or beta radioactive or X-ray ionization source, although various other ion generators may be used. The neutralizer first ionizes gas molecules which subsequently transfer charge to the sampled aerosol particles, resulting in a distribution of aerosol particles which are variously positively charged 3, negatively charged 5 and uncharged 2, as illustrated in Figure 1(a). The fraction of total particle concentration at each discrete charge level tends to follow an asymmetric Boltzmann distribution centred near neutral, though negative on average, as shown in Figure 1(b).

[0131] Importantly, in low-cost sensors and high-resolution instrumentation the charge levels acquired by particles must be known, predicted, or calibrated as a function of particle diameter (or other particle properties, such as surface area) in order to interpret measurements.

[0132] Bipolar charging is often used for aerosol instruments and applications in part because it is generally understood that a well-defined steady-state charge distribution is achievable and that this steady-state distribution provides a known distribution of charges, enabling repeatable and reliable measurements.

[0133] In the dynamic charging process under consideration, positive and negative ions are generated while simultaneously either recombining with one another, transferring charge to the aerosol particles, or being lost to the walls of the charger housing. If more ions are generated (e.g., using a higher activity radioactive source), more ion recombination occurs, but a point is eventually reached where, due to the ion generation and recombination rates converging, higher ion generation provides less benefit for aerosol charging as the proportional increase in the concentration of free ions is less.

[0134] Initially uncharged particles are charged to increasingly high levels. As the charge distribution develops, the particle charge levels tend to a steady-state distribution centred near neutral. For example, a +2 charged particle is more likely to attract a -1 ion than a +1 ion due to electrostatic attraction. That process is similar for other charge levels, therefore particles are increasingly unlikely to acquire higher charge levels. The ion-to-particle charge transfer probabilities balance as the steady-state charge distribution is established.

[0135] Conventional understanding of bipolar charging suggests that once the ‘steady-state’ is established inside the charger, this charge distribution is also maintained in the tubing between the charger and a downstream classifier / detector. However, the present inventors have shown experimentally that this is not the case and that this effect occurs over fractions of a second. This error results in significant measurement uncertainties in aerosol instruments that use bipolar diffusion charging.

[0136] We have found that the steady-state charge distribution described above is only one of three charging regimes relevant to practical bipolar charge conditioners. Figure 2 shows the development of mean charge per aerosol particle for particles moving through a charger and into a tubing region downstream of the charger, and illustrates these three regimes. Particles acquire charge in the charging / ionizing region (i.e. ’’charging regime”), eventually developing a steady-state charge distribution (i.e. ’’steady-state regime”). We have shown experimentally that if ions are present downstream of the charging region (e.g. in tubing), the charge distribution will change from its steady-state distribution (i.e. enter the ’’discharge regime”) in fractions of a second. The discharge regime explains at least some of the unreliability of known devices which have assumed that the steady state is maintained once it has been reached. The charger may be configured to operate in any of these three regimes depending on the charge levels and properties of the charge distribution desired.

[0137] Charging Regime The charging regime refers to particle charging while its steady-state charge distribution is developing. Operating a charge conditioner solely within this regime, for example by quenching the charging process before the steady-state charge distribution is reached, represents a new mode of operation for low-cost sensing or high-resolution instrumentation depending on the desired outcome. The primary advantage of operating a charge conditioner in the charging regime is that there is no minimum requirement for the product of ion concentration, ni, and ion-particle interaction time, t, (nit product), as is required for reaching a steady-state distribution. Consequently, chargers can be operated with:

[0138] • higher flow rates;

[0139] • smaller charger volumes (ideal for miniaturization);

[0140] • lower activity sources (important for safety and regulatory requirements); and

[0141] • less residence time in charging region.

[0142] At the same time, charge levels desired for a given application can still be achieved.

[0143] In some embodiments we deliberately quench the charging process before the charge distribution has reached steady state by applying a potential gradient between electrodes, thereby removing ions but leaving the majority of charged particles, and avoiding the discharge regime.

[0144] The present inventors are not aware that anyone has intentionally stopped the charging process by removing the ions before a steady-state bipolar charge distribution is reached. This operation can be considered a quenching operation.

[0145] Steady-State Regime

[0146] By measuring mean charge (q) for a known diameter (d) of initially uncharged particles, it is possible to quantify the nt product or conditions for which a steady-state is reached. This quantification can be used to determine operating specifications of an arbitrary charger design (e.g. during a design phase, quality assurance or calibration procedure).

