GB2283316A

GB2283316A - Method and apparatus for detecting fibrous particles

Info

Publication number: GB2283316A
Application number: GB9421346A
Authority: GB
Inventors: Phillip Edward Harley
Original assignee: HARLEY SCIENT Ltd
Current assignee: HARLEY SCIENT Ltd
Priority date: 1993-10-28
Filing date: 1994-10-24
Publication date: 1995-05-03
Anticipated expiration: 2014-10-24
Also published as: GB9322242D0; GB9421346D0; GB2283316B

Abstract

A method of detecting the presence of fibrous particles in a fluid comprises the steps of feeding the fluid under detection through a sensing zone 6, transmitting a beam of radiation from a source 1 through the sensing zone 6 to illuminate fluid within the sensing zone 6, collecting radiation scattered by fibrous particles in the sensing zone 6 by a plurality of receiving means 12 disposed about the sensing zone 6, and transmitting signals received by the receiving means 12 to electronic detection means for interpretation to provide an indication of the presence of fibrous particles in the fluid passing through the sensing zone 6. Prior to passage through the sensing zone 6, turbulence is created in the fluid whereby the fibrous particles in the fluid adopt random orientations therein and relative to one another, each fibrous particle substantially maintaining its random orientation during passage through the sensing zone 6. <IMAGE>

Description

METHOD AND APPARATUS FOR DETECTING FIBROUS PARTICLES This invention relates to a method and apparatus for detecting the presence of fibrous particles in a fluid, and has particular, though not exclusive, application to the detection of asbestos fibres in the atmosphere.

Microscopic airborne particles, commonly referred to as aerosols, can include particles of fibrous morphology as well as those of a compact spherical shape, and the concentration of such fibrous particles in the approximate size range 0.3 to 10 microns is of particular interest in the field of environmental monitoring for health and safety purposes as well as in industrial contamination control. A wide range of instruments exist for the characterisation of such particles in terms of their equivalent spherical diameter, their mass and their aerodynamic properties.

Statutory methods for, for example, monitoring asbestos fibres in the atmosphere specify 'off-line' laboratory techniques which rely upon collecting a sampled aerosol by a filtration process, followed by preparation of the sample and analytical microscopy in a laboratory. Such methods are extremely time consuming, the analysis often taking more than 24 hours.

A number of 'on-line' instruments have also been developed which provide a substantially instantaneous measure of the presence of low concentrations of fibrous aerosols against a background of a much higher concentrate of compact, non-fibrous particles.

Existing 'on-line' instruments for fibrous aerosol detection commonly utilise the directionally asymmetric nature of light scattered by fibrous particles illuminated by a suitably intense light source, coupled with a deliberate pre-orientation of the particles using either electric or magnetic fields or an aligning flow gradient in the supporting air. The deliberate pre-orientation of the particles ensures that the fibrous particles are illuminated in such directions that the scattered light cylindrical asymmetry is maximised. An optical detection device can then register the presence of fibrous particles in a sensing zone.

Such instruments are however complicated and expensive, incorporating several specialised components including, specifically, means for aligning the fibrous particles in a predetermined orientation prior to illumination thereof. The necessity for preorientation is a major obstacle in the integrity of the detection, since it relies upon arbitrary interactions between the particle and the orienting force, and it is not 'fail-safe' in that a particle which does not align will fail to be detected.

Other existing instruments use multiple laser sources to bathe the particle in light from different directions, thereby enabling the particle's cross section to be assessed from various perspectives, while it is also known to use parabolic or elliptical mirrors to collect asymmetrically scattered light from aligned particles injected into a sensing zone, at an angle normal to the illuminating axis. However there are attendant practical problems with such arrangements associated with degradation of reflectivity, inherent asymmetric effects resulting from shadows cast by the particle injection system, and also, as mentioned above, the requirement for some form of particle alignment, either through a flow gradient, or an electric or magnetic field.

