GB2583765A

GB2583765A - Filter unit for an opitcal particle counter

Info

Publication number: GB2583765A
Application number: GB1906627.3A
Authority: GB
Inventors: Henry Kaye Paul; Roy Stanley Warren; Harry Scott Girdwood Joseph
Original assignee: Alphasense Ltd
Current assignee: Alphasense Ltd
Priority date: 2019-05-10
Filing date: 2019-05-10
Publication date: 2020-11-11
Also published as: GB201906627D0

Abstract

A filter unit 1 for demountable fitting at the inlet of an optical particle counter (OPC), wherein the filter comprises a central conduit 3 for supplying a central sample flow 11 to the counter, and a filtering portion for supplying a filtered sheath flow 13 surrounding the central sample flow. The filter unit may comprise a sheath flow conduit 5, which may be tapered and extend further in the direction of intended gas flow than the central conduit, for supply of the filtered sheath flow to the counter. In normal operation, the filtering portion can comprise a filter 9 which may be annular, located around the central conduit, and which offers low flow impedance. The filter may be housed within a housing 7, comprising a base and separable cap portion to allow replacement of the filter. The cap is preferably impermeable and the base portion allows ingress of atmospheric gas, to provide the filtered sheath. An optical particle counter comprising the filter unit is disclosed wherein a fan may be used to draw air through the inlet. A method of using the filter unit and a method of supplying sample gas to an OPC are also described.

Description

1 Filter Unit for an Optical Particle Counter

3 Field of the invention

The invention relates to the field of optical particle detection. Specifically, the 6 invention relates to a filter unit for fitting to the inlet of an optical particle counter.

8 Background to the invention

Optical particle counters (OPCs) are known in the art which detect the passage of 11 particles through a beam of light (typically from a laser). Typically, a narrow column of 12 air is passed through a light beam and the column of air is sufficiently narrow that the 13 volume within which particles are detected and counted, referred to as the particle 14 detection zone, will only rarely contain more than a single particle. The pulse of light which is scattered by each respective particle is detected and the magnitude of each 16 pulse of light is used to estimate the size of the particle which has been detected.

17 Accordingly, the number of particles within various size ranges can be determined 18 and the concentration of those particles in an air sample can be determined from the 19 known velocity of the column of air through the light beam.

21 One such OPC is known from WO 2012 056217. There is provided an apparatus for 22 the detection of a fluid-borne particle in an optically defined particle detection zone, 23 the apparatus comprising a scattering chamber; a means for providing a sample of 24 fluid, containing the fluid-borne particle, in the form of a flow through the optically defined particle detection zone; a means for generating a beam of radiation through 1 the optically defined particle detection zone; a single reflector or refractor means 2 having a primary focus within the optically defined particle detection zone and a 3 secondary focus located outside the beam of radiation; a detector means comprising 4 a first photosensitive detection area and a second photosensitive detection area; a means for deriving data from the radiation detected by the first photosensitive 6 detection area and second photosensitive detection area of the detection means 7 wherein the single reflector or refractor means is adapted to direct radiation scattered 8 from the fluid borne particle passing through the beam of radiation within the optically 9 defined particle detection zone to the detection means located at the secondary focus of the single reflector or refractor means.

12 These known OPCs suffer from the problem that if operated in a heavily polluted 13 environment the sensitive optical surfaces and elements present in the counter can 14 become contaminated. Similar damage can also occur following extended use of an OPC in a lightly polluted environment. Friction between the sample flow and the 16 stationary chamber air causes some of the outer portion of the sample flow to 17 become entrained into the chamber air and begin circulating inside the chamber.

18 Fugitive particles carried in this entrained flow may slow down and become deposited 19 on the internal surfaces of the chamber, including the optical surfaces. This reduces the optical efficiency of the optical surfaces and this ultimately results in an under- 21 sizing of particles that are being measured by the OPC. Electrical fans which are 22 sometimes used in such devices are also subject to damage through such 23 contamination. Contamination of optical surfaces and elements leads to a 24 deterioration in the performance of the counter and to costly maintenance and cleaning in order to put the device to good again. A damaged fan can also lead to a 26 deterioration in the functioning of the device through unreliable sampling.

28 The present invention addresses these problems which arise particularly in heavily 29 polluted environments or in connection with extended use of an OPC in a polluted environment.