[0147] Discharging Regime The steady-state regime relies on ions of both polarities having a constant ratio of ion concentrations (i.e. n+ / n-=constant) and mobilities (i.e. Z+ / Z-=constant). This constant concentration ratio is typically accomplished by the rate of ion generation matching the rate of ion-ion recombination, leaving a constant concentration of free ions of both polarities in the charging region. However, that process also requires the ion generation rate to be relatively constant and sufficiently high for both ion polarities. In conventional bipolar chargers, particles and ions are convected downstream of the charger’s ionization / charging region (where ions are produced by the ion generator). In many cases, this convection of both particles and ions through additional volume (often still incorrectly referred to as part of the charging region) is intentional to increase the interaction time between the two, thereby leading-to a higher likelihood of reaching a steady-state charge distribution. However, it turns out that this common approach is misguided and adversely affects the steady-state charge distribution of the aerosol particles.

[0148] In reality, outside the ionization section of the charger, the remaining free ions diffuse and are captured at the walls of the charger housing or downstream tubing / volumes due to their high diffusivity. For example, in air negative ions are usually significantly more mobile than positive ions, and as a result diffuse more readily to the charger / tubing walls. Thus, positive ions remain convected in the flow with the particles longer, producing a shift in the charge distribution relative to its state at the outlet of the ionizing region. The ion imbalance becomes appreciable and affects the charge of the aerosol particles rapidly downstream of the ionization / charging region. The present inventors are the first to fully characterize this effect experimentally by controlling the time the free ions downstream of the ionization / charging region remain with the aerosol particles and measuring the mean charge of the particles.

[0149] By considering ion diffusion losses in a numerical model, the inventors have been able to show that a steady-state charge distribution is reached in the same charger at different flow rates but that the final charge state sampled downstream of a charger (with associated tubing) depends on the downstream volume (e.g. tube diameter and length). This work led the inventors to discover the steady-state charge distribution is more asymmetric (i.e. less neutral) than previously assumed. Although conventional chargers are intended to operate at steady-state, this ’discharging’ effect, which disturbs the operation in a steady-state regime, is not common knowledge, and thus not accounted for in conventional bipolar chargers.

[0150] Mean charge as a function of time in the tubing downstream of the charging is shown in Figure 2. The Figure shows that the ’downstream ion’ effect occurs within milliseconds of the particles exiting the charging area. This effect can also occur when operating in ‘charging regime’ unless free ions are removed after the charging / ionizing region.

[0151] In the present disclosure, volumetric flow rates and tube lengths were varied to assess the dependence of mean charge on convective time in the tubing between the charger and where the mean charge is sampled. The results are shown in Figure 3a for a given chargerand particles of 217 nm in diameter. For a wide range of time between charger and ion trap (i.e. convective time of remaining free ions with the aerosol particles), the data fall on a consistent curve, which is further supported by modelling results shown in Figure 3b. That the position of the mean charge peak does not occur at the exit of the charger indicates that the charger was operated in the charging regime. The peak mean charge in this figure represents sampling at the steady-state regime. Moving to the right portion of Figure 3a or 3b, which corresponds to longer convective times (i.e. lower flow rates and / or longer tubing), the effect of downstream ions discharging reduces the asymmetry of mean charge towards neutral.

[0152] First Example Embodiment

[0153] A first embodiment of an apparatus for charge conditioning is illustrated in Figure 4.

[0154] The apparatus 1 comprises body 4 which has an inlet 17 through which an aerosol, being a gas with a suspension of aerosol particles therein, enters the apparatus. As the aerosol enters the apparatus it starts on a flow through path through the apparatus, the flow through path beginning at the apparatus body inlet 17, and finishing at the apparatus outlet 19. A flow regulator 25 is positioned shortly after the apparatus input 17. The flow regulator 25 may be used to control the velocity of the aerosol within the apparatus. It may take the form of a fan or a pump, for example. The flow regulator may alternatively be positioned in a downstream position, for example downstream of the electrometer 27. The flow regulator is typically controlled by a processor. There may also, or alternatively, be a flow rate sensor.

[0155] The flow transitions from an inlet 17 to a charging region 33 in a charger, where charge conditioning takes place. The inlet tube 43 has a small diameter and the charging region 33 is maximized to maximize the effectiveness of the (low-activity) source. It is critical to consider the flow of the aerosol / gas in the expansion from the inlet 17 to the charging region 33 to avoid any adverse pressure gradient which would causes zones of flow recirculation and to ensure particles have consistent exposure to the ionization / charging region 33.

[0156] An aerosol entering the apparatus, passes the flow regulator 25 and then enters into the conditioning chamber 13 of the apparatus. The aerosol enters the conditioning chamber 13 through conditioning chamber inlet 21 and into charging region 33 within the conditioning chamber. Conditioning chamber inlet 21 comprises a flow expansion region 35 between the inlet of the apparatus inlet 17 and the charging region 33. Flow expansion region 35 is configured to increase the cross-sectional area of gas flowing through the charging region 33. In this example the flow expansion region is defined by a frusto-conical section of a diameter which increases in the direction of flow. Flow expansion is achieved without the introduction of vortex flow into the flow of the aerosol through the apparatus.

