WO2021058873A1

WO2021058873A1 - Apparatus and method for detecting particles

Info

Publication number: WO2021058873A1
Application number: PCT/FI2020/050628
Authority: WO
Inventors: Joonas ENROTH
Original assignee: Airmodus Oy
Priority date: 2019-09-24
Filing date: 2020-09-24
Publication date: 2021-04-01
Also published as: FI130261B; FI20195801A1; EP4034858A1

Abstract

A method for detecting particles, the method comprising at least providing a sample flow comprising particles, providing the sample flow with a saturating agent, directing at least a portion of said sample flow to an expansion chamber, sealing at least the expansion chamber from the environment, and increasing a pressure inside at least the expansion chamber from a first pressure to a second, higher pressure. The method additionally comprises decreasing the pressure inside at least the expansion chamber from the second pressure to the first pressure to produce activated particles by inducing supersaturation of the saturating agent and thus condensation of the saturating agent onto the particles of the sample flow to increase the size of selected particles from a first size to a second size, unsealing at least the expansion chamber, directing the sample flow comprising the activated particles of second size from the expansion chamber to a detector, and detecting at least an amount of single activated particles in the sample flow.

Description

APPARATUS AND METHOD FOR DETECTING PARTICLES

TECHNICAL FIELD OF THE INVENTION

The invention is related to detecting of particles. More specifically, the invention is related to an apparatus and method for detecting an amount of particles by increasing the size of selected particles of a sample flow.

BACKGROUND OF THE INVENTION

Detection of particles, especially aerosol particles, is a field with various important applications. For detection of particles which have a size that is smaller than can be detected through optical methods, there exist various solutions where the size of the particles is increased before detection.

Often, a working fluid is condensated onto the particles of a sample to be detected to increase the size of the particles and enable their detection. The sample particles may be subjected e.g. to various different saturation ratios depending on the location of the particles in the flow profile of the sample flow of gas. Therefore, particles of widely different sizes can be increased in size, i.e. activated making it difficult to select a narrow size range or single selected size from the original size distribution.

The prior art provides methods e.g. based on laminar flow techniques that make it slow and difficult to detect a size range of particles in a sample. Mixing type solutions also for instance exhibit difficulties in control of gas flows and mixing.

Many existing solutions, especially those based on adiabatic expansion, also require a minimum concentration of particles in the sample for detection to be feasible. For instance with optical detectors, the extinction of an optical signal is detected as it passes through the sample gas. The optical signal is passed through the entire sample and these methods are limited to samples with a concentration of particles that exceeds a certain threshold. The background noise associated with these optical detectors is of such magnitude that small concentrations of particles, such as below 300 #/cc cannot be reliably detected. Particle concentrations of such small magnitudes are however readily present in the air and especially in many research environments, such as northern areas or mountain areas, even smaller particle concentrations in surrounding air is common. Many traditional techniques for determining size distribution of particles rely on charging the particles and classifying them by size based on their electrical mobility. However, the charging of small particles involves difficulties.

There is a need for a method of detecting particles where the size distribution of a sample gas may be determined reliably, in a simple and timely manner, which also allows detection of particles of a sample with small particle concentration.

SUMMARY OF THE INVENTION

A purpose of the invention is to alleviate at least some of the problems relating to the known prior art. In accordance with one aspect of the present invention, a method is provided for detecting particles, the method comprising at least providing a sample flow comprising particles, providing the sample flow with a saturating agent, directing at least a portion of said sample flow to an expansion chamber, sealing at least the expansion chamber from the environment, and increasing a pressure inside at least the expansion chamber from a first pressure to a second, higher pressure. The method additionally comprises decreasing the pressure inside at least the expansion chamber from the second pressure to the first pressure to produce activated particles by inducing supersaturation of the saturating agent and thus condensation of the saturating agent onto the particles of the sample flow to increase the size of selected particles from a first size to a second size, unsealing at least the expansion chamber, directing the sample flow comprising the activated particles of second size from the expansion chamber to a detector, and detecting at least an amount of single activated particles in the sample flow.

There is also provided an apparatus for detecting particles according to the independent claim 18 and a method of manufacturing an apparatus for detecting particles according to the independent claim 26.

Having regard to the utility of the present invention, according to an embodiment, the present invention may provide a reliable method for detecting an amount of (activated) particles comprised in a sample flow.

Embodiments of a method may comprise varying the second pressure to obtain activated particles with different initial size to detect/determine a size distribution of the particles comprised in the sample flow. In one advantageous embodiment, the detecting of an amount of particles may comprise detecting a number of particles with a single-particle counter, preferably through an optical measurement.

The detector may comprise or be coupled to a nozzle through which the activated particles are directed essentially one at a time to achieve a flow of single particles into a detection chamber, wherein the detecting of the number of particles is conducted by observing scattering of light directed at the flow of single particles. The nozzle may be of such dimensions that the particles may be directed through the nozzle essentially one at a time. The combination of increasing the size of at least a portion of selected particles in an expansion chamber and detecting the thus activated particles with a single-particle detector, i.e. detecting the amount of particles of second size by detecting them individually, preferably optically, may provide several benefits. Small concentrations of particles may be observed, and an apparatus and method may be utilized e.g. in environments where such small concentrations of particles prevail in the air, for instance.

Embodiments of the present invention also provide a possibility to detect when the concentration of particles in a sample flow (or at least the concentration of activated particles) is zero or essentially zero. Concentrations of e.g. 0, 10, and 100 #/cc may be detected or observed through the invention, and samples having such concentrations may be differentiated from each other.

With the invention, small particles, e.g. particles with diameter of under 100 nm, may be detected/observed.