32 Summary of the invention

34 According to a first aspect of the invention, there is provided a filter unit for demountable fitting at the inlet of an optical particle counter, said filter unit comprising 36 a central conduit for supplying a central sample flow to the counter, and a filtering 1 portion for supplying a filtered sheath flow surrounding said central sample flow. The 2 sample flow is central to the filtered sheath flow.

4 The central sample flow, being the gas to be measured, is typically an aerosol, but it may be any gas containing particles. The central sample flow and the filtered sheath 6 flow pass into the inlet of the optical particle counter in use. The flow gas (comprising 7 the central sample flow and the filtered sheath flow) may be drawn by a fan with the 8 optical particle counter in use.

Particles present in the central sample flow are constrained by the filtered sheath flow 11 of clean air to remain within that sheath flow as the sample passes through the optical 12 particle counter chamber. In this way, the particles are prevented from depositing 13 themselves on optical surfaces, which are located around the cylindrical periphery of 14 the optical particle counter chamber. The optical surfaces are thus protected from degradation. Due to the optical nature of the particle detection, the measurement of 16 the particles is not affected by the presence of the filtered sheath flow. The operation 17 of the optical particle counter in, for example, a heavily polluted environment is 18 protected, while its performance is maintained.

The gas entering the filtering portion may come from the ambient gas to be measured 21 by the optical particle counter, or it may come from a separate source, for example a 22 cylinder of clean compressed gas.

24 The filtered sheath flow of the filter unit may be produced by a sheath flow conduit arranged around, typically coaxial with, the central conduit. The sheath flow conduit 26 may be shorter than the central conduit, or it may be longer, or it may be substantially 27 equal in length.

29 The sheath flow conduit may extend further in the direction of intended gas flow (the direction of gas flow in use) than the central conduit, and may do so with a tapering 31 cross-section. In this possible configuration, the clean sheath-flow air not only 32 surrounds the sample air flow joining from above, but also constrains the sample flow 33 radius down in diameter. The sheath flow conduit may have a tapered cross-section 34 which narrows as the sheath gas egresses the filter unit. This can have the effect of constraining the diameter of the sample flow entering the OPC. As an example, the 36 sample flow may be constrained from a diameter of,1.2mm to a diameter of -.0.6mm 37 as it passes through the optical particle zone (e.g. laser beam or region thereof) of 1 the optical particle counter. The sample flow radius may be reduced at least 10% or 2 at least 25% from where it exits the central conduit to the particle detection zone (e.g. 3 a laser beam, or region thereof) of the optical particle counter. In this way, more 4 uniform illumination of particles can be ensured and correspondingly better determination of particles sizing.

7 For laminar flow the velocity of the central sample flow and that of the sheath flow 8 should be substantially equal at the point where they initially meet. With the filter unit 9 in its normal operational configuration (not necessarily mounted on the OPC), the filtering portion of the filter unit may comprise a filter. The filter may be replaceable.

11 The allows the filter to be replaced after it becomes clogged with deposited aerosol 12 particles. The filter unit itself may be replaceable. The filter unit enables an OPC 13 which is not configured to provide a filtered sheath flow to be operated with a filtered 14 sheath flow around the central sample flow.

16 A filter unit may be configured in such a way that the filter is located (e.g. entirely) 17 around the central conduit. The central conduit may thereby pass through an 18 aperture in the filter. This serves to ensure that the sheath flow (e.g. entirely) 19 surrounds the central sample flow.

21 The filter may be of annular form, possibly of cylindrical symmetry. Such an 22 arrangement helps to ensure that the sheath flow is uniform around the central 23 sample flow.

The filter unit is particularly suitable for use with OPCs having a so-called "open path" 26 design, i.e. one with no narrow delivery tubes to constrain the particles to pass 27 through the particle detection zone as is usual in the art. Such a device has a particle 28 detection chamber through which gas flows, which is typically formed by a tube, with 29 an optically-defined particle detection zone (where particles are detected, which detected particle are counted) which is within the particle detection chamber (e.g. 31 bore of the tube) with a gap between the particle detection zone and the inner wall of 32 the particle detection chamber (e.g. inner wall of the tube), all around the particle 33 detection zone. Typically, the cross-sectional area of the particle detection zone (i.e. 34 perpendicular to the direction of gas flow in use) is at most 50% of the cross-sectional area of the particle detection chamber (e.g. bore of a tube) through the particle 36 detection zone.