[0157] A source of ionizing radiation, such as a low activity radioactive source 11 , is sited within the conditioning chamber 13. The radioactive source 11 emits radiation which ionises gas molecules in the charging region 33 of the conditioning chamber 13, thereby generating ions. The ionized gas molecules subsequently transfer charge to the aerosol particles. In this way the aerosol particles are conditioned. In the case of bipolar charging, this conditioning results in a distribution of positively charged aerosol particles 3, negatively charged aerosol particles 5 and uncharged aerosol particles 2 as discussed above. Examples of radiation sources which are suitable for ion generation are Am-241 (preferably < 0.27 pCi / 10 kBq) and Ni-63 (preferably < 2.7 mCi / 100 MBq).

[0158] The conditioned aerosol particles exit the conditioning chamber 13 through the conditioning chamber outlet 23. The conditioned aerosol particles enter quenching region 37. Situated in quenching region 37 are two electrodes 15a, 15b. The application of a potential difference across these electrodes creates an electric field in the quenching region 37, forming an ion filter which serves to remove ions. The aerosol particles have a much lower electrical mobility than the ions and because of this they are effectively uninfluenced by the electric field of the ion filter. The ion filter 15a, 15b therefore enables the charge conditioning process to be stopped (quenched), preventing or minimising aerosol particle charge state varying downstream in a discharge zone.

[0159] The aerosol, having been quenched in the quenching region 37 then exits the apparatus via outlet 19.

[0160] The device illustrated in Figure 4 is shown in use with a Faraday cup type electrometer 27, called a Faraday cup aerosol electrometer, situated at the outlet 19 of the apparatus. Charged aerosol particles which have been conditioned and quenched of ions within the apparatus exit the apparatus and are collected and detected at the electrometer 27.

[0161] Alternative to a Faraday cup electrometer is a Faraday cage electrometer. This consists of a cylinder through which charged particles flow and induce a charge on the cylinder. The electrometer detects the change in charge that passes through the cylinder, and not the absolute flux of charge as in a Faraday cup Electrometer. For this reason, a Faraday cage electrometer can be configured to detect successive clouds of positively, then negatively charged particles. The change in signal is approximately twice that of going from uncharged to unipolarly charged, providing the additional advantage of ~2x the signal of a conventional Faraday cup electrometer. Furthermore, Faraday cages are advantageous because they avoid drift in signal because it is the change in current that matters, not a current offset (which may drift over time). Finally, use of a Faraday cage is also advantageous because it avoids the need for maintenance (i.e. replacement of a filter in a Faraday Cup Electrometer due to fouling).

[0162] A Faraday based electrometer positioned at the outlet of the apparatus enables the charged aerosol particles exiting the apparatus to be detected. This enables the dependence of the output of the apparatus on various design parameters to be measured. Such design parameters include the relative locations of the charging region and the quenching region, the form of the flow in the flow contraction region 39, should one be present, and length of the outlet tube 41 which culminates in the apparatus outlet 19.

[0163] Also shown in the apparatus illustrated in Figure 4 is a controller 29. Controller 29 typically takes the form of a microprocessor. Controller 29 enables control of flow regulator 25. It also enables control of the voltages applied to the electrodes 15a, 15b, of the ion trap. The controller 29 can be interfaced to register charged particles which are detected by the electrometer 27. It is also possible to detect charges collected by the electrodes 15a, 15b of the ion trap.

[0164] Figure 4 shows placement of the ion filter electrodes 15a, 15b downstream of the ionizing radiation source 11. In a variation of the first embodiment, by appropriate positioning of ion filter electrodes 15a, 15b within the body of the apparatus, the removal of ions from the aerosol flowing through the flow-through path can be effected within any of the three regimes identified above in connection with Figure 2, i.e. within the charging regime, within the steady-state regime, or within the discharging regime.

[0165] The present inventors have demonstrated that discharging of a charged aerosol sample is dependent on several elements, including:

[0166] • particle size;

[0167] • particle concentration;

[0168] • ion concentration within the aerosol (therefore ion generation rates / source activity);

[0169] • volume downstream of the ionization / charging region (e.g. downstream tube lengths);

[0170] • tube diameters (due to interaction with electrical space charge and diffusion losses);

[0171] • volumetric flow rate; and,

[0172] • residence time of particles and free ions downstream of the ionization / charging region (i.e. combination of downstream volume and flowrate outlined in points above). These elements have not been proven experimentally nor taken properly into account in the prior art and this has resulted in increased uncertainty and often lower signals in high-resolution instruments and in low-cost sensors.