There is a need for an instrument which, although not capable of providing an accurate measurement of the precise level of contamination of fibrous particles, can give an indication of the presence of such particles in the atmosphere. A typical example of the benefit of such an instrument is in the stripping of asbestos where leakage through protective sheeting might well go unnoticed over a relatively prolonged period until the statutory laboratory methods determine the situation. During that period many workers or members of the public could have been exposed to excessive ambient concentrations of asbestos. An 'online' instrument could give a warning of the high concentrations present almost immediately and could therefore work alongside, and in support of, the laboratory methods.

Thus it would be desirable to be able to provide a method and apparatus for detecting the presence of fibrous particles in the atmosphere which gives a substantially instantaneous indication of said presence, the apparatus being of relatively compact form and simple construction compared with existing 'on-line' instruments, and which enables an assessment of the particle regardless of its orientation.

According to one aspect of the present invention there is provided a method of detecting the presence of fibrous particles in a fluid comprising the steps of feeding the fluid under detection through a sensing zone, transmitting a beam of radiation through said sensing zone to illuminate fluid within said sensing zone, collecting radiation scattered by fibrous particles in the sensing zone by a plurality of receiving means disposed about said sensing zone, and transmitting the signals received by the receiving means to electronic detection means for interpretation to provide an indication of the presence or otherwise of fibrous particles in the fluid passing through the sensing zone, the method further comprising the step of, prior to passage through the sensing zone, creating turbulence in the fluid whereby the fibrous particles in the fluid adopt random orientations therein and relative to one another, each fibrous particle substantially maintaining its random orientation during passage through the sensing zone.

It will be appreciated that the turbulent nature of the fluid flow inhibits any tendency for the fibrous, non-spherical particles to adopt a consistent alignment relative to the flow direction, and thereby ensures scattering of radiation therefrom in a sideways direction characteristic of the physical asymmetry of the illuminated particle's scatter profile.

In a preferred method, the direction of the beam of radiation and the direction of the flow of fluid through the sensing zone are coaxial, the receiving means being located in a common plane normal to said direction of flow.

According to a further aspect of the present invention, there is provided apparatus for detecting the presence of fibrous particles in a fluid comprising a source of radiation for transmitting a beam of radiation through a sensing zone, means for feeding the fluid under detection through the sensing zone, a plurality of receiving means disposed about said sensing zone to receive radiation from said source scattered by fibrous particles in the sensing zone, and electronic detection means connected to the receiving means for interpreting the signals created thereat by said scattered radiation to provide an indication of the presence or otherwise of fibrous particles in the fluid passing through the sensing zone, the apparatus further comprising, prior to the sensing zone, means for creating turbulence in the fluid flow whereby the fibrous particles in the fluid adopt random orientations therein and relative to one another, each fibrous particle substantially maintaining its random orientation during passage through the sensing zone.

The dimensions of the sensing zone are chosen such that the probability of multiple occupancy by fibrous particles in the fluid flow is likely to be low at the expected measurement concentration, and also sufficiently small such that the illumination intensity by the source of radiation is high enough to ensure adequate detection of small single fibrous particles.

In a preferred apparatus, the beam of radiation and the flow of fluid are arranged to be coaxial within the sensing zone.

Conveniently the receiving means are located in a plane normal to the common direction of the fluid flow and the beam of radiation within the sensing zone.

The receiving means may each comprise a photodetector, or, alternatively, a fibre-optic light guide interconnected with a photodetector mounted remotely from the sensing zone.

The apparatus may further comprise a receiving means beyond the sensing zone and substantially coaxial with the fluid flow and the beam of radiation through the sensing zone to receive radiation scattered forwardly by particles in the sensing zone.

By way of example only, the aspects of the invention will now be described in greater detail with reference to the accompanying drawings of which: Fig. 1 is a longitudinal section through apparatus according to the invention; Fig. 2 is a transverse section on line 11-11 in Fig. 1; Fig. 3 is a detail of Fig. 1 with the addition thereto of a forward-scatter receiving means; Fig. 4 is a block diagram of electronic detection means of apparatus according to the invention, and Fig. 5 is a block diagram of a system incorporating apparatus according to the invention.