1 Thus, particles in the sample flow of an open-path OPC can pass through the 2 (optically defined) particle detection zone or around it, allowing for much wider flows 3 and therefore much lower flow impedances. This enables a small fan (or even no 4 fan) to generate the flow, but it also results in the possibility of the release into the chamber of fugitive particles. The filter unit of the present invention serves to restrict 6 this release of fugitive particles. It is not necessary to eliminate all fugitive particles 7 from the chamber, just the majority of them. A filter with 80% capture efficiency 8 would give the OPC a five-fold increase in run-time before chamber contamination 9 became a problem.

11 It is preferable that the filter is one which offers low flow impedance. A correct 12 functioning of the filter unit requires that the flow impedance for the sample air 13 passing through the central conduit into the optical particle counter is similar in 14 magnitude to that experienced by the flow of air through the filter, i.e. that air which becomes the sheath flow. If the impedance of the filter is too high with respect to that 16 of the central conduit, the sheath will be too thin and will not efficiently prevent 17 particles from the sample flow escaping into the OPC chamber. If the filter 18 impedance is too high, then the total flow into the OPC will be dominated by the 19 sheath flow, and particle counting rates will be reduced. The flow impedance of the filtering portion shall thus be such that the filtered sheath flow effectively prevents 21 particles from the sample flow escaping into the OPC chamber. The flow impedance 22 of the filtering portion shall also be such that the presence of the filtered sheath flow 23 does not significantly reduce the particle counting rates of the OPC.

The flow impedance affects the pressure drop and so, typically, the filter unit will be 26 confined so that the pressure drop through the central conduit in use will be within 27 50%, or 25% or 10% of the pressure drop through the filtering portion in use.

29 The person skilled in the art understands that flow impedance for the air passing through the filter depends on the properties of the filter. One important property of a 31 filter is the material from which it is made. Examples of suitable filter material include 32 needlefelt. Needlefelt is a filter material made from plastic fibres of typically 33 polypropylene, polyester, polyamide. Further similar materials are possible. For 34 example, another possible filter material is TEMISH (®), a fluoroplastic based non-woven fabric material made by the Nitto Denko Corporation (Osaka, Japan).

1 The flow impedance of the filter is also influenced by the physical dimensions of the 2 filter. The person skilled in the art is aware that the flow impedance of the filter varies 3 in inverse proportion to the cross-sectional area presented to the gas flow.

Having a good understanding of these parameters upon which the flow impedance of 6 the filtering portion depends, the person skilled in the art would be able to construct 7 the presently disclosed filter unit such that it functions in the manner described herein 8 without any difficulty. The proportions of the central conduit, and those of the sheath 9 flow conduit shall be such as to produce the laminar gas flow which the person skilled in the art will understand is necessary for the correct operation of the filter unit. The 11 corresponding dimensions can be arrived at without undue experimentation on the 12 part of the person skilled in the art. Furthermore, it is well within the capabilities of 13 the person skilled in the art to engineer the filter unit to achieve a sample flow of 14 desired diameter in the particle detection zone of the OPC.

16 In a preferred arrangement, the filter is housed within a housing comprising a base 17 upon which the filter is located and, mechanically engageable with said base, a cap 18 portion, wherein said cap portion is separable from said base. Such an arrangement 19 allows easy and quick replacement of the filter when this becomes necessary due to clogging through accumulation of captured particles.

22 It is also conceivable that the cap portion be impermeable and that the base portion 23 be configured to allow the ingress of gas from the atmosphere external to the filter 24 unit, which gas is filtered by the filter to provide the filtered sheath flow. This geometry could facilitate use of separate sources for the sample flow and for the 26 sheath flow which the person skilled in the art might find useful depending on 27 circumstances.

29 Further facilitation of the possibility to use separate sources can come from a filter unit geometry in which the central sample flow enters the filter unit at a first filter unit 31 side and exits the filter unit at a second filter unit side, which is typically opposite the 32 first filter side. and wherein said base portion is at the said second filter unit side.