[0173] Figure 5 illustrates the use of an apparatus of the type described above in one such high-resolution instrument. One of the most common measurement instrument which incorporates a bipolar charge conditioner is the Scanning Mobility Particle Size Spectrometer (SMPS). The SMPS follows an ISO standard for measuring aerosol particle size distributions and is widely used. It combines a bipolar charge conditioner, Differential Mobility Analyzer (DMA) and a particle counter such as Condensation Particle Counter (CPC). In the DMA, which is a particle classifier, the charged particles flow through a column comprising two concentric electrodes. When an electric field is applied, particles of one polarity pass through a sheath flow, resisted by a drag force, towards a slit at the end of the DMA and are then counted by the CPC. Particles within a particular range of electrical mobility travel through the DMA to be counted at the CPC, while those particles outwith the range of electrical mobility are filtered out and do not pass through for counting. By stepping or scanning through a range of electrical mobility setpoints, the particle size distribution, or the particle concentration at each mobility, is determined. If all particles were singly-charged, interpretation of this measurement would be straightforward. However, as the electrical mobility of a particle depends on its charge state, to yield correct concentrations at each mobility, the number of multiply-charged particles must be deconvoluted. Furthermore, to yield accurate particle concentrations of the sampled aerosol particles, the concentration of charged particles at each charge level relative to the total sampled concentration must be known (i.e. the charging efficiency). Therefore, to account for multiply-charged particles and charging efficiency, the charge states as a function of particle mobility must be known a priori. This renders the stability provided by the steady-state charge distribution of the present invention highly valuable.

[0174] In general, in SMPS measurements a charge distribution independent of sample flow rate is assumed. However, the standard configuration of this instrument is a 13-in long, 0.25-in tube from the charger outlet to the inlet of the classifier resulting in an average time in the tube of 0.217 and 1.08 s for 0.3 and 1.5 L / min sample flows, respectively. These times between the charger and the classifier (which acts as an effective ion trap) result in operating in the discharge regime (and its associated sensitivities as described in the list above), and not in the steady-state regime as assumed. As a result, the charge distribution of the aerosol particles at each of the two flow rates. This is also reflected by the average charges on the curve in Figure 3, whose data points show different charge distributions measured at different sample flow rates and for a range of tube lengths. This also explains observations by the inventors that SMPS measurements at 0.3 and 1.5 L / min sample flow rates usually do not agree with each other.

[0175] The present invention enables charge conditioning a sample of aerosol particles to a known charge distribution that can be controlled. Free ions can be removed utilising the ion filter before a steady state regime is reached (i.e. quenching). Free ions can also be removed during a steady state regime, or during a discharge regime to stop the effect of the remaining free ions downstream of the charging region on the charge distribution.

[0176] In this way, previous measurements which have been based on assumptions which have been shown to be unrealistic, can be put on a strong basis.

[0177] Example Embodiment 2

[0178] A second embodiment of an apparatus for charge conditioning is illustrated in Figure 6.

[0179] In this embodiment the region immediately after the inlet 21 to the conditioning chamber is tapered outwards, forming a flow expansion region 35, to enable a more uniform flow expansion near the radiation source, i.e. to avoid flow recirculation which can cause large differences in particle residence time which causes major uncertainty in temporal response and charge levels (due to differences in nt product, this being the product of the concentration of ions and the interaction time between ions and aerosol particles). The tapering at the inlet constitutes a flow expansion region which increases the cross-sectional area of the flow. The cross-sectional area of the flow reaches a maximum at the charging region in a maximum cross-sectional area region 47. Downstream of the maximum cross-sectional area region 47 the chamber tapers and the cross-section of the flow decreases in a flow contraction region 39. The expansion region is substantially longer, for example 5x the length, of the flow contraction region to facilitate the provision of a relatively large charge conditioning volume followed by rapid quenching of the charging process. Thus, there is no plane of symmetry of the chamber perpendicular to the flow direction / longitudinal axis of the chamber.

[0180] The conditioning chamber has a large volume near the radiation source. This is to maximise the ionizing effect of the radiation since the nt product is proportional to chamber diameter to power of 4 (until reaching the maximum penetration distance of the radiation in the gas).