Of course, embodiments of the invention may also be used for detection of larger particles and/or higher concentrations. In the case of larger particles, the invention may also be advantageous in detection of the particles individually/singularly. Amounts/numbers of particles of different sizes may be detected and the size distribution of particles in a sample flow may be determined efficiently from samples comprising e.g. small particles and small concentrations of such particles. By controlling the difference between the first and second pressure, the level of supersaturation may be controlled to control the second size of the particles. For instance, the second pressure may be between 0-100 kPa, e.g. advantageously 0-50 kPa, larger than the first pressure (pressure difference lower range endpoint is given at 0 kPa but of course it should be understood that the pressure should be increased by larger amount than 0 kPa). By controlling this pressure difference, the level of supersaturation may be controlled quickly and to a high degree of precision. Thereby, particles of different sizes may be effectively activated. In one embodiment, the directing of at least a portion of the sample flow to the expansion chamber may be done through a first valve, wherein the first valve may be a valve that provides an essentially unobstructed flow path for the particles, such as a valve selected from the group of a pinch valve, a ball valve, and a butterfly valve. Here, such a valve may allow the particles of the sample flow to flow through without being obstructed and without particle losses. Valves that are traditionally used in this field do not allow such an unobstructed flow of particles to pass through, and considerable losses of sample particles may be experienced.

The sealing of at least the expansion chamber from the environment may be carried out utilizing at least a first valve (i.e. closing the valve), such as a pinch valve (which may also be used for directing the sample flow to the expansion chamber).

The sealing of at least the expansion chamber may also involve use (closing) of a second valve, which may in one embodiment also be used to direct the sample flow comprising activated particles to the detector.

At least a first valve and a second valve may constitute a sealing arrangement for an apparatus and unsealing may comprise opening of at least one of said valves, so that the sample flow and activated particles are able to be directed towards the detector via a transfer member comprised between the expansion chamber and the detector.

A transfer member may refer to at least a nozzle which may in some embodiments be considered to be part of the detector. In some other embodiments, a transfer member may additionally or alternatively refer to e.g. a valve and/or transfer tube that may be placed between the expansion chamber and detector.

The inventors have apprehended that after particles have been activated (here grown adiabatically from a first size to a second size), they remain in an activated state where they have a size that is larger than the first size for some period of time after the adiabatic activation process is discontinued. While the particles remain in this activated state, they will have a size that is large enough for the particles to be detected optically. Thus, it could be possible to use adiabatic expansion to increase the size of particles and thereafter lead the activated particles to a detector and then detect the particles, instead of utilizing a detector that detects the particles while they are still inside an expansion chamber.

It was realized by the inventors that by controlling the conditions in the method and/or by selecting suitable components used in an apparatus and characteristics therof, the time period in which the particles remain in an activated state (activation window/particle lifetime) may be extended and/or the activated particles may be directed to the detector rapidly (so that they do not have time to evaporate during the directing to no longer be in the activated state). Features described herein may be utilized to extend the aforementioned activation window time period and/or direct the activated particles to the detector more efficiently or quickly in seclusion or these features may be combined, possibly extending said time period or expediting the directing to the detector even more efficiently. If the activation window is longer than the time that is required for the activated particles to reach the detector, the activated particles can be efficiently detected. The providing of the sample flow with a saturating agent may comprise directing at least a portion of the sample flow through a saturator or combining the sample flow with a second, sheathing, flow that has been directed through a saturator.

A benefit that may be attained with the present invention is that an apparatus may be used with any saturating agent. Creation of the supersaturated condition does not depend on the geometry of the apparatus, but on the used temperatures and pressure difference. A multitude of different saturation ratios or levels may also be easily obtained with an apparatus.

In some embodiments, a by-pass-flow of the sample flow may be provided that bypasses the expansion chamber and the detector. A by-pass-flow may be advantageously used to ensure that the flow in the apparatus may be constant and even. An apparatus may then be utilized in different conditions and environments. In embodiments of the invention, the pressure may be decreased by opening of a fast valve (such as opening of at least a second valve used for sealing) and preferably additionally using a vacuum. The decreasing of the pressure may then be quick and efficient.

In one embodiment, increasing and decreasing of pressure may be done by utilizing a piston arrangement.

The increasing and decreasing of pressure (and thus supersaturation of saturating agent and condensation onto particles, i.e. activation of particles based on adiabatic expansion) and subsequent detection of particles may be suitable for precise size classification of particles based on activation. With the expansion chamber of the present invention, an essentially homogeneous supersaturation field (level of supersaturation of the saturating agent) may be achieved.

Compared to other expansion techniques, the present invention may provide an apparatus and method that is faster in its operation. In one embodiment, an expansion chamber may be provided where the expansion chamber is designed or adapted to exhibit advantageous or optimized/selected flow characteristics. With selected flow characteristics or selected flow field, a change or replacement of the sample flow in the expansion chamber (i.e. change or replacement of the quantity of the sample flow that is sealed into the expansion chamber as it is sealed from the environment) may be fast and/or efficient. A selected flow field may be one that is optimized to provide a selected expansion chamber evacuation time and/or selected flow velocity of particles or gas in the expansion chamber or combination of these. A combination of parameters influencing the flow field may be optimized to provide a suitable combination of these parameters.

A selected flow field may be one that is selected to provide faster evacuation time and/or minimized zones of slower flow velocity in the expansion chamber and/or provide a laminar sample flow. The selection or optimization of the flow field may be obtained by modeling a plurality of expansion chamber geometries and selecting a geometry that provides a selected expansion chamber evacuation time and/or selected flow velocity in the expansion chamber.

The modeling of the geometry of the expansion chamber may involve e.g. modeling of different volumes and/or shapes of the expansion chamber and a homogeneity of the expansion chamber (to ensure that locations of nonhomogeneous saturation are avoided or reduced), and the impact of these on the evacuation time of the expansion chamber.

An expansion chamber may in some embodiments be adapted to exhibit a selected flow field through providing one or more tapers and/or flow guides in the expansion chamber geometry.

In an embodiment, at least the expansion chamber and the detector may be sealed from the environment, such that they are sealed from the environment together. Here, the expansion chamber and the detector may be considered as being part of the same space (although they may still be provided as separate bodies or chambers, they may be part of the same air space in the sense that e.g. they may be separated only by a small orifice and nozzle and/or other transfer member through which particles may be able to traverse). A first valve may be operated such as described above. A second valve may be used after the detector to direct the activated and detected particles away from the detector and this second valve may also be used (closed) when sealing the expansion chamber and detector.

