GB2265217A

GB2265217A - Apparatus for detecting fibrous particles

Info

Publication number: GB2265217A
Application number: GB9304893A
Authority: GB
Inventors: Phillip Edward Harley
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-03-17
Filing date: 1993-03-10
Publication date: 1993-09-22
Anticipated expiration: 2013-03-10
Also published as: GB9205883D0; GB2265217B; GB9304893D0

Abstract

Apparatus for detecting the presence of fibrous particles in a fluid, particularly asbestos fibres in the atmosphere, comprises an optical light scatter zone (Z) through which a sample of fluid under test is fed and which is illuminated by a plurality of sources (2, 4, 6, 10, 12) transmitting beams of light at angles to one another and extending transversely of the sensing zone (Z). Light scattered by a fibrous particle is collected by detectors (8, 14) located at angles to the associated sources (2, 4, 6, 10, 12), the signals triggered by the detected scattered light being fed to electronic processors (not shown) which interpret the signals and provide an indication of the presence or otherwise of fibrous particles. Particulate sphericity and concentration may also be determined. <IMAGE>

Description

APPARATUS FOR DETECTING FIBROUS PARTICLES This invention relates to 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.

Although these statutory methods are unlikely to be replaced by direct reading instruments utilising 'on-line' techniques, such 'on-line' instruments have been developed which provide a substantially instantaneous measure of the presence of low concentrations of fibrous aerosols against a background of a much higher concentration 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 particle 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 pre-orientation 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.

There is a need for an instrument which, although not capable of providing an accurate measurement of the precise level of contamination by 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 'on-line' 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 apparatus for detecting the presence of fibrous particles in the atmosphere which gives a substantially instantaneous indication of said presence, which is 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 the present invention there is provided apparatus for detecting the presence of fibrous particles in a fluid, the apparatus comprising at least three sources of radiation located at angles to one another and each adapted to transmit an associated beam of radiation substantially transversely through a sensing zone, the fluid under detection being fed through said sensing zone, at least two receiving means located at an angle to one another and to each of the sources and adapted, in combination, to receive light from all the sources scattered by fibrous particles in said sensing zone, the radiation sources being actuated whereby the receiving means receive signals from the radiation sources associated therewith the magnitudes of which are determined by the orientations of the fibrous particles in the sensing zone relative to the beams incident thereon, and electronic detection means connected to said receiving means for interpreting said signals and providing an indication of the presence or otherwise of fibrous particles in the fluid passing through the sensing zone.

Such sensing apparatus is capable of discriminating substantially all orientations of fibrous particles which enter the sensing zone even if they are oriented with their major axes aligned closely with any one of the receiving means. In such a case the receiving means with which the major axis is aligned could receive an equal amount of scattered light from each of the sources associated therewith and would detect the particle as spherical, but the other receiving means, being at an angle to the aforementioned receiving means would receive varying amounts of light from the sources associated therewith and would therefore provide the desired indication of the presence of a fibrous particle.

A first embodiment of the invention incorporates three sources of radiation each of which has its own receiving means, the receiving means each being located at the same predetermined angle relative to its associated source.

Conveniently the axes of the three sources are mutually perpendicular to one another, as are the axes of the receiving means, the axis of a source and the axis of its associated receiving means also being perpendicular to one another.

In an alternative embodiment of the invention, there are a plurality of first sources of radiation located at angles to one another in a first common plane, first receiving means located at an angle to said first plane, a plurality of second sources of radiation located at angles to one another in a second common plane, and second receiving means located at an angle to said second plane.

In such an arrangement, the first and second planes are preferably perpendicular to one another, while it is further preferred that the axis of each receiving means is perpendicular to the associated plane, and the axes of the first and second receiving means are perpendicular to one another.

There may be three sources in each of the first and second planes, the beams transmitted by adjacent sources in a plane being at an angle of 600 with each other.

In one such embodiment, one of the sources is common to both the first and second planes.

The sources of radiation may each comprise a semiconductor laser diode, each diode supplying an associated fibre optic light guide, or, alternatively a collimating lens assembly.

The receiving means may each comprise a photodetection device, for example a photomultiplier tube, a solid state junction photodiode or an avalanche photodiode. The photodetection device may be remote from the sensing zone and arranged to view a fibre optic light guide adjacent the sensing zone.