33 This geometry has the further advantage that it would prevent rain, for example, from 34 entering the filter unit region. Thus, the OPC could be used in, for example, an outdoor environment where rain is a possibility.

1 The filter unit features are configured such that the flow impedance for gas forming 2 the sheath flow and the flow impedance for gas forming the central sample flow are 3 similar in magnitude. Typically, the sheath flow should be approximately equal to the 4 central sample flow, although an acceptable range of sheath flow is typically from half the value of the central sample flow, to four times the value of the central sample flow 6 (expressed in dimensions of volume per unit time). In an example, the sample and 7 sheath flows would be 1 and 3 litres/minute respectively. The sample flow and 8 sheath flow should be laminar.

The filter may sit on a base, and the filter housing may create a cavity space between 11 the inner-side of the filter and the entrance to the sheath conduit. In such an 12 arrangement, the cavity space maximizes exposure of the surface area of the filter, 13 thus minimizing flow resistance. In another option, the filter may occupy the whole of 14 the internal space of filter housing.

16 Typically, the filtered sheath flow of the filter unit serves to encourage flow of the 17 central sample flow, by creating a decrease in fluid pressure around the central flow 18 according to Bernoulli's principle.

For reliable operation of the OPC, it is generally preferable that the filter unit of the 21 present disclosure is arranged such that the diameter of the sample flow is wider than 22 the optically defined particle detection zone of the OPC. For example, an inlet tube 23 diameter of 2-3mm could be appropriate for a detection zone diameter of P-1.2mm if 24 the filter impedance and the central conduit impedance are comparable.

26 A second aspect of the invention provides an optical particle counter comprising a 27 particle detection chamber (which may be formed from a tube), an optically defined 28 particle detection zone within the particle detection chamber, and an entrance inlet for 29 receiving gas (to be sampled) into the optical particle counter and thereby the particle detection chamber. There is further provided a filter unit according to the first aspect 31 of the invention. The filter unit may be provided in a kit with the optical particle 32 counter. The filter unit may be demountably attached to the entrance inlet of the 33 optical particle counter. The filter unit provides the central sample flow and the 34 filtered sheath flow. The sample flow passes through the particle detection zone in use.

1 It may be that the optical particle counter comprises a tube, the particle detection 2 chamber is a region of the tube and the said optically defined particle detection zone 3 (where particles are detected, which detected particle are counted) is located within 4 the tube.

6 The optically-defined particle detection zone is within the particle detection chamber 7 (e.g. bore of the tube) with a gap between the edge of the particle detection zone and 8 the inner wall of the particle detection chamber (e.g. inner wall of the tube), all around 9 the particle detection zone. Typically, the cross-sectional area of the particle detection zone (i.e. perpendicular to the direction of gas flow in use) is at most 50% of 11 the cross-sectional area of the particle detection chamber (e.g. bore of a tube) 12 through the particle detection zone.

14 The filter unit may be demountably attached to the optical particle counter.

16 The optical particle counter unit may comprise a tube, wherein the particle detection 17 chamber is formed within the tube and there is a gap between the particle detection 18 zone and the inner wall of the tube.

Typically the diameter of the sample flow is wider than the optically defined particle 21 detection zone of the optical particle counter.

23 The optical particle counter may additionally comprise a fan to draw air through the 24 entrance inlet and into the particle detection chamber.

26 The invention extends in a third aspect of the invention to the use of a filter unit 27 according to the first aspect in conjunction with an optical particle counter.

29 The filter unit may be mounted at the entrance inlet of an optical particle counter to provide a filtered sheath for gas whose particle content is to be counted.

32 The invention extends in a fourth aspect to a method of supplying sample gas to an 33 optical particle counter wherein the sample gas is supplied through a (typically 34 central) conduit of a filter unit, forming a sample gas flow, and wherein additionally ambient gas passes through a filtering portion of the filter unit, which filtering portion 36 surrounds said central conduit, thereby forming a filtered sheath flow which sheaths 37 said sample gas flow as it passes through the optical particle counter. The optical 1 particle counter is typically an optical particle counter according to the second aspect 2 of the invention. The sample gas flow, which has passed through the conduit of the 3 filter unit, passes through the optically defined particle detection zone. Particles in 4 the sample gas flow which pass through the optically defined particle detection zone are detected and counted. The filtered sheath flow around the sample gas flow also 6 passes through the particle detection chamber and may or may not pass through 7 (typically the periphery of) the particle detection zone.