When the expansion chamber and the detector are sealed from the environment, they may be considered to be sealed, pressurized, depressurized, and unsealed simultaneously. After the increasing and decreasing of pressure, the particles that are activated in the expansion chamber may be directed to the detector by reversing the sealing, e.g. by opening at least a second valve that separates the detector form the environment (or rest of the apparatus after the detector). During the sealing, particles do not move in the sealed space and therefore do not move from the expansion chamber to the detector but after unsealing, the particles advance from the expansion chamber to the detector. Sealing of the expansion chamber together with the detector may be advantageous as the moving or transferring of activated particles may be difficult. These larger particles may easily collide with parts of an apparatus and may evaporate easily. Therefore, allowing the particles to be directed directly from the expansion chamber to the detector (directly here meaning that there is no closing arrangement such as valve that is operated therebetween, but e.g. some type of transfer member may still be used), particle losses may be avoided.

Simultaneous sealing of the expansion chamber and the detector from the environment also allows rapid evacuation of the expansion chamber so that the activated particles may be directed to the detector while they are still in an activated state. The longer the evacuation time from the expansion chamber, the more time particles have to evaporate and the activation window may be over before the particles reach the detector.

In some embodiments, the expansion chamber and detector do not have to be sealed from the environment simultaneously (it could be only the expansion chamber that is sealed), yet rapid evacuation of the expansion chamber may be facilitated through use of a fast valve between the expansion chamber and detector, and the activation window could still exceed the time required for the particles to reach the detector.

The exemplary embodiments presented in this text are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this text as an open limitation that does not exclude the existence of unrecited features.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific example embodiments when read in connection with the accompanying drawings.

The previously presented considerations concerning the various embodiments of the method may be flexibly applied to the embodiments of the apparatus and method for manufacturing the apparatus mutatis mutandis, and vice versa, as being appreciated by a skilled person.

BRIEF DESCRIPTION OF THE DRAWINGS

Next the invention will be described in greater detail with reference to exemplary embodiments in accordance with the accompanying drawings, in which:

Figure 1 shows one exemplary apparatus according to an embodiment of the present invention, Figure 2 shows a portion of an apparatus according to an embodiment of the invention,

Figure 3 exhibits one other exemplary apparatus according to an embodiment of the present invention,

Figure 4 illustrates one more exemplary apparatus according to an embodiment of the present invention,

Figure 5 shows a flow chart of a method according to the present invention, and

Figure 6 illustrates one exemplary embodiment of an expansion chamber with a selected geometry. DETAILED DESCRIPTION

Figure 1 shows one example of an apparatus according to an embodiment of the present invention. A sample flow of gas comprising particles is provided via a first inlet 102. Particles may herein refer to e.g. aerosols, molecules, molecule clusters, or single particles. The sample flow is provided with a saturating agent. The sample flow may be provided with a saturating agent to obtain a saturated sample flow. A saturated sample flow refers here in the following text to a sample flow that is at least partially saturated. A partially saturated sample flow may herein refer to a sample flow that is obtained by combining e.g. two or more flows, of which at least one has been saturated. A “saturated sample flow” may thus have different levels of saturation at different portions of an apparatus or during different steps in a method. A saturated sample flow may refer to a fully saturated vapor or a gas which has a partial vapor pressure that has been elevated by using a saturator. The saturating agent may be provided by directing the sample flow or at least a portion thereof through a saturator 104. In one embodiment, such as that of Fig. 1 , a portion of the sample flow is directed through the saturator 104 while the remaining portion does not pass through the saturator 104. The portion of the sample flow that is passed through the saturator 104 may for instance be over 50%, and be relatively high, such as e.g. over 80% and around 90%.

A portion of the sample flow that is directed through a saturator 104 may advantageously be relatively high, so that the apparatus may be less sensitive to the temperature and/or humidity of the sample flow. The temperature and humidity of the final obtained saturated sample flow is an average of the sample flow that is not passed through the saturator 104 and the sample flow that comes out of the saturator 104. A high percentage or portion of the sample flow that is passed through the saturator 104 then contributes more to this average temperature and humidity, so that the apparatus may be more stable even in varying conditions when the temperature and/or humidity of the sample flow may change.

The portion of the sample flow that is passed through the saturator 104 forms a protecting flow that may reduce losses of small particles. A relatively high portion of the sample flow being passed through the saturator 104 aids in faster passing of the sample particles to the expansion chamber. The sample particles may be surrounded by the saturator flow, so that the particles have a larger distance to diffuse in before colliding into walls of the apparatus and thus being lost.

The sample flow may advantageously be directed to the saturator 104 through a filter 106. The filter 106 may essentially remove the particles from the portion of the sample flow that is passed through the saturator 104 or at least reduce the number of particles so that the particle concentration in the portion of the sample flow that is passed through the saturator 104 is at least substantially reduced. Preferably, the sample particles that reach an expansion chamber as will be described below are obtained essentially only from the portion of the sample flow that is not passed through the saturator 104.

In some embodiments, providing the sample flow with a saturating agent may comprise directing the entire sample flow through a saturator.

In the exemplary apparatus 100 of Fig. 1 , the portion of the sample flow that has not been directed through the saturator 104 and the portion of the sample flow that has been directed through the saturator 104 are combined, after which the then obtained sample flow, which may be called a saturated sample flow, is directed to an expansion chamber 108.

The portion of the sample flow that has been directed through a saturator may then act as a sheathing flow for the remaining portion of the sample flow that has not been directed through the saturator Here, it is the remaining portion that has not been directed through saturator comprises the sample particles.

The sample flow or saturated sample flow or other fluid utilized may in the apparatus 100 be transferred inside a tube/conduit as may be easily comprehended by the person skilled in the art, shown in the figures as lines.