[0181] The radiation source 11 in this embodiment is placed near the outlet. This contrasts with conventional bipolar chargers, in which the radiation source is situated in the middle or at the inlet. The arrangement in the presently described embodiment minimises the opportunity for preferential ion loss to walls by diffusion. Consequently, charge asymmetry and the intended charge states are preserved. The mean position 45 of generated ions in the charging region is closer to the outlet 23 of the conditioning chamber than to the inlet 21 of the conditioning chamber. The embodiment also shows an ion trap comprising electrodes 15a and 15b connected directly downstream of the conditioning chamber 13. An electrostatic classifier (such as an DMA) may also be similarly connected to the apparatus. These are options. The Faraday cup electrometer 27 of the figure is also a possible option.

[0182] The second embodiment has a conditioning chamber outlet which defines a flow contracting region.

[0183] A modification of the second embodiment is illustrated in Figure 7. In this modification the electrodes 15a and 15b which form the ion trap are positioned in the flow contracting region 39. In the embodiment illustrated, the electrodes 15a and 15b encroach into conditioning chamber 13.

[0184] In a further alternative arrangement, the electrodes 15a and 15b which form the ion trap are positioned after the radioactive source, but upstream of the flow contracting region 39. This alternative arrangement is illustrated in Figure 8.

[0185] The apparatus illustrated in Figure 6 may also be operated as a static eliminator, with no potential across electrodes 15a and 15b. In this mode of operation, the aim is to produce an output of ions of both polarities which deposit on a surface in order to neutralise the surface from static / electrostatic charges. Instead of passing an aerosol through the apparatus, a gas is passed through the apparatus. Deposition can occur by convection or diffusion. Static elimination is used in the art to minimize the electrostatic charge, e.g. for analytical balances laboratory settings, or for conditioning a surface (e.g. of an automobile) prior to painting.

[0186] Any of the conditioning devices presently disclosed may be operated as a static eliminator. When running a device as a static eliminator, gas is passed though the device instead of an aerosol and the ion trap is not utilised. Figure 9 illustrates an alternative modification of the second embodiment, which corresponds to a static eliminator. The static eliminator of Figure 9 has no ion trap electrodes, and no flow contracting region is included in this static eliminator.

[0187] Example Embodiment 3

[0188] A third embodiment of an apparatus for charge conditioning is illustrated in Figure 10.

[0189] This configuration utilises a radioisotope 11 electro-plated onto an electrically conductive surface 16b, which makes up one of two electrodes 16a, 16b. The radioactive source need not form one of the electrodes as illustrated. Other arrangements are possible. The critical aspect is the introduction of an electric field in the charging region, as this enables control of the charging of the aerosol particles. The outlet 23 of the conditioning chamber in this illustrated embodiment is positioned away from the main central axis 31 of the apparatus 1 , close to one of the two electrodes 16a. The flow through path, which in the previously described embodiments follows the main central axis, in this third embodiment passes through the conditioning chamber outlet 23 and thereby deviates away from the central axis 31.

[0190] By applying an external voltage between the two electrodes 16a, 16b, an electric field results which transports ions of a selected polarity within the conditioning chamber 13 towards the electrode 16a nearest the flow of particles. This has the effect of generating a sample of aerosol particles with a charge distribution which is principally unipolar. Since ions of both polarities are generated throughout the volume, some bipolar charging could occur based on the operation conditions of the charger. This configuration could also be used to condition a highly-asymmetric bipolar charge distribution (i.e. higher aerosol currents, while recognizing the benefits of reaching steady-state).

[0191] By combining an electric field in the charging region the present embodiment generates an output of charged aerosol particles with known charge levels either bipolar or unipolar. This contrasts with conventional chargers which are either unipolar or bipolar. The polarity of the output is also adjustable (either positive, negative or a combination of both).

[0192] Although the conditioning chamber can have other shapes, and may for example be axisymmetric, in the present example, the use of a conditioning chamber outlet which is offset towards one of the electrodes producing the electric field has the effect that the flow path for aerosol particles through the conditioning chamber is inherently closer to one electrode than the other, facilitating controllable charging with a selectable ratio of positive to negative ions.

[0193] Amongst other advantages are that the apparatus is less sensitive to initial charge than conventional unipolar chargers since some bipolar charging still typically occurs. High charge levels are possible with this embodiment, thereby increasing signal significantly, e.g. 4x bipolar. Further advantages of this embodiment include the nt product (and charge levels) being controllable over a wide range, providing control over charge levels and charge polarity, and a low level of particle loss at walls (>95% penetration).

[0194] The apparatus of the third embodiment may be operated the charger in any of the three bipolar regimes, charging, steady-state or discharging, with the associated benefits.

[0195] Although in the examples above, the particle charging is fully quenched by the ion trap in the quenching region, for some applications it is sufficient to partially quench the charging process by removing most, e.g. 90% or 99% of ions.