9 It may be that in use the filter unit is removed from the optical particle counter and replaced with a filter unit comprising a fresh filter.

12 Features described in respect of any aspect of the invention are optional features of 13 each aspect of the invention.

Description of the Drawings

17 Example embodiments of the present invention will now be illustrated with reference 18 to the following Figures in which: Figure 1 shows a schematic diagram of a known optical particle counter; 22 Figure 2 shows a schematic diagram of (a) a first embodiment and (b) a second 23 embodiment of the filter unit in cross-section, showing the manner of function.

Figure 3(a) shows an exploded view of the filter unit of the first embodiment. An 26 annular filter is housed in a housing comprising a base and cap portion. Figure 3(b) 27 shows the filter unit with the base and cap portions engaged; and 29 Figure 4 illustrates a third embodiment of the filter unit.

31 Detailed Description of Example Embodiments

33 The present invention is conceived to be used in conjunction with an optical particle 34 counter (OPC), particularly one with an optically defined detection region and not a thin tube. An example of such an OPC is schematically shown in Fig. 1 (WO 36 2012/056217). Such particle counters take in sample gas from, typically, particle 37 laden ambient air. With an eye to Fig. 1, the sample gas enters the optical particle 1 counter apparatus and flows through scattering chamber, where it passes through 2 laser beam (12). Particles (10) present in the sample gas are illuminated by the laser 3 beam (12). Laser light scattered by the particles is collected by an elliptical mirror 4 (14), from where it is focussed onto a photodetector (24) for detection.

6 The air to be sampled can be drawn into the counter by using a fan (34), typically a 7 small low-power electrical fan (not illustrated). The presence of a fan is optional 8 however and other alternatives exist. For example, air flow can be generated simply 9 by an operator waving the counter around, thereby creating a flow through the scattering chamber.

12 It can often be of interest to monitor heavily polluted environments with an OPC.

13 However, measuring such environments can have a detrimental effect on the 14 measuring device, particularly arising from particle induced contamination of the optical surfaces. A possibly lesser, but nevertheless still significant, problem arising 16 from monitoring heavily polluted environments is that the fan can become affected by 17 contamination, leading to operational problems and problems with memorised 18 contamination.

First Embodiment 22 Figure 2(a) illustrates a first embodiment of the filter unit (1) forming the subject of 23 this invention in cross-section. The filter unit is shown mounted on the gas inlet of an 24 optical particle counter (OPC) of the type referred to above.

26 The filter unit (1) comprises a main central conduit (3), coaxially surrounded by a 27 second sheath conduit (5) of annular cross-section. When the filter unit is attached to 28 the OPC, the gas from each of the central conduit (3) and the sheath conduit (5) 29 egresses the filter unit (1) directly into the OPC scattering chamber. This is illustrated in Fig. 2(a).

32 The gas ingress of the sheath conduit (5) is located within a housing (7), serving to 33 house a filter (9). The filter (9) generally has a form with a hole in the middle through 34 which the central conduit (3) passes. In this embodiment, the filter (9) is of annular form and the central conduit (3) passes through the inner circle of the annular filter.

36 The filter is situated within the path of flow of air (gas) which becomes the sheath flow 37 (13). Thus the filter cleans the air prior to it ingressing the sheath conduit (5).

2 In the present embodiment, the unit housing (7) the filter (9) comprises a gas 3 impermeable cap portion (17) which engages with a base (19) upon which the filter 4 (9) is located in the normal operational set up. In the embodiment illustrated in Fig. 2(a), the filter (9) is shown sitting on base (19), with the filter housing (7) creating a 6 cavity space between the inner-side of the filter and the entrance to the sheath 7 conduit (5). The cavity space maximizes exposure of the surface area of the filter, 8 thus minimizing flow resistance. In another option, the filter (9) can occupy the whole 9 of the internal space of filter housing (7). The cap portion (17) and the base (19) are separable, allowing the housing to be opened, and the filter accessed and replaced.