The saturated sample flow may be directed to the expansion chamber 108 through at least one first valve 110. The first valve 110 may advantageously be a valve that provides an essentially unobstructed flow path for the particles, such as a pinch valve, a ball valve, or a butterfly valve. Yet, any type of valve(s) may be utilized, but in advantageous embodiments the valve is of such a type that particle losses are avoided or minimized by allowing the flow to be essentially unobstructed through said valve.

A suitable quantity of saturated sample flow is passed to the expansion chamber 108, wherein the suitable quantity may be e.g. about 30 cm³ or an amount of gas that may flow through the expansion chamber in 1s, which is enough to replace the gas previously existing in the chamber, and depends on the design of the expansion chamber. The expansion chamber 108 is then sealed from the environment (i.e. sealed at least from the incoming saturated sample flow and at least a portion of the rest of the apparatus 100). The sealing may be done e.g. by closing of the at least one first valve 110 and closing of at least one second valve 112 that may be placed in the apparatus 100 after at least the expansion chamber 108.

A second valve 112 may e.g. be a valve that is similar to the first valve 110.

The e.g. valves that may be utilized to (reversibly) seal at least the expansion chamber 108 from the environment may constitute a sealing arrangement for an apparatus 100.

The shape of the expansion chamber 108 may affect the functioning of the apparatus, such as its speed and/or how long the attained supersaturation is maintained in the chamber after adiabatic expansion. A spherical expansion chamber with a large volume to surface area ratio may be advantageous, as the supersaturation ends when the heat from the walls of the expansion chamber has returned the temperature inside the chamber to the original temperature.

After sealing the expansion chamber 108 from the environment, the pressure inside the expansion chamber is increased from a first pressure to a second, higher pressure.

In one embodiment, the second pressure may be 0-100 kPa larger (or e.g. 1- 100 kPa larger) than the first pressure. In some embodiments, the second pressure may be 0-50 kPa, e.g. 0-25 kPa, larger than the first pressure (yet being e.g. at least 1 kPa larger). The difference between the first pressure and the second pressure may depend on the intended use of the apparatus, i.e. , the size of the particles that are to be detected.

In some embodiments of the invention, the second pressure may be smaller than the first pressure, thus an apparatus 100 may be operated so that a pressure inside an expansion chamber is first decreased and then increased to induce growth of sample particles.

The pressure inside the expansion chamber 108 may be increased by a pressure increasing arrangement, such as by directing pressurized air into the sample line, i.e. the conduit in which the saturated sample flow is transferred (into the portion of the sample line which is located between the first valve 110 and the expansion chamber 108 and after the first valve has been closed). The pressure may be increased by directing air into the sample line via a third valve 112.

After increasing the pressure inside the expansion chamber 108, the pressure is decreased from the second pressure to a lower pressure, preferably the pressure is decreased from the second pressure essentially back to the first pressure.

The decreasing of the pressure may involve opening of the at least first valve 110 and the at least one second valve 114. The at least second valve 114 may be a fast valve. The decreasing of the pressure may then also mean that sealing of at least the expansion chamber 108 is reversed, i.e. the expansion chamber 108 is unsealed. In an embodiment of an apparatus 100, a pressure decreasing arrangement may also involve use of a vacuum 116 which may be connected to the sample line after at least the expansion chamber 108.

The decreasing of the pressure in the expansion chamber 108 causes simultaneous adiabatic expansion and cooling of the saturated sample flow that is contained in the expansion chamber 108. As a result of the cooling, the saturated sample flow reaches a temperature which is below the dew point of the saturated sample flow, leading to supersaturation of the saturating agent. The saturating agent is then condensed onto selected particles of the saturated sample flow.

The particles onto which saturating agent condenses increase in size from a first size to a second, larger size. The particles that increase in size are herein termed activated particles. The activated particles may start to evaporate and decrease in size even immediately after reaching a maximum second size, while the particles may still be considered to be activated particles and exhibit a second size while they are of a size that is larger than the first size and exceeding a detection threshold size (which may be a threshold size for optical detection of the particles).

The induced level of supersaturation that is achieved in the expansion chamber 108 may be determined by the relative humidity (or partial vapor pressure in the case of a saturating agent other than water) of the sample flow and/or by the difference between the first pressure and the second pressure.

Different levels of supersaturation will activate particles of different size, with the level of supersaturation required for activating particles being inversely dependent on the size of the particles. E.g. a first level of supersaturation may cause particles having size between a first range or being larger than a first activation threshold size to increase in size from a first size to a second size, thus being activated. A second, higher level of supersaturation may then cause particles having size between a second range or being larger than a second activation threshold size to increase in size from a first size to a second size, where the second range comprises particles that are smaller than the particle in the first range or the second activation threshold size is smaller than the first activation threshold size. For example, if water is used as a saturating agent, a supersaturation level or value of about 1.4 could activate particles having a first size that is larger than an activation threshold size of about 10 nm. All particles in a saturated sample flow having size at or above this activation threshold size may then be activated, while possible smaller particles comprised in the saturated sample flow would require a higher level of supersaturation to be activated. Still as an example for water as a saturating agent, if the value of supersaturation is about 1.56, then particles having a size larger than an activation threshold size of about 8 nm will be activated. The second size does not need to be the same for different particles comprised in the size range of particles that are activated by a specific level of supersaturation. Thus, a first level of supersaturation may activate particles such that their sizes after adiabatic expansion are different, i.e. the second size is different for the different particles, but the first level of supersaturation only activates particles that have a first size within a first range or above a first activation threshold size. Maximum second sizes of particles may for instance be between 1 and 10 micrometers.

The selected particles that are activated are thus particles that have a first size in a selected range or above a selected activation threshold size, wherein the selected range or activation threshold size is determined by the humidity or level of saturation of the saturated sample flow and/or by the difference between the first pressure and the second pressure.

Then, the saturated sample flow (or at least the sample quantity of saturated sample flow that is comprised in the expansion chamber 118 as it is sealed from the environment) comprising the activated particles is directed to a detector 118. The detector 118 is used to detect an amount of activated particles in the saturated sample flow.