The detection means connected to the receiving means may utilise synchronous demodulation, phase sensitive detection or tracking filter techniques utilising a reference signal coherent with the enabling sequence of the sources of radiation to interpret the signals received by the receiving means.

By way of example only, embodiments of the invention will now be described in greater detail with reference to the accompanying drawings of which: Fig. 1 is a schematic perspective view of an apparatus according to the invention; Fig. 2 is a cross-section through an apparatus according to the invention.

Fig. 3 is a schematic block diagram of the apparatus of Fig. 2, and Fig. 4 is a circuit diagram of the electronic detection means associated with the apparatus of Fig. 2.

Referring to Fig. 1 and by way of initial illustration, there are shown schematically and in perspective a system of light transmitting elements or sources and light collection elements or receiving means arranged around a sensing zone.

More particularly, transmitting elements 2,4 and 6 are arranged in a horizontal plane to transmit beams adjacent ones of which converge at an angle of 600 with one another into a sensing zone 'Z'. A light collection element 8, the axis of which is perpendicular to the plane of the elements 2,4,6, collects orthogonally scattered light initially transmitted by the elements 2,4,6 and subsequently scattered from a fibrous particle at that time in the sensing zone. In this respect, a continuous flow of sampled air is fed through the sensing zone Z as will be explained in more detail below.

A second similar array of light transmitting elements 10,4,12 is arranged in a vertical plane (the element 4 being common to both planes) and a light collection element 14 is arranged to collect orthogonally scattered light initially transmitted by the elements 10,4,12 and subsequently scattered by each fibrous particle at that time in the sensing zone 'Z'.

The size of the sensing zone 'Z' is chosen such that the probability of multiple occupancy by fibrous particles is likely to be low at the expected measurement concentration. The light transmitting elements 2,4,6,10,12 may each comprise a semiconductor laser diode associated with either a fibre optic light guide together with lens and light baffles, or with a collimating lens assembly. A light absorber or beam dump (not shown in Fig. 1) is located radially opposite each light transmitting element to absorb the majority of the light from its associated element and thereby minimise reflection of light back into the sensing zone 'Z'.

The light collection elements 8,14 each comprise an arrangement of irises and lenses, and a photodetection device or, optionally, a further fibre optic light guide positioned to allow a remote photodetection device to view the detection zone. Excluded from the photodelection devices1 view by means of the irises are the walls of the sensing zone, the light transmitting elements and the beam dumps. Such an arrangement is commonly referred to as a 'dark field' optical system. The photodetection devices may each comprise a photomultiplier tube, a solid state junction photodiode or an avalanche photodiode.

In operation, each laser diode 2,4,6, of the first array is switched on sequentially in turn at a high rate in a first switching cycle, while the diodes 10,4,12 are then switched on sequentially in turn at a high rate in a second switching cycle, the first and second switching cycles alternating with one another. The sensing zone is thus sequentially illuminated by light from the various radial directions in turn. The time taken for the first and second cycles to be completed is arranged to be short compared to the residence time of a fibrous particle in the sensing zone 'Z'.

Any fibrous particle present in the sensing zone will scatter a small portion of the incident illuminating light into the associated light collection element. A spherical particle will scatter a similar amount of light into the collection element regardless of which transmitting element is illuminating it, whereas a fibrous particle will deliver more light when illuminated from directions to which it presents greater scatter cross-sections. Thus, when a fibrous particle is in the sensing zone, the photodetection devices will provide a modulated signal the modulation frequency of which will be related to the sequential illumination rate of the sensing zone, and the modulation amplitude of which will be related to the aspect ratio of the fibrous particle.

Because the modulation frequency and phase are known, very powerful narrow band-electronic filtration techniques, such as phase sensitive detection, digital filtering and correlation, or analogue tracking filter designs, may be used to recover an electronic signal at adequate signal to noise ratio for successful fibre detection to be achieved.

The system illustrated in Fig. 1 can detect all fibrous particles of all conceivable orientations. As the transmitting element 4 is common to both arrays, the switching cycles of the two arrays are alternated.

However, with no common elements, the elements of the two arrays can be operated simultaneously by using different emission wavelengths for the elements of the two arrays and incorporating appropriate wavelength filtration in the collection elements.