[0196] Method of design Figure 11 shows a schematic diagram of a method of designing an apparatus for conditioning a sample of aerosol particles, in an aerosol, with a controlled charge distribution. The apparatus comprises: a body having an inlet and an outlet and a conditioning chamber there between, the body defining a flow-through path for an aerosol from the body inlet and through the conditioning chamber to the body outlet.

[0197] Step 101 of the method is a step of modelling, simulating or experimentally determining the charge distribution of aerosol particles, in the aerosol, passing through a charging region of the conditioning chamber due to ions in the charging region colliding with the aerosol particles.

[0198] In step 103, one or more parameters of the apparatus are varied, the parameters comprising one or more of: the shape of the conditioning chamber, the rate of flow of gas sample along the flow-through path, and the concentration or mobility of ions in the charging region.

[0199] In step 105, a selection is made of one or more parameters of the apparatus, whereby according to the selected one or more parameters the charge distribution of the aerosol particles remains such that > 0.05.

[0200] Qin Qss

[0201] In step 107, an apparatus having the selected parameters is provided (e.g. manufactured).

[0202] Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to and do not exclude other components, integers, or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0203] Features, integers, characteristics, or groups described in conjunction with a particular aspect, embodiment, or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0204] The disclosure extends to the following numbered clauses:

[0205] 1. A method of charge conditioning a sample of aerosol particles, in a gas, to have a controlled charge distribution, the method comprising: providing a body having an inlet and an outlet and a conditioning chamber therebetween, the body defining a flow-through path for a gas from the body inlet and through the conditioning chamber to the body outlet; causing a gas comprising aerosol particles to flow along the flow-through path, through the conditioning chamber at a known flow rate; causing the gas to contain ions of both positive and negative polarity in a charging region of the conditioning chamber; the ions colliding with the aerosol particles and thereby charge conditioning the charge distribution of the aerosol particles in the gas; the charge distribution of the aerosol particles progressing towards a steadystate as they flow along the flow-through path; and removing some or all of the remaining free ions from the gas within or downstream of the conditioning chamber to at least partially quench the charging conditioning process without the charge distribution of the aerosol particles reaching the steady state. 2. A method according to clause 1 , wherein the charge conditioning is at least partially quenched by removal of the ions from the flowing gas before the charge distribution of the aerosol particles has proceeded more than 95% (optionally 90%, 80% or 75%) of the way to the steady state.

[0206] 3. A method according to any preceding clause, comprising the steps of measuring the charge of a portion of the charge-conditioned aerosol particles with an electrometer which generates a current and calculating one or more parameters of the size and concentration of the aerosol particles by processing both the measured current and the known flow rate.

[0207] 4. A method according to clause 3, comprising the step of measuring the flow rate.

[0208] 5. A method according to clause 3 or clause 4, whereby the flow rate is controlled.

[0209] 6. A method according to any preceding clause, comprising expanding the cross- sectional area of the flow of the gas upstream of, within, or overlapping the charging region and then contracting the cross-sectional area of the flow of gas downstream of, within or overlapping the charging region, prior to at least partial quenching, whereupon the contracting takes place over a shorter distance than the expansion.

[0210] 7. A method according to clause 6, whereby majority of the generation of ions by radiation, in the flow of the gas, takes place after the cross-sectional area of the gas has expanded to a maximum cross-sectional area.

[0211] 8. A method according to any preceding clause, wherein the charging region is within the conditioning chamber and extends to the outlet from the conditioning chamber, but does not extend to the inlet to the conditioning chamber.

[0212] 9. A method according to any of clauses 2 to 8, wherein the at least partial quenching takes place within or at the outlet from the conditioning chamber, optionally wherein the at least partially quenching takes place in a quenching region which overlaps with the charging region. 10. A method according to any preceding clause, wherein the body comprises a radioactive source configured to irradiate the charging region, wherein the radioactive source has an activity which is exempted from notification under “Radiation Protection, R. Safety of Radiation Sources: International Basic Safety Standards. IAEA Safety Standards Series No. GSR Part 3. Vienna: IAEA: (2014)”.

[0213] 11. A method of designing an apparatus for conditioning a sample of aerosol particles, in a gas, with a controlled charge distribution, the apparatus comprising: a body having an inlet and an outlet and a conditioning chamber therebetween, the body defining a flow-through path for a gas from the body inlet and through the conditioning chamber to the body outlet, the method comprising modelling, simulating or experimentally determining the charge distribution of aerosol particles, in a gas, passing through a charging region of the conditioning chamber due to ions in the charging region colliding with the aerosol particles, and then passing through a quenching region within or downstream of the conditioning chamber in which ions are removed, and varying one or more parameters of the apparatus, the parameters comprising one or more of: the shape of the conditioning chamber, the rate of flow of gas sample along the flow-through path, and the concentration of ions in the charging region, selecting one or more parameters of the apparatus, whereby according to the selected one or more parameters the charge distribution of the aerosol particles remains such that > 0.05, and in ss providing an apparatus having the selected parameters.