The detector 118 may comprise means for optically detecting an amount of activated particles. A detection threshold size or minimum second size of particles may be determined by the type of detector that is used. For instance, with optical detectors a minimum size of particles that may be detected is typically around 300 nm.

The detector 118 may advantageously be a single-particle counter, so that the detecting/measurement may be indicative of a number/quantity/numerical quantity of activated particles.

In one embodiment of the invention, an apparatus 100 also provides a by- pass-flow of the sample flow or saturated sample flow that bypasses at least the expansion chamber 108 and the detector 118. The by-pass-flow may be directed past at least the expansion chamber 108 and detector 118 into a vacuum-providing tube, which may also be utilized in the decreasing of pressure as disclosed hereinbefore. The by-pass-flow may ensure that the sample flow in the apparatus 100 is maintained substantially constant and continuous. After the amount of activated particles has been detected, at least part of the above process/method may be repeated with a different level of supersaturation so as to activate particles having a different first size. For instance, the second pressure may be varied.

Thus, a new quantity of sample flow may be obtained via the inlet and at least partly passed though the saturator, the saturated sample flow then being directed to the expansion chamber, the expansion chamber being sealed and then pressurized to a different second pressure. The pressure is then again dropped and unsealing is performed, and the saturated sample flow is directed to the detector 118. A numerical quantity of activated particles with a different first size may then be detected. The activation of particles with different size and subsequent detection may be conducted any number of times. Thus, the numbers of particles having at least different first sizes may be measured. With the apparatus 100 and related method, a size distribution of particles in a sample flow may therefore be obtained.

With advantageous embodiments of the invention, the detector 118 may be able to indicate an absence of activated particles, i.e. , if the number of activated particles is zero. The apparatus 100 may then determine if a sample flow does not comprise particles or at least if a sample flow does not comprise particles having at least a selected first size.

In some embodiments, temperatures may be detected at one or more locations of the apparatus 100 and temperatures may also be controlled.

In one embodiment, the temperature of the saturated sample flow and/or the temperature of the detector (e.g. the temperature in a detection chamber comprised in a detector) may be controlled. Controlling of a temperature in a detector may reduce or prevent condensation on surfaces of a detector which may hinder optical detection.

In one embodiment, the temperature of the sample flow inside the expansion chamber 108 is controlled. Advantageously, the temperature of the sample flow inside the expansion chamber is maintained at or close to the dew point temperature of the saturated sample flow to allow activation of particles at a suitable time and they can be detected before evaporating.

In one embodiment, the temperature of the detector 118 is controlled to maintain a temperature that is slightly, e.g. 1-5 degrees centigrade, higher than that of the expansion chamber. This temperature difference may prevent unnecessary condensation of the saturating agent onto the walls of the detector 118.

One embodiment of a detector 118 is depicted in Figure 2, showing a portion of an apparatus 100. Here, a detector 118 may comprise at least a detection chamber 118a and a nozzle 120 through which the saturated sample flow is directed so that the activated particles 122 are directed through the nozzle 120 essentially one at a time to achieve a flow of single activated particles 122 into the detection chamber 118a (or at least a very narrow flow which does not allow many particles to pass through the nozzle simultaneously). A detector 118 may additionally comprise a laser 124. The amount of activated particles 122 may be detected by directing a preferably narrow laser beam 126 at the flow of single particles, advantageously in the vicinity of the nozzle 120 (such as at a distance of under 1 cm away from the nozzle 120), and observing the scattering of the laser beam as it impacts the activated particles flowing through the nozzle 120 and into the detection chamber 118a. By monitoring the pulse height distribution, information on the initial particle number size distribution can be obtained.

An embodiment of an apparatus 100 may be constructed so that the sealing of at least the expansion chamber 108 also comprises sealing of the detector 118. In one advantageous embodiment, such as that depicted in Figs. 1-2, the expansion chamber 108 and the detection chamber 118a may be sealed, opened, pressurized, and depressurized together. The expansion chamber 108 and the detection chamber 118a may be considered as being part of the same space. The chambers may be provided as separate bodies but they may be part of the same air space in the sense that they may be separated only by a small orifice and the nozzle or transfer member 120. In one embodiment, the diameter of the nozzle 120 may be about 0.1 mm, but may be larger if larger sample flows are used, such as up to 1 mm.

In one embodiment, the nozzle 120 may also comprise a valve, yet the valve should preferably be such that the valve is a fast valve, opening essentially instantaneously to a diameter that substantially corresponds to diameter/geometry of the rest of the nozzle 120 (or e.g. differs from it by under 5%), such that the activated particles have an unobstructed flow path to the detector 118. The valve may be a pinch type valve, for instance.

If the expansion chamber 108 and detector 118 are sealed together, then it is clear that the saturated sample flow does not move inside the sealed space, but the activated particles that are obtained after pressurizing and depressurizing may be able to traverse to the detector 118 after unsealing.

Figure 3 shows one other exemplary apparatus 200 according to an embodiment of the invention. Here, the increasing and decreasing of pressure may be conducted via a piston arrangement inside the expansion chamber 108. The pressure increasing and pressure decreasing arrangement may thus comprise the piston arrangement. The volume inside the expansion chamber 108 that holds the sample particles to be detected may therefore be decreased and increased via operation of the piston arrangement.

This embodiment of an apparatus 200 may comprise elements of the apparatus 100 of Fig. 1 , such as a first valve 110 and a bypass flow may be arranged to ensure that the flow in the apparatus 200 may be constant and even.

In the embodiment of Fig. 3, a fourth valve 128 may be arranged between the expansion chamber 108 and the detector 118. Yet, the apparatus 200 with the piston arrangement may also resemble that as discussed previously, so that the expansion chamber 108 and the detector 118 essentially reside in the same air space and are separated only by a small nozzle.