Generally speaking, the greater the number of transmitting elements per plane, and the greater the number of planes containing a plurality of transmitting elements, the greater is the resolution of fibrous particle discrimination and the degree of independence of fibre orientation.

Fig. 2 illustrates an embodiment of the invention engineered in its simplest form having three light transmitting elements one in each of the x,y and z planes.

The optical arrangements along the x and y axes are shown with the z axis being in the direction of view. The components associated with a particular plane or axis are shown with a subscript denoting the relevant plane or axis.

A cubic housing 16 defines therein a sample chamber 18 into which, by means of a small tube 20, sampled air under investigation is injected. In each of three adjoining faces of the housing 16 are mounted three laser diode light sources and their associated collimation and beam forming optics, the x and y axis sources being referenced 22x and 22y respectively. The light beams from the sources converge into the chamber 18 to intersect with one another and define the sensing zone. The housing 16 is bored with three accurately converging holes two of which are shown at 24x and 24y, along the three optical axes, each of which, as well as providing a path for the associated illuminating beam, serves to support the beam forming optical components mounted on accurately machined annuli and a circular polarising element 26x,26y.

More particularly, the optical components include a series of square cross-section apertures 28x,28y arranged such that the beam profile in the region of the sensing zone is also of square cross-section with fairly uniform optical flux across the area.

The sample volume formed by the intersection of the three beams thus forms a cubic shape. In this example, the width of the cubic volume is chosen to be about 100 microns, being the optimum size for practical beam formation and alignment, and also providing a sufficiently small volume to ensure single fibrous particle occupancy conducive with a reasonable probability of occupancy in environments containing particles at the threshold limit values of concentration for hygiene monitoring applications, especially those appropriate to asbestos fibre hazard evaluation.

Light collection is performed by means of three silicon photo diodes 30 each perforated by a small central aperture. Each photo diode 30 is arranged axially with one of the laser beams from a source 22, the latter passing through the central aperture on its journey to the sensing volume in the chamber 18, and each diode 30 being adapted to receive scattered light from a laser beam orthogonal thereto. An apertured cone 32 is located in front of each diode 30 to limit its field of view.

Light absorbers or beam dumps 34 serve to dissipate light from the associated laser beam once it has traversed the sensing zone.

The described device detects fibrous particles independently of their orientation and there is thus no necessity for the establishment of laminar flow through the chamber 18. The sensing zone dimensions and the rate of air flow provided through the tube 20 ensure that there is a minimal change of orientation while a fibrous particle is traversing the zone.

Fig. 3 shows a schematic block diagram of the embodiment of Fig. 2. A timing sequencer 36 derives the sequential enabling pulses for the three laser diodes of the sources 22, the switching frequency of the diodes being optimally set in the region of 1MHz. Signal amplifiers 38 are provided for each of the three collection photodiode signal channels, the resultant signals from each channel being presented to a shape discriminator circuit 40.

A logic element 42 is included in the circuitry in which a decision is made as to whether the sphericity value for a particle falls between two specified thresholds, and also whether the mean signal falls between two defined thresholds.

A pump 44 ensures a continuous flow of sampled air through the sensing volume, while a flow monitoring device 46 indicates to the electronic processing circuitry that air flow through the apparatus is within acceptable limits.

By the provision of an air flow rate signal derived from the monitoring device 46, means are further provided for continuously calculating the concentration of detected fibrous particles in the air sample, and an annunciation of the measured concentration is provided to the user in the form of a numeric display 48 of fibrous particle concentration, and an audible alert 50 in the event of an unacceptable level being present.

Fig. 4 demonstrates how particle sphericity is measured. The amplified signals from the three photo diodes 30 are gated by switches 52 under the control of the timing sequencer 36 into three summing sample and hold circuits 54 in such a manner that three signals are derived each of a magnitude proportional to the orthogonally scattered energy from a particle illuminated by each of the three respective light sources 22. The three signals so derived are summed in order to provide a mean energy representative of the total scattered energy independent of the direction of illumination. Also, each of the three signals are differenced from the others and the modulus of the result is obtained. The maximum value of these three raw sphericity signals is selected and is divided by the mean energy to provide a rationalised representation of the sphericity of the particle.