[0214] 12. A particle charge conditioner designed by the method of clause 11.

Claims

Claims1. An apparatus comprising: a body having an inlet and an outlet and a conditioning chamber therebetween, the body defining a flow-through path for a gas from the body inlet and through the conditioning chamber to the body outlet; the conditioning chamber having an inlet and an outlet, the apparatus comprising an ion generator and the conditioning chamber defining a charging region receiving generated ions; the conditioning chamber comprising a flow expansion region between the inlet of the conditioning chamber and a maximum cross sectional area region, the flow expansion region configured to increase the cross-sectional area of gas flowing through the conditioning chamber, wherein the charging region is within or overlaps the maximum cross sectional area region and the mean position of generated ions in the charging region is closer to the outlet of the conditioning chamber than to the inlet of the conditioning chamber.

2. An apparatus according to claim 1 , wherein the flow expansion region is at least 3.25 times longer axially than the maximum cross sectional area region and / or the flow expansion region has an internal volume at least 1.25 times the volume of the maximum cross sectional area region.

3. An apparatus according to claim 1 or claim 2, wherein the charging region includes at least 99% of the internal volume of the conditioning chamber which is between the mean position of the generated ions within the conditioning chamber and the outlet.

4. An apparatus according to any preceding claim, wherein the charging region extends from the conditioning chamber through the outlet of the conditioningchamber, wherein the external radiation exposure at the conditioning chamber outlet is approximately 0 mSv / h.

5. An apparatus according to any one preceding claim, wherein the flow expansion region extends along at least 60% of the length of the body and / or has a volume which is at least 50% of the internal volume of the body.

6. An apparatus according to any preceding claim, which is a static eliminator.

7. An apparatus according to any preceding claim, further comprising at least two electrodes which define a quenching region, the quenching region overlapping or downstream of the charging region, the apparatus comprising a circuit to generate a potential difference between the at least two electrodes and thereby remove ions from the gas in the quenching region.

8. An apparatus according to claim 7, wherein the quenching region overlaps with the outlet of the chamber.

9. An apparatus according to any of claims 1 to 7, for charge conditioning a sample of aerosol particles in a gas to have a controlled charge distribution, wherein the gas is an aerosol comprising aerosol particles.

10. An apparatus according to any preceding claim, wherein the removal of ions by the quenching region preserves the asymmetry of the charge distribution of the aerosol particles in the gas whether not a steady-state charge distribution is reached.

11. An apparatus according to any of claims 6 to 10, comprising an electrometer to measure the charge of aerosol particles captured by the electrometer or remaining in the flow gas, within a measurement region, the measurement region being downstream of the quenching region.

12. An apparatus according to any preceding claim, comprising an air flow rate regulator and / or an air flow rate sensor and comprising a processorprogrammed to determine a particle size, surface area and / or concentration parameter taking into account the flow rate of air through the apparatus.

13. An apparatus according to any preceding claim, wherein the mean position of the generated ions within the chamber is within a maximum cross sectional area region of the chamber, and is closer to the outlet of the chamber than to the inlet of the chamber.

14. An apparatus according to any preceding claim, further comprising a flow contracting region between the maximum cross sectional area region and the outlet of the chamber, the flow contracting region configured to decrease the cross-sectional area of flowing gas in the conditioning chamber, whereupon the contracting takes place over a shorter distance than the expansion.

15. An apparatus according to claim 14, where the charging region extends into at least 75% of the flow contraction region’s axial length and / or at least 95% into the flow contraction region’s internal volume.

16. An apparatus according to claim 15, comprising an ionizing radiation source, located downstream of the flow expansion region and upstream of the flow contraction region.

17. An apparatus according to claim 15 or claim 16, wherein the flow expansion region and the flow contracting region each have frusto-conical cross sections, wherein the decrease in radius of the flow contracting region per unit of axial length is at least 5 times the increase in radius of the flow expansion region per unit of axial length.

18. An apparatus according to any of claims 15 to 16, configured to avoid flow recirculation.

19. An apparatus according to any of claims 16 to 18, wherein the ion generator comprises an ionizing radiation source which generates radiation that causes gas particles to become ionized, optionally wherein the ionizing radiationsource is located closer to the flow contracting region than the flow expansion region.