In the embodiment of Fig. 3, the entire sample flow is directed through a saturator 104. Of course, also here the arrangement may be constructed so that only a portion of the sample flow is directed through a saturator. Figure 4 shows one more exemplary apparatus 300 according to an embodiment of the invention. The functionality of the apparatus 300 of Fig. 4 is similar to that explained in connection with Figs. 1-3, with Fig. 4 showing a more detailed construction of an apparatus 300 with more components illustrated. As may be easily understood by those skilled in the art, an apparatus 100, 200, 300 according to the present invention may be constructed in various ways regarding e.g. how the sample lines, bypass flows, possible different valves, pressure decreasing and increasing arrangements etc. are arranged, yet still exhibiting the same functionality relating to the adiabatic expansion and detection of activated particles that may be conducted.

Figure 5 shows a flow chart of a method according to one embodiment of the invention. A sample flow comprising particles is provided 402 and the sample flow is provided with a saturating agent at 404, to obtain a sample flow which may be called a saturated sample flow where the sample flow may be at least partially saturated, so that the sample flow may be saturated during at least some part of the method.

At least a portion of the sample flow is directed 406 to an expansion chamber 108 and at least the expansion chamber 108 is sealed 408. The pressure inside the expansion chamber 108 is increased 410 from a first pressure to a second pressure produce activated particles by inducing supersaturation of the saturating agent and thus condensation of the saturating agent onto the particles of the sample flow to increase the size of selected particles from a first size to a second size. The pressure inside the expansion chamber is then decreased 412 from the second pressure to the first pressure and the at least expansion chamber is unsealed 414. The unsealing 414 and the decreasing of pressure 412 may occur essentially simultaneously and/or the unsealing may also be considered to be part of the stage of decreasing the pressure. The sample flow is directed 416 to a detector and an amount of activated particles is detected 418.

At least part of the stages above, such as 406-418, may be repeated to activate selected particles of different size as disclosed hereinbefore. A size distribution of particles in the sample flow may then be determined 420. In an embodiment of the invention, the expansion chamber 108 may be adapted to exhibit a selected flow field. The selected flow field may be obtained by selecting a geometry for the expansion chamber 108 that provides selected expansion chamber evacuation time and/or selected flow velocity of particles in the expansion chamber, e.g. through modeling a plurality of expansion chamber geometries.

The selection or adaptation of an expansion chamber 108 may be a selection or optimization of several features that in combination provide a suitable expansion chamber 108 (which may be carried out through modeling geometries of the expansion chamber). A suitable balance may be obtained through consideration of particle losses before and/or after the activation of the particles, temperature gradients, homogeneity of supersaturation field, and/or dimensions of the expansion chamber. All the aforementioned parameters may be affected by e.g. the selection of working fluid and/or a total flow of the apparatus.

A suitable expansion chamber 108 may involve a selected combination of parameters which offers advantageous properties of the expansion chamber 108 for a selected use case scenario. For instance, differing magnitudes of sample flow may lead to considerably differing geometries of expansion chamber 108 which could be considered to be suitable.

In the selection of flow field, the flow of sample gas (sample gas flow) is advantageously laminar into and out of the expansion chamber 108.

A selected geometry of an expansion chamber 108 may involve selection of diameter and volume of the expansion chamber based on a selected total flow. The discussed selections may be based on modelling different geometries of expansion chamber 108 and taking into account parameters in different use case scenarios.

In the adaptation of the expansion chamber 108, the starting point is by considering the requirements of single particle aerosol counting/detecting that may be utilized. With an optical single-particle detector 118, the sample or expansion chamber 108 should be such that it contains enough sample gas particles to provide sufficient counting statistics for particles even for low concentrations (< 1000 #/cm³).

Further, as the sizing of particles is based on observing the difference in concentrations between the different supersaturations, the measured sample flow should be even larger than is required for simply counting purposes, as the size information can be found in the observed difference between different generated supersaturations. In practice, for an apparatus with a flow rate of 1 Ipm (or 16.667 cm³/s), the expansion chamber should preferably be around 5 - 20 cm³. In one exemplary use case scenario, a chamber with a volume of 12 cm³, with a radius of 0.9 cm, and a total height of 8 cm, including tapers and an internal flow guide may be used. Of course also differing radii and height may also be utilized.

The lower end of the expansion chamber volume range may be limited by the counting statistics required to separate the size bins in low concentration environments, and the upper end of the expansion chamber volume range may be limited by the activated particle lifetime. Based on numerical growth and evaporation models, wetted aerosol particles in the 10 micron range have a lifetime around 1 - 2 seconds (in RH > 90 % environments) prior to evaporating. This means that in order to have accurate counting/detecting, and to avoid the undeterminable evaporative losses, preferably essentially the entire sample volume of sample gas particles contained in the expansion chamber 108 should be detected before the evaporation occurs. Therefore the expansion chamber volume is preferably chosen to be such that it is fully evacuated in around 1 s, as occurs with the exemplary chamber and situation described above.

Further, the expansion chamber 108 may preferably be designed or adapted such that once the detecting occurs, the entire expansion chamber 108 is evacuated of the old sample prior the new air or sample gas flowing through it. This is opposed to the case where new air or sample gas flows through the chamber while some of the old sample gas still remains, as this will affect the determined particle count.

The expansion chamber 108 may also advantageously be such that the sample gas flow during the evacuation of the expansion chamber 108 is laminar, in order to minimize the losses of the activated sample particles, which have a tendency of colliding and being lost on the walls of the expansion chamber in turbulent flows. Laminarity may also aid in the controlled evacuation process.

Understanding the limitations considered above and adapting an expansion chamber 108 based on these design limitations, yet also preferably considering a tolerance of 1 - 2 s to observe the activated particles (before evaporation, i.e. the expansion chamber 108 should be evacuated e.g. in under 2 - 3 s or preferably in under 1 s or under 2 s or in order to be able to observe the particles at the detector 118), may be key considerations in adapting the expansion chamber 108 to exhibit selected flow field.

The present invention may in one embodiment differ from and improve on the prior art by utilizing and preferably lengthening or maximizing this tolerance time or activation window, in order to channel/direct the activated particles to a single particle detector 118 from the expansion chamber 108. In prior work, the particles are detected only in the expansion chamber. There, the activation of particles is produced, maintained for an extended time, the particles are detected, and an integrated response is measured over the entire sample containing all activated particles that are present in the expansion chamber, which are discarded after the detection. This is opposed to the methodology presented here, where the detection of the activated sample particles begins at the stage where in the previous approaches the sample was discarded.