Referring to Figs. 1 to 3, there are shown the optical components of a detector according to the invention.

A semiconductor laser diode 1 incorporating a collimation lens assembly and circular polariser is mounted at one end of a support tube 2 such that, when energised, it projects a parallel beam of light along the axis of the tube 2. A converging lens 3, and an aperture 4, cause the laser beam to be focused to a diffraction limited waist in a sensing zone 6 within the tube 2 after passing through the central axis of an injection nozzle 5.

After emerging from the sensing zone 6, the beam is absorbed on the walls of a beam dump 7. The support tube 2 is provided with an accurately machined bore within which all the optical components are positioned to the required tolerances.

Air from the environment to be tested is admitted through an inlet port 8 into the tube 2 and is accelerated in the injection nozzle 5 to a velocity sufficiently high to ensure turbulent flow conditions in the bore of the nozzle 5 and for a distance extending beyond the mouth of the nozzle 5 into the sensing zone 6. After traversing the sensing zone 6 the air is exhausted from a port 9 situated at the end of the beam dump 7.

An exhaust nozzle 14 defines the axial length of the sensing zone 6 from which light scattered by a fibrous particle may be admitted to a series of photodetectors 12 disposed about the sensing zone 6.

Microscopic particulate matter supported by the air will be transported through the same path. Particles of matter at the central core of the flow through the nozzle 5 will be illuminated by light from the laser diode 1, and each will scatter a proportion of light at various angles to the axis, as determined by the laws of light scatter physics. Some of this scattered light will be collected by the photo-detector devices 12.

A supply of clean air drawn through a filter is admitted at a port 10, and is injected around the sensing zone 6 in order to protect the optical elements from contamination. The clean air is mixed with the sampled air beyond the sensing zone, and shares the same exhaust port 9.

In the illustrated arrangement, five photodetectors 12 are each located radially and at right angles to the axis of the laser beam. Each of the detectors 12 is conveniently mounted on a printed circuit board 13 which may also support other electronic components associated with the measurement system. The detectors 12 are shown schematically for clarity. Each detector measures the scattered light from a particle as it traverses its field of view. The sensing zone 6 is thus defined by the field of view of the photo-detectors 12 and the waist of the laser beam.

A spherical particle traversing the sensing zone 6 will thus scatter equal amounts of light into each photo-detector 12 for the duration of the transit of the particle through the sensing zone 6. A fibrous particle whose major axis is aligned with that of the laser beam will similarly scatter light uniformly into each photo-detector 12.

A fibrous particle which traverses the sensing zone 6 with its major axis not aligned with the laser beam will result in an asymmetry in the scattered intensity delivered to the photo-detectors 12.

Turbulent flow conditions in the nozzle 5 ensure that non-spherical particles are presented to the sensing zone 6 at random orientations with respect to the laser beam axis. Although some fibrous particles will fail to result in an asymmetric scatter intensity as measured by the detectors 12, provided statistical conditions on the minimum number of particles examined are adhered to, then useful fibrous particle detection may be achieved. A glass window, (not shown) is incorporated between the photo-detectors 12 and the support tube 2 to ensure integrity of the fluid containment.

It is to be emphasised that the air from the environment that is injected into the sensing zone 6 is of a turbulent nature so as to inhibit any tendency for non-spherical particles to adopt a consistent alignment relative to the flow direction. This is achieved by designing the bore of the nozzle 5 to be of a dimension such that a high Reynolds Number exists under the required sample flow velocities.

Further, the sensing zone 6 is of relatively small volume and is such that any given fibrous particle, as it passes through the sensing zone 6, maintains its random orientation during said passage through the sensing zone 6 whereby the light scattered thereby and received by the photo-detectors 12 remains consistent.

The photo-detectors 12 adjacent the sensing zone 6 may be replaced by, for example, fibre-optic light guides which collect the scattered light and convey it to remote photo-detectors.