The precise construction of the aparatus can vary from those detailed above, providing there are a plurality of light sources each illuminating the sensing zone from a plurality of different directions and there are a series of scattered-light collection elements each at an angle to the associated source or sources and interconnected with electronic circuitry capable of interpreting the signals received at the collection elements.

Thus there is provided 'on line' apparatus for detecting, substantially instantaneously, the presence of fibrous particles in an atmosphere, and which can give an indication of the concentration of such particles at any given time, the invention having particular application in the determination of the degree of asbestos pollution in the atmosphere.

Claims

1. Apparatus for detecting the presence of fibrous particles in a fluid, the apparatus comprising at least three sources of radiation located at angles to one another and each adapted to transmit an associated beam of radiation substantially transversely through a sensing zone, the fluid under detection being fed through said sensing zone, at least two receiving means located at an angle to one another and to each of the sources and adapted, in combination, to receive light from all the sources scattered by fibrous particles in said sensing zone, the radiation sources being actuated whereby the receiving means receive signals from the radiation sources associated therewith the magnitudes of which are determined by the orientations of the fibrous particles in the sensing zone relative to the beams incident thereon, and electronic detection means connected to said receiving means for interpreting said signals and providing an indication of the presence or otherwise of fibrous particles in the fluid passing through the sensing zone.

2. Apparatus as claimed in claim 1 and incorporating three sources of radiation each of which has its own receiving means, the receiving means each being located at the same predetermined angle relative to its associated source.

3. Apparatus as claimed in claim 2 in which the axes of the three sources are mutually perpendicular to one another, the axes of the receiving means are mutually perpendicular to one another, and the axis of a source and the axis of its associated receiving means are perpendicular to one another.

4. Apparatus as claimed in claim 3 in which the sources of radiation are actuated sequentially.

5. Apparatus as claimed in claim 3 in which the radiation emitted by each source is of a different wavelength and the receiving means are each provided with an optical filter appropriate to the wavelength of the associated source, the sources of radiation being actuated simultaneously.

6. Apparatus as claimed in claim 1 in which there are a plurality of first sources of radiation located at angles to one another in a first common plane, first receiving means located at an angle to said first plane, a plurality of second sources of radiation located at angles to one another in a second common plane, and second receiving means located at an angle to said second plane.

7. Apparatus as claimed in claim 6 in which the first and second planes are perpendicular to one another.

8. Apparatus as claimed in claim 7 in which the axis of each receiving means is perpendicular to its associated plane and the axes of the first and second receiving means are perpendicular to one another.

9. Apparatus as claimed in any one of claims 6 to 8 in which there are three sources in each of the first and second planes, the beams transmitted by adjacent sources in a plane being at an angle of 600 with each other.

10. Apparatus as claimed in claim 9 in which the sources of radiation in the first plane are actuated sequentially in a first switching cycle, and the sources of radiation in the second plane are actuated sequentially in a second switching cycle, the first switching cycle alternating with the second switching cycle.

11. Apparatus as claimed in claim 10 in which one source is common to both the first and second planes.

12. Apparatus as claimed in claim 9 in which the radiation emitted by the sources of the first and second planes are of different wavelengths, the receiving means being provided with optical filters appropriate to the wavelengths of the associated sources, the sources of radiation in the first plane being actuated sequentially in a first switching cycle, and the sources of radiation in the second plane being actuated sequentially in a second switching cycle, the first and second switching cycles operating simultaneously.

13. Apparatus as claimed in any one of claims 1 to 12 in which the sources of radiation each comprise a semiconductor laser diode.

14. Apparatus as claimed in any one of claims 1 to 13 in which the receiving means each comprise a photodetection device which is, for example, a photomultiplier tube, a solid state junction photodiode or an avalanche photodiode.

15. Apparatus as claimed in any one of claims 1 to 14 in which the detection means utilise synchronous demodulation, phase sensitive detecting or tracking filter techniques involving a reference signal coherent with the enabling sequence of the sources of radiation to interpret the signals received by the receiving means.

16. Apparatus for detecting the presence of fibrous particles in a fluid substantially as described with reference to Fig. 1 or Figs. 2 to 4 of the accompanying drawings.