In prior art evaporation-based instrumentation, a sample chamber used may be roughly of similar size as is the expansion chamber 108 utilized in embodiments of the present invention. However, some crucial differences may be found. One key difference may be in the geometry of the chamber, which in the prior art is an empty cylinder, as opposed to here, where there may be a diamond shaped flow guide in the inlet side of the expansion chamber 108. Because the prior measurement method of recording the integrated response of the aerosol sample, there could not be any flow guides present in the chamber. This restriction also means that the measurements with this type of method will inherently be slower, as the complete evacuation or flushing of a chamber is a relatively slow process as compared to an expansion chamber 108 that is advantageously adapted to exhibit a selected flow field. Also, the design of the expansion chamber 108 differs from the sample chamber of the prior art in the sense that for the prior art sample chambers, also those utilized in expansion type particle counters, an optical signal must be passed through the (expansion) chamber, which limits the geometry. Here, there is no such limitation, allowing for more complex geometries of expansion chambers 108 and thereby making possible a more comprehensive selection of expansion chamber to exhibit selected flow field.

In one embodiment, a sloped/tapered inlet or first end portion of an expansion chamber 108 and/or tapered outlet or second end portion of an expansion chamber may be provided, where flow guides may optionally be utilized.

Figure 6 illustrates an exemplary selected geometry of an expansion chamber 108 that may be utilized in one embodiment of the invention. The expansion chamber 108 of Fig. 6 has a tapered first end portion I of the expansion chamber having expanding cone-like/funnel shape. A first angle a is exhibited by the first end portion I. The expansion chamber 108 may have a homogenous portion II that is provided between the first end portion I and a second end portion II. The second end portion III may also advantageously be tapered to have a narrowing cone-like shape.

In an embodiment, the geometry of an expansion chamber 108 at the inlet side to the expansion chamber (the first end portion I of the expansion chamber) may be tapered/sloped and comprise an expanding cone-like shape and the shape may essentially may be limited by two factors. First it may preferably be such that the incoming flow spreads equally throughout the expansion chamber 108, and therefore the presence of a flow guide may be beneficial. Second, the first end portion I of the expansion chamber 108 is preferably such that the sample flow remains essentially laminar and there is substantially no flow separation, as defined by the Reynold number. In practice for an instrument with 1 Ipm flow, and the customary 4 mm inner channel size, this means that a first angle a for the expanding cone-like shape (angle between a central axis of the expansion chamber 108 and the wall of the expansion chamber at the cone-like shape portion) should be less than 70 degrees, preferably less than 45 degrees, even more preferably less than 30 degrees. Yet if the first angle a or slope exhibited by the first end portion I is too small (e.g. smaller than 5 degrees), excessive particle losses and an increase in difficulty in the expansion process may follow. It is advantageous that the volume that is experienced by the particles in the sample flow is seen as a large, homogenous volume. The volume experienced by the particles may be affected by the first angle a.

In one embodiment, the geometry on the expansion chamber outlet side (the second end portion III of the expansion chamber 108) may exhibit a narrowing cone-like shape such as in Fig. 6. The second angle b for the narrowing cone- like shape can in one embodiment advantageously be less than the first angle a for the expanding cone-like shape on the inlet side (first end portion I of the expansion chamber). This is due to the activated particles in the expansion chamber 108 having higher inertia, and are therefore easily being lost on sharp bends. For good design practices perspective, second angles b less than e.g. 20 degrees may be used. However, the exact first and second angles a and b may be dependent on the total flow, the shape of the flow guides, and/or the ratio of expansion chamber 108 volume to the volumetric flow of the apparatus. The second angle b also should not be too shallow, below 10 degrees or so, as this means that the evacuation time will be increased, and the wall effects during this second end portion III will increase, enhancing the evaporation of the particles prior to detection. Yet, first and second angles could also be essentially equivalent or close to each other.

The narrowing cone-like shape of the second end portion III of the expansion chamber 108 may be advantageous because the flow of activated particles may be efficiently provided into a smaller volume and the flow of particles to the detector 118 (possibly through orifice or nozzle 120) may be efficient.

In preferable embodiments, the homogenous portion II of the expansion chamber 108 is preferably cylinder-like, which radius is a function of the total flow of the apparatus. The radius should be such, that the average flow velocity in the homogenous portion II of the chamber should be between 3 - 10 cm/s. This assumes that the expansion chamber 108 can be evacuated in around 1 second (which in some embodiments may be a critical assumption, as the lifetime of activated particles in subsaturated conditions can be limited to 1-2 s). If a desired expansion chamber volume is achieved with a larger flow to radius ratio, it means that the wall effects are beginning to distort the homogeneity of the saturation field and the size selectivity of the apparatus may be lost. Conversely, if the ratio is much smaller than this it means that the narrowing cone-like shape or outlet funnel may have too steep of a second angle b, and substantial particle losses may occur, and thus lessening the ability of the apparatus to observe/detect an exact particle number concentration.

A fast valve may in this description refer to a valve that may be opened and/or closed in under 500 ms, preferably under 300 ms, more preferably under 100 ms, and most preferably under 10 ms. The invention has been explained above with reference to the aforementioned embodiments, and several advantages of the invention have been demonstrated. It is clear that the invention is not only restricted to these embodiments but comprises all possible embodiments within the spirit and scope of inventive thought and the following patent claims.

The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated.

Claims

1. A method for detecting particles, the method comprising at least

- providing a sample flow comprising particles,

- providing the sample flow with a saturating agent, - directing at least a portion of said sample flow to an expansion chamber,

- sealing at least the expansion chamber from the environment,

- increasing a pressure inside at least the expansion chamber from a first pressure to a second, higher pressure, - decreasing the pressure inside at least the expansion chamber from the second pressure to the first pressure to produce activated particles by inducing supersaturation of the saturating agent and thus condensation of the saturating agent onto the particles of the sample flow to increase the size of selected particles from a first size to a second size,

- unsealing at least the expansion chamber,

- directing the sample flow comprising the activated particles of second size from the expansion chamber to a detector, and

- detecting at least an amount of single activated particles in the sample flow.