Fig. 3 shows in more detail part of the embodiment of Figs. 1 and 2 but with the addition of an optional perforated photo-detector 15 coaxial with the nozzle 5 to the side of the sensing zone 6 remote from said nozzle 5. The photo-detector 15 receives light scattered forwardly from the sensing zone 6 and, in conjunction with the electronic detection means to be detailed below, enables the sizes of the particles traversing the sensing zone 6 to be accurately assessed.

Fig. 4 is a block diagram of the electronic detection means of the embodiment detailed above. A set of five transimpedance amplifiers 13 converts the photo currents generated by the scattered light in each detector 12 to a voltage. Because of the transitory nature of each particle, the scattered light will be in the form of a pulse whose duration is determined by the particle's transit time in the sensing zone 6. Thus a voltage pulse will appear at the output of each trans impedance amplifier 13 in correspondence to the scattered light pulse. The amplitude of the pulse will correspond to the intensity of scattered light received by the detector 12. In Fig. 4, the five transimpedance amplifier outputs are designated A,B,...E.

The trans impedance amplifier outputs are made available to the inputs of ten differential amplifiers 14. The connections are made in such a way that all combinations of the difference in voltage between all of the five channels are computed. Thus, voltage pulses of durations equivalent to the transit time of a particle through the sensing zone 6, and of amplitudes equivalent to the difference in scattered light intensity between the two presented channels, are computed. In Fig. 4, the ten difference amplifier outputs are designated A-B, A-C, A-D,....D-E.

The output signals from the differential amplifiers 14 may be either positive or negative.

Because it is the magnitude of the difference between channels which is of importance in determining the scatter asymmetry of a particle, it is necessary to take the modulus of the differential amplifier outputs.

This is achieved by means of the modulus circuit 15.

In Fig. 4, the ten modulus circuit outputs are designated (A-B), (A-C), (A-D),...(D-E).

The ten modulus circuit outputs are applied to an analog gate 16, which is a circuit designed to pass the signal channel with the greatest magnitude, and no other. The output of this circuit will be a pulse whose duration is that of the transit of the particle through the sensing zone 6, and whose amplitude represents the greatest difference in scattered light intensity between any pair of the five channels. It is designated Raw Asphericity in Fig. 4. A large pulse would be expected at this point if a fibrous particle was to traverse the sensing zone 6 broadside on to the laser beam. A spherical particle would result in a very small amplitude pulse. The Raw Asphericity signal amplitude does not, however, represent the absolute asphericity of the particle.For instance, a very large, nearly spherical particle could result in a signal of similar amplitude to that produced by a very small, highly fibrous one. To achieve an absolute value of asphericity, the signal must be compared with the mean value of scattered light received by all the detectors.

An alternative method of achieving the modulus, not shown in Fig. 4, would involve a duplication of all ten differential amplifiers, but configured as B-A, C A, D-A ...E-D. In such an arrangement, all twenty difference signals would be presented to the ensuing analog gate 16 which would choose the greatest in magnitude for a given sign.

A mean value circuit 17 is included in order to generate a signal whose amplitude is proportional to the mean of all five input channels, A, B....E, and is designated in Fig. 4, as (A+B+C+D+E)/5. This signal serves two purposes. Firstly, it provides a pulse for all particles traversing the sensing zone 6, thus enabling the instrument to totalise the number of particles, of any shape, detected over a given length of time. This is of importance in determining the statistical significance of any fibrous particle detection. Secondly, it provides a mean amplitude of scattered light for each individual particle, against which the Raw Asphericity can be adjusted to provide an absolute asphericity measurement. This is achieved in a Ratio Circuit 18, the output of which is a pulse whose amplitude is proportional to the asymmetry of the particle, independent of its size.

Fig. 5 is a block diagram of a complete fibrous aerosol detector. Air is drawn from the environment under test through an inlet port 19 across a flow measurement device 24 and into a particle separation device 20, which may take the form of a cyclone or elutriator whose purpose is to remove the larger aerosol particles of a size greater than those of interest. The sample air is then drawn into an optics module 21 which is equivalent to the detector described in greater detail above with reference to Figs 1 and 2.