11 Fig. 3 illustrates the housing unit in an exploded view (Fig. 3(a)) and in an assembled 12 view (Fig. 3(b)).

14 In the embodiment illustrated in Fig. 2(a), the gas enters the filter housing through the base portion, i.e. from the side of the housing facing the OPC. Other arrangements 16 are easily conceivable. One example of a variant is shown in the second 17 embodiment below.

19 The filter unit (1) exists as a separate unit and is arranged to be fitted, in a demountable manner, to the entrance port of an OPC. Typically, the filter unit (1) is 21 configured to form a friction fit over the OPC entrance inlet.

23 Once the filter unit is in place on the OPC, main central conduit (3) serves to pass 24 gas from the environment being tested (21) into the OPC where it is measured (see Fig. 2(a)). The ambient aerosol (21) to be measured (otherwise called the particle 26 laden ambient air) can, for example, be drawn into the central conduit (3) and thus 27 into the OPC using a fan (not illustrated).

29 As the ambient aerosol enters the OPC it is provided with a flowing sheath of clean air (13). This clean sheath flow (13) originates from air which passes through filter 31 (9), where it is cleaned of particles. The air thus cleaned then enters the sheath 32 conduit (5) where it is formed into a (typically concentric) laminar flow which sheaths 33 the central sample flow (11) upon exit from the filter unit (see Fig. 2(a)).

The clean sheath flow (13) constrains aerosol particles present in the central sample 36 flow (11) to remain in that sheath flow as the sample passes through the optical 37 particle counter chamber. In this way, the particles are prevented from depositing 1 themselves on optical surfaces (14, 24), which are located around the cylindrical 2 periphery of the optical particle counter chamber. The optical surfaces (14, 24) are 3 thus protected from degradation. This is particularly important if measurements are 4 being made in an environment which is heavily polluted with aerosol particles, for example. The measurement of the aerosol is an optical one based on the scattering 6 of light from the particles, the presence of the clean sheath flow (13) does not affect 7 the measurement made by the OPC as there is no interaction between the laser 8 beam (12) and the sheath flow (13). The particle detection zone is a part of the laser 9 beam and is defined by the optcrical elements, typically by a parabolic reflector, a detector and an aperture. Accordingly, the particle detection zone is an optically 11 defined particle detection zone. Preferably, the particle detection zone is centrally 12 located within the region of the scattering chamber through which the gas sample 13 flows in use.

In order that the filter unit (1) provides the desired function, i.e. protection of the OPC 16 optics with simultaneous non-interference in the measurement, the clean sheath flow 17 should be neither too little, nor too much. If the clean sheath flow is too little, the 18 sheath will be too thin to fulfil its purpose and will simply not reliably prevent particles 19 from the sample flow escaping into the OPC scattering chamber. If the sheath flow is too high, then the majority of the flow through the OPC will be in the sheath flow and 21 particle count rates will be reduced. Ideally, the sample flow (11) and the sheath flow 22 (13) should be broadly similar in magnitude for reliable operation. Increased sheath 23 flow, relative to the central sample flow, has the effect of squeezing and thus 24 reducing the diameter of the sample flow it surrounds.

26 For a given strength of suction generated by the fan, the sample flow (11) depends 27 on the cross section of the central conduit (3). The sheath flow (13) is dependent on 28 the (annular) cross-section of the sheath flow conduit (5) and also the flow resistance 29 of the filter (9). The flow resistance of any filter material is inversely proportional to the surface area of the filter exposed to the incoming air. The person skilled in the art 31 is well aware of all the parameters affecting flow resistance, and it is well within their 32 capabilities to select conduit and filter dimensions which ensure the broad flow 33 equivalence condition referred to above.

Another aspect which impacts the dimensions of the filter unit is the material making 36 up the filter. In general terms, a filter material with a low flow impedance is preferred.

37 One filter material which has been successfully employed is needlefelt. Needlefelt is 1 a filter material made from plastic fibres, of which polypropylene, polyester and 2 polyimide are typical examples. It has suitably good particle retention properties 3 (typically close to 100% for particles >1um and >50% efficiency for particles 0.5um to 4 1 um) but it achieves this with very low flow impedance. One supplier of needlefelt is Andrews Webron Ltd (Altham, UK).

7 Other suitable materials offering a combination of good particle retention and low flow 8 impedance are also available. For example, TEMISH NTF9000 Series from Nitto 9 Denke Corporation (Osaka, Japan) is a PTFE based material offering suitable filtering properties.