2. The method of claim 1, wherein the detecting an amount of single activated particles comprises detecting a number of particles with a single-particle counter, preferably through an optical measurement.

3. The method of claim 2, wherein the detector comprises a detection chamber and a nozzle through which the activated particles are directed essentially one at a time to achieve a flow of single activated particles into the detection chamber, wherein the detecting of the number of particles is conducted by observing scattering of light directed at the flow of single particles.

4. The method of any previous claim, wherein an indication of absence of activated particles is obtained if the detected number of particles is essentially zero.

5. The method of any previous claims, wherein by controlling the difference between the first and second pressure, the level of supersaturation is controlled to control the second size of the particles, and preferably wherein the second pressure is between 0-50 kPa larger than the first pressure.

6. The method of any previous claims, wherein the directing of at least a portion of the sample flow to the expansion chamber is done through a first valve, wherein the first valve is a valve that provides an essentially unobstructed flow path for the particles, such as a valve selected from the group of a pinch valve, a ball valve, and a butterfly valve.

7. The method of any previous claims, wherein providing the sample flow with a saturating agent comprises directing the sample flow through a saturator or directing a portion of the sample flow through a saturator for providing a sheathing flow for the remaining sample flow that has not been directed through the saturator.

8. The method of any previous claims, wherein the method additionally comprises providing a by-pass-flow of the sample flow that bypasses the expansion chamber and the detector.

9. The method of any previous claims, wherein the pressure is increased by directing a gas into the expansion chamber through a third valve and wherein the pressure is decreased by opening of a fast valve and preferably additionally using an additional vacuum.

10. The method of any previous claims, wherein the method comprises increasing and decreasing of pressure utilizing a piston arrangement.

11. The method of any previous claims, wherein the method comprises repeating at least part of the steps and varying the second pressure to obtain activated particles with different initial size to detect a size distribution of the particles comprised in the sample flow.

12. The method of any previous claims, wherein the method comprises controlling the temperature of the sample flow and/or the temperature of the detector.

13. The method of any previous claims, wherein the method comprises controlling the temperature of the sample flow inside the expansion chamber so that the temperature of the sample flow inside the expansion chamber is essentially equivalent to the condensation point of the sample flow.

14. The method of any previous claims, wherein the expansion chamber is adapted to exhibit a selected flow field.

15. The method of claim 14, wherein the selected flow field is obtained by selecting a geometry that provides selected expansion chamber evacuation time and/or selected flow velocity of particles in the expansion chamber and/or a laminar flow of the sample flow into and/or out of the expansion chamber, optionally through modeling a plurality of expansion chamber geometries.

16. The method of any of claims 14-15, wherein the expansion chamber is adapted to exhibit a selected flow field through providing one or more tapers and/or flow guides in the expansion chamber geometry.

17. The method of any previous claims, wherein the method comprises sealing at least the expansion chamber and the detector from the environment.

18. An apparatus for detecting particles, the apparatus comprising at least

- a first inlet for receiving a sample flow comprising particles,

- a saturator for providing a saturating agent to the sample flow,

- an expansion chamber for receiving the sample flow,

- a sealing arrangement for reversible sealing of at least the expansion chamber from the environment,

- a pressure increasing arrangement for increasing pressure inside at least the expansion chamber from a first pressure to a second pressure,

- a pressure decreasing arrangement for decreasing the pressure inside at least the expansion chamber from the second pressure to the first pressure to produce activated particles by inducing condensation of the saturating agent onto the particles of the sample flow to increase the size of selected particles from a first size to a second size,

- a detector for detecting at least an amount of single activated particles in the sample flow, and - a transfer member between the expansion chamber and the detector for directing the activated particles from the expansion chamber to the detector.

19. The apparatus of claim 18, wherein the pressure increasing arrangement is provided through directing pressurized air into a conduit into which the saturated sample flow is transferred, optionally via a third valve.

20. The apparatus of claim 18 or 19, wherein the pressure decreasing arrangement is provided through opening of a first valve that is provided between the saturator and expansion chamber and opening of a second valve that is provided after an outlet of the detector, optionally wherein a vacuum is utilized.

21. The apparatus of claim 18, wherein the pressure increasing arrangement and pressure decreasing arrangement is provided through a piston arrangement.

22. The apparatus of any of claims 18-21 , wherein the expansion chamber comprises a taper at a first end portion of the expansion chamber, wherein the taper comprises an expanding cone-like shape.

23. The apparatus of any of claims 18-22, wherein the expansion chamber comprises a taper at a second end portion of the expansion chamber, wherein the taper comprises a narrowing cone-like shape.

24. The apparatus of any of claims 18-23, wherein the detector is an optical single-particle counter.

25. The apparatus of any of claims 18-24, wherein the sealing arrangement seals at least the expansion chamber and the detector from the environment.

26. A method for manufacturing an apparatus for detecting particles, the method comprising at least

- providing a first inlet for receiving a sample flow comprising particles,

- providing a saturator for providing a saturating agent to the sample flow,

- providing an expansion chamber for receiving the sample flow, - providing a sealing arrangement for reversible sealing of at least the expansion chamber from the environment,

- providing a pressure increasing arrangement for increasing pressure inside at least the expansion chamber from a first pressure to a second pressure,

- providing a pressure decreasing arrangement for decreasing the pressure inside at least the expansion chamber from the second pressure to the first pressure to produce activated particles by inducing condensation of the saturating agent onto the particles of the sample flow to increase the size of selected particles from a first size to a second size,

- providing a detector for detecting at least an amount of single activated particles in the sample flow, and

- providing a transfer member between the expansion chamber and the detector for directing the activated particles from the expansion chamber to the detector.