Environmental air is also drawn through an absolute filter 22 and a flow control valve 23 to provide the clean air for the optics module 21. On emerging from the optics module 21, the air is passed through a pump 25 before being returned to the environment at outlet port 31.

A controlled power supply 26 is included to drive the semiconductor laser diode 1 mounted within the optics module 21. Measurement electronics 27 are as described above and as shown in Fig. 4. Control electronics 28 accept the mean and asphericity signals from the measurement electronics, and also the signal from the flow measurement device 24. The control electronics provide the user interface, in the form of fibrous particle concentration information, and annunciation, on display devices 29, and control the operation of the pump 25, and the acquisition of particle measurement, under command from user controls 30.

Thus there is provided a method for detecting the presence of fibrous particles in a fluid which gives a substantially instantaneous indication of said presence utilising apparatus of a relatively compact form and simple construction and operation. In particular the apparatus utilises a single source of radiation illuminating fibrous particles of random orientation in a flow of fluid into which turbulence has been induced prior to passage through the sensing zone, said particles maintaining their random orientation during passage through the sensing zone and scattering radiation asymmetrically to photo-detectors surrounding the sensing zone.

The photo-detectors, which may be photomultiplier tubes, solid state junction photodiodes, avalanche photodiodes or fibre-optic light guides, are shielded from a direct view of the laser optical components to provide a 'dark field' optical system.

Claims

1. A method of detecting the presence of fibrous particles in a fluid comprising the steps of feeding the fluid under detection through a sensing zone, transmitting a beam of radiation through said sensing zone to illuminate fluid within said sensing zone, collecting radiation scattered by fibrous particles in the sensing zone by a plurality of receiving means disposed about said sensing zone, and transmitting the signals received by the receiving means to electronic detection means for interpretation to provide an indication of the presence or otherwise of fibrous particles in the fluid passing through the sensing zone, the method further comprising the step of, prior to passage through the sensing zone, creating turbulence in the fluid whereby the fibrous particles in the fluid adopt random orientations therein and relative to one another, each fibrous particle substantially maintaining its random orientation during passage through the sensing zone.

2. A method as claimed in claim 1 in which the direction of the beam of radiation and the direction of the flow of fluid through the sensing zone are coaxial, the receiving means being located in a common plane normal to said direction of flow.

3. Apparatus for detecting the presence of fibrous particles in a fluid according to the method of claim 1 or claim 2, the apparatus comprising a source of radiation for transmitting a beam of radiation through a sensing zone, means for feeding the fluid under detection through the sensing zone, a plurality of receiving means disposed about said sensing zone to receive radiation from said source scattered by fibrous particles in the sensing zone, and electronic detection means connected to the receiving means for interpreting the signals created thereat by said scattered radiation to provide an indication of the presence or otherwise of fibrous particles in the fluid passing through the sensing zone, the apparatus further comprising, prior to the sensing zone, means for creating turbulence in the fluid flow whereby the fibrous particles in the fluid adopt random orientations therein and relative to one another, each fibrous particle substantially maintaining its random orientation during passage through the sensing zone.

4. Apparatus as claimed in claim 3 in which the beam of radiation and the flow of fluid are arranged to be coaxial within the sensing zone.

5. Apparatus as claimed in claim 4 in which the receiving means are located in a plane normal to the common direction of the fluid flow and the beam of radiation within the sensing zone.

6. Apparatus as claimed in claim 4 or claim 5 in which the receiving means each comprise a photodetector, or a fibre-optic light guide interconnected with a photo-detector mounted remotely from the sensing zone.

7. Apparatus as claimed in any one of claims 4 to 6 and further comprising a receiving means beyond the sensing zone and substantially coaxial with the fluid flow and the beam of radiation through the sensing zone to receive radiation scattered forwardly by particles in the sensing zone.

8. A method of detecting the presence of fibrous particles in a fluid substantially as described with reference to the accompanying drawings.

9. Apparatus for detecting the presence of fibrous particles in a fluid substantially as described with reference to and as illustrated by the accompanying drawings.