12 For successful operation of the OPC the sample flow diameter through the OPC 13 needs to fully enclose the virtual detection zone used to establish the measurement 14 space (the optically defined particle detection zone). To a large extent, this sample flow diameter is determined by the cross-section of the central conduit. Thus, the 16 dimension which tends to dictate all the other parameters of the filter unit is the cross- 17 section of the central conduit. The virtual detection zone in a typical OPC has a 18 diameter of approximately 1.2 mm, more generally 0.8 to 1.5 mm. For such a virtual 19 detection zone diameter it has been found that a central conduit inner diameter of approximately 2-3mm would suffice as this would result in a wide enough sample flow 21 diameter at the detection zone providing the filter impedance was comparable to that 22 of the inlet tube. A corresponding typical sheath flow conduit diameter would be 23 approximately 4-6mm. The dimensions are not critical, but as a rule of thumb the 24 diameter ratio between the central (sample) flow and the outer sheath flow would be of the order of 2:1. The diameter of the central flow has to be sufficient to cover all of 26 the cross-sectional area of the particle detection zone where the flows are intersected 27 by the laser beam; the diameter of the outer flow has to be such that the 'sheath' of 28 clean air surrounding the central particle-laden flow is a thick enough barrier to 29 prevent particles transgressing from the central flow into the OPC chamber. Both the central flow and the sheath flow are ideally laminar, so will not mix, but the outer 31 surface of the sheath flow will be in contact with the relatively static air within the 32 OPC chamber so some of the sheath air will be entrained (i.e. dragged away) from 33 the sheath flow and therefore pass into the chamber. This has no detrimental effect 34 as the sheath flow is particle free, but had the sheath flow not been present, the same thing would have happened to the central 'particle' flow and that is what causes 36 particulate contamination in the chamber and ultimately of the optical surfaces within 1 the chamber. Typical examples of sample and sheath flows are 1 and 3 litres/minute, 2 respectively.

4 Although for reliable operation of the OPC the sample flow and the sheath flow should be broadly similar in magnitude, in practice the allowable tolerances are 6 relatively wide. Furthermore, the person skilled in the art who wishes to reduce the 7 diameter of the sample flow at the detection zone, can do this by increasing the size 8 of the filter, thus reducing its impedance and increasing the sheath flow. The effect of 9 the increased sheath flow is to 'squeeze' the sample flow it constrains, reducing its diameter by effectively 'stretching' the flow (by virtue of the increased flow velocity).

12 The orientation of the filter housing illustrated in Fig. 2(a), wherein ingress of air from 13 the side of the housing facing the OPC, provides a protection of the filter inlet. This 14 allows the OPC to be used in an outdoors environment with more confidence as the filter is shielded from rain or other possible impediments to operation.

17 Second Embodiment 19 Figure 2(b) shows a filter unit design suitable for a wide inlet OPC.

21 Third Embodiment 23 Further variants of the filter unit described in the first embodiment are possible. One 24 such variant is illustrated in Fig. 4.

26 In this third embodiment the air flow which will form the filtered sheath flow and the 27 sample flow each enter the filter unit from the same side. This is shown in Fig. 4(c).

29 The embodiment illustrated in Fig. 4 shows further aspects of the present invention, which, as the person skilled in the art will recognize, are combinable with the first two 31 embodiments.

33 This third embodiment illustrates an arrangement in which the sheath flow conduit 34 extends further in the direction of the OPC than the central conduit. According to Bernoulli's principle, the sheath flow in general produces an area of low pressure at 36 the exit end of the central conduit. This helps to suck the sample gas through the 1 central conduit. The extended sheath flow conduit illustrated in Fig. 4 enhances this 2 effect.

4 The embodiment of Fig. 4 also shows a sheath flow conduit of tapered cross-section.

Such a nozzle arrangement helps to constrain the central sample flow. For example, 6 a sample flow exiting a central conduit of --.1.2mm diameter can be constrained down 7 to a diameter of ---0.6mm as it passes through the laser beam.

