WO1989009392A1

WO1989009392A1 - Fluid pollution monitor

Info

Publication number: WO1989009392A1
Application number: PCT/AU1989/000138
Authority: WO
Inventors: Martin Terence Cole
Original assignee: Martin Terence Cole
Priority date: 1988-03-30
Filing date: 1989-03-30
Publication date: 1989-10-05
Also published as: EP0407429A4; EP0407429A1; KR900700871A

Abstract

A pollution monitoring device including a fluid sampling chamber (21), a collimated light source (14, 15) directing a light beam (10, 11) into the chamber, a port (23) for introducing sample fluid into the chamber exposed to the light beam, a light detector cell (16) separated or shaded from the light beam and focussing apparatus (17) for directing scattered light produced by the presence of suspended particles and molecules in the chamber towards the detector. The source may be two lasers with co-linear beams or an LED.

Description

FLUID POLLUTION MONITOR BACKGROUND OF THE INVENTION

The present invention relates to detection and monitoring equipment for fluid pollution, including smoke and air pollution by light scatter techniques.

Devices are known for the detection of smoke by light scatter techniques. Such devices include a light source configured to irradiate through a volume of air provided in a sampling region in which smoke, dust or like particles may be suspended. Light scattered off said particles is collected on a light detector means. The amplitude of the signal from said light detector is an indication of the quantity of particulates in the fluid.

Particularly sensitive versions of such detectors are capable of monitoring low levels of fluid pollution and thus may be a useful tool for monitoring general atmospheric pollution. Such high sensitivity enables detection of fires at the earliest possible (incipient) stage, whereby the fire may be controlled by local personnel using portable extinguishers or by removal of the source of heat (e.g. by disconnection of electric current) before smoke levels become dangerous to life. Such detectors require a sensitivity as high as twenty microgra s of wood smoke per cubic metre for example, which is equivalent to a visual range of 40 kilometres.

The monitors disclosed in my earlier Australian Patent Specification Nos. 31843/84, 31842/84, 34537/84, 31841/84 and 42298/85 were developed primarily to detect the very earliest traces of smoke from overheating substances before fire develops. This has nowadays become a critical requirement because of the widespread use of synthetic materials in furniture and furnishings, wiring and equipment. Synthetic materials burn more fiercely and produce toxic fumes at rates considerably higher than their outmoded natural counterparts. Very early detection of smouldering has become vital to the preservation of life (e.g. dormitories) and valuable equipment (e.g. computers).

This prior apparatus can and does summon human intervention before smoke levels become dangerous to life or delicate equipment, it can cause an orderly shutdown of power supplies so that equipment overheating will subside (thereby preventing a fire), or it can operate automatic fire suppression and personnel evacuation systems.

The prior art utilizes a sampling chamber as described in Australian Specification No. 31843/84 through which a representative sample of air within the zone to be monitored, is continuously drawn by an aspirator. The air sample is normally irradiated by an intense, wideband light pulse from a Xenon lamp. A miniscule proportion of the incident photons are scattered off airborne particles towards a very sensitive detector, to produce an analog signal which, after signal processing, represents the level of pollution (smoke) present in the air. The instrument is so sensitive that photons scattered off air molecules alone are detected. Therefore, minor pollution is readily detected as an increased signal. Despite the greater sensitivity of the known apparatus the rate of false alarms is much lower than for conventional smoke detectors (which are comparatively insensitive, by two or three orders of magnitude) . OBJECTIVES OF THE INVENTION

Photons scattered off air molecules are invisible to the naked eye, like faint starlight, whereas the incident light is of similar brilliance to sunlight (thus in the one instrument, the range of light levels spans "cosmic proportions"). The task is to detect the equivalent of faint starlight in the presence of sunlight. This requires a chamber of advanced optical design, to separate the desired scattered light from the incident light, and advanced electronics to detect the miniscule scattered light component without resort to cryogenics or photomultipliers. The prior art has utilized a Xenon lamp as a suitable source of intense wideband light with low energy input. However, in view of the reducing cost of current laser technology it is the object of this invention to utilize the properties of coherent collimated light to simplify the design of the said sampling chamber, to improve the stability or sensitivity, and to reduce the power consumption of the whole instrument. SUMMARY OF THE INVENTION

There is provided according to the present invention a fluid pollution monitoring apparatus including a fluid sampling chamber, means for projecting a coherent collimated light beam into said sampling chamber, means introducing sample air from an area to be monitored into said chamber to be exposed to said light beam, a light detector cell positioned at a location separated or shaded from the axis of said light beam, and means in said sampling chamber for directing any scattered light produced by the presence of airborne particles in the chamber towards said detector.