20. An apparatus according to any preceding claim, which is a bipolar charge conditioner.

21. A method of charge conditioning of a gas comprising aerosol particles, the method comprising: providing a body comprise a conditioning chamber, the conditioning chamber having an inlet and an outlet, receiving a flowing gas comprising aerosol particles into the conditioning chamber through the inlet of the conditioning chamber, expanding the cross-sectional area of the flowing gas within the conditioning chamber, generating ions mixed with the expanded flowing gas, the ions colliding with aerosol particles within the gas and thereby charge conditioning the charge distribution of the aerosol particles within the gas, the charging process progressing towards a steady-state, wherein the charging process is ongoing when either (i) the gas flows out of the conditioning chamber through the outlet of the conditioning chamber, or (ii) ions are removed from the flow gas in a quenching region defined between at least two electrodes.

22. A method according to claim 21 , wherein the charge conditioning is at least partially quenched by removal of the ions from the flowing gas before the charge distribution of the aerosol particles has proceeded more than 95% of the way to the steady state.

23. A method according to claim 22, comprising expanding the cross-sectional area of the flow of the gas upstream of, within, or overlapping the charging region and then contracting the cross-sectional area of the flow of gas downstream of, within or overlapping the charging region, prior to at least partial quenching, whereupon the contracting takes place over a shorter distance than the expansion.

24. A method according to claim 23, whereby the majority of the generation of ions by radiation, in the flow of the gas, takes place after the cross-sectional area of the gas has expanded to a maximum cross-sectional area.

25. A method according to any of claims 21 to 24, wherein the charging region is within the conditioning chamber and extends to an outlet from the conditioning chamber, but does not extend to the inlet to the conditioning chamber, and wherein the at least partial quenching takes place within or at an outlet from the conditioning chamber, optionally wherein the at least partially quenching takes place in a quenching region which overlaps with the charging region.

26. An apparatus for charge conditioning a sample of aerosol particles to have a controlled charge distribution, the apparatus comprising: a body having an inlet and an outlet and a conditioning chamber therebetween, the body defining a flow-through path for a gas, comprising aerosol particles, from the body inlet and through the conditioning chamber to the body outlet; wherein the conditioning chamber comprises first and second electrodes, a controller configured to apply a potential difference between the first and second electrodes, the first electrode having an ion generator adjacent or integrated therewith, the conditioning chamber and ion generator defining a charging region within the conditioning chamber, the conditioning chamber configured to define a flow path for aerosol particles therethrough from the inlet to the outlet, through the charging region, the controller being configured to select the potential difference between the first and second electrodes to thereby control the distribution of positively andnegatively charged ions and thereby control the balance between unipolar and bipolar charging.

27. An apparatus according to claim 26, further comprising at least two electrodes which define a quenching region, the quenching region overlapping or downstream of the charging region, the apparatus comprising a circuit to generate a potential difference between the at least two electrodes and thereby remove ions from the gas in the quenching region.

28. An apparatus according to claim 26 or claim 27, configured such that aerosol particles at the outlet will on average have been closer to the second electrode than the first.

29. An apparatus according to claim 26 or claim 27, whereby the at least two electrodes are configured such that the potential gradient generated between them in use crosses the direction of flow of gas along the flow-through path.

30. An apparatus according to claim 27 or 28, wherein the quenching region overlaps with the outlet of the chamber, typically wherein the quenching region removes ions from the gas after charge conditioning in the charging region.

31. An apparatus according to any of claims 29 to 30, wherein the quenching region is located at the outlet of the conditioning chamber and / or overlaps with the charging region.

32. An apparatus according to any of claims 27 to 31, wherein the removal of ions by the quenching region preserves the asymmetry of the charge distribution of the aerosol particles in the gas whether not a steady-state charge distribution is reached.

33. An apparatus according to any of claims 26 to 32, wherein the mean position of the generated ions is within the region of maximum cross-section of the conditioning chamber and is closer to the outlet of the conditioning chamber than to the inlet of the conditioning chamber.

34. An apparatus according to any of claims 27 to 33, wherein the charging region comprises a variable potential gradient region defined by the first and second electrodes and wherein the volume of the conditioning chamber between the variable potential gradient region and the quenching region is less than the volume of the variable potential gradient region.

35. An apparatus according to any of claims 26 to 34, whereby the ion generator comprises a radioactive material coated on the first or second electrode.

36. A method of operating the apparatus of any of claims 26 to 34, the method comprising causing a gas, comprising aerosol particles, to flow through the body, along the flow-through path and controlling the potential difference across the first and second electrodes to thereby select the balance between unipolar and bipolar charge conditioning of aerosol particles in the gas.

37. A method according to claim 36, wherein the charge conditioning is at least partially quenched by removal of the ions from the flowing gas before the charge distribution of the aerosol particles has proceeded more than 95% of the way to the steady state.

Images