Claims

1 Claims 3 1. A filter unit (1) for demountable fitting at the inlet of an optical particle 4 counter, said filter unit comprising a central conduit (3) for supplying a central sample flow (11) to the counter, and a filtering portion for supplying 6 a filtered sheath flow (13) surrounding said central sample flow (11).8

2. A filter unit according to claim 1, wherein the filter unit comprises a sheath 9 flow conduit (5), arranged around the central conduit (3), for supply of the filtered sheath flow (13) to the counter.12

3. A filter unit according to claim 2, wherein the sheath flow conduit (5) 13 extends further in the direction of intended gas flow than the central 14 conduit (3).16

4. A filter unit according to either claim 2 or claim 3, wherein the sheath flow 17 conduit (5) has a tapered cross-section which narrows as the sheath gas 18 egresses the filter unit.

5. A filter unit according to any of the previous claims, wherein with the filter 21 unit in its normal operational configuration, the filtering portion comprises 22 a filter (9).24

6. A filter unit according to claim 5, wherein the filter (9) is located around the central conduit (3).27

7. A filter unit according to either claim 5 or claim 6, wherein said filter (9) is 28 of annular form.

8. A filter unit according to any one of claims 5 to 7, wherein the filter (9) 31 offers low flow impedance.33

9. A filter unit according to any one of claims 5 to 8, wherein the filter (9) is 34 housed within a housing (7) comprising a base (19) upon which the filter is located and, mechanically engageable with said base (19), a cap portion 36 (17), wherein said cap portion (17) is separable from said base (19) to 37 allow replacement of said filter (9).2

10. A filter unit according to claim 9, wherein said cap portion (17) is 3 impermeable and said base portion (19) is configured to allow the ingress 4 of gas from the atmosphere external to the filter unit, which gas is filtered by the filter to provide the filtered sheath (13).7

11. A filter unit according to claim 10, wherein said central sample flow (11) 8 enters the filter unit at a first filter unit side and exits the filter unit at a 9 second filter unit side, and wherein said base portion (19) is at the said second filter unit side.12

12. A filter unit according to any one of the above claims, wherein the filter unit 13 features are such that the flow impedance for gas forming the sheath flow 14 and the flow impedance for gas forming the central sample flow are similar in magnitude.17

13. A filter unit according to any one of the preceding claims wherein the 18 filtered sheath flow (13) serves to encourage flow of the central sample 19 flow (11).21

14. An optical particle counter comprising a particle detection chamber, an 22 optically defined particle detection zone within the particle detection 23 chamber, an entrance inlet for receiving gas, and a filter unit according to 24 any one preceding claim.26

15. An optical particle counter according to claim 14 comprising an optically- 27 defined particle detection zone within said particle detection chamber, 28 wherein there is a gap between the edge of the particle detection zone 29 and the inner wall of the particle detection chamber.31

16. An optical particle counter according to claim 14 or claim 15, wherein the 32 filter unit is demountably attached to the optical particle counter.34

17. An optical particle counter according to any one of claims 14 to 16, wherein the optical particle counter comprises a tube, and wherein said 36 particle detection chamber is formed within said tube and there is a gap 37 between the particle detection zone and the inner wall of the tube.2

18. An optical particle counter according to any one of claims 14 to 17, 3 wherein the diameter of the sample flow (11) is wider than the optically 4 defined particle detection zone of the optical particle counter.6

19. An optical particle counter according to any of claims 15 to 17, additionally 7 comprising a fan to draw air through the entrance inlet and into the particle 8 detection chamber.

20. Use of a filter unit according to any one of claims 1 to 14 in conjunction 11 with an optical particle counter.13

21. A method of using a filter unit according to any one of claims 1 to 14, 14 wherein the filter unit is mounted at the entrance inlet of an optical particle counter to provide a filtered sheath for gas whose particle content is to be 16 counted.18

22. A method of supplying sample gas to an optical particle counter wherein 19 the sample gas is supplied through a central conduit (3) of a filter unit (1), forming a sample gas flow, and wherein additionally ambient gas passes 21 through a filtering portion of the filter unit, which filtering portion surrounds 22 said central conduit (3), thereby forming a filtered sheath flow which 23 sheaths said sample gas flow as it passes through the optical particle 24 counter.26

23. A method according to claim 21, wherein the filter unit is removed from the 27 optical particle counter and replaced with a filter unit comprising a fresh 28 filter.