The advantage of a coherent collimated light source is that light is not visible beyond the axis of the light beam, except as a result of light scattering off airborne particles. Therefore if a laser light beam is projected through the sampling chamber, its beam is invisible to the off-axis light detector cell. The introduction of airborne particles which scatter the light beam, produces light capable of detection by said off-axis light detector cell. The incident light beam can be projected or reflected out through and beyond said chamber, such that it may be conveniently absorbed outside the said chamber and thereby readily separated from scattered light within the said chamber. Thus the detector cell positioned within the chamber is responsive only to the scattered light component. Output from said detector cell varies in proportion to the level of scattered light, providing a measure of the concentration of particles within said chamber. To enhance the operation of the laser beam and detector cell combination, said chamber is conveniently in the form of a highly reflective elliptical tube. The tube includes a flat wall at each end to confine the air sample, perpendicular to the axis of said light beam. The sampling tube preferably in the end walls contains an induction or exhaust port to enable the continuous renewal of the air sample. The laser beam is projected along one focus of said elliptical tube. Any particles present in the path of the said laser beam would cause light scattering in all directions, but the elliptical chamber design would cause much of the scattered light to pass through the second focus of said elliptical tube, after one reflection, and again after subsequent multiple reflections. This is especially so for light scattered at 90° to said light beam, as is often the predominant scattering direction in the case of gas molecules.

The said second focus forms a line, parallel to the said laser beam. Ideally the detector cell should form the shape of a thin rod to collect light arriving in all directions, at all points along its length. One form of such a detector could be an optical fibre or preferably a long thin laser rod with a detector cell mounted at one end. This can be a difficult or costly requirement which can be simplified by the novel modification of tapering the said elliptical tube. Because the walls of said elliptical tube are not now parallel, then by multiple reflections most of the scattered light would be focussed at a line or point on an end wall. It should be noted that with a tapered tube, one end wall is larger than the other and focussing will occur at the second focus of the larger end wall. A comparatively simple detector cell is therefore placed at the second focus of the larger end wall. Said detector cell requires a wide reception angle, between a few degrees and approaching 90 degrees from its perpendicular, depending upon the length and taper of said elliptical chamber. For optimum performance, a suitable light-collecting lens (e.g. "fisheye"), cone or prism may be used with the said detector cell.

Whereas light may be predominantly scattered at 90° for gas molecules, in the case of larger particles, such as smoke, the majority of scattering occurs within 0° to 30° of the axis of said light beam.

In an alternative form of the invention it is proposed to enhance the capture of light scattered in the forward direction (that is, at small angles to the axis of the laser beam). This approach would provide greater sensitivity of the overall instrument when the greatest proportion of light is scattered in the forward direction at small angles.

One method for detecting the light scattered at small angles to the axis of said light beam could be to position a very large detector cell at one end of said chamber such as a disc with a relatively small central.hole to permit the passage of said light beam. Such cells are not usually available with sufficient sensitivity and low noise, so a comparatively small cell must be used. To capture and focus sufficient scattered light onto said small cell, a concave mirror or lens could be used.

Thus, there is also provided according to the present invention a light sensing device for use in a fluid pollution monitoring apparatus, including a fluid sampling chamber, means for providing or projecting a coherent collimated light beam into said sampling chamber, means introducing sample fluid from an area to be monitored into said chamber to be exposed to said light beam, a light detector cell positioned at a location separated or shaded from said light beam, reflecting means in said chamber focussing as much as possible the available scattered light towards said reflecting means onto said detector cell. Preferably said reflecting means is an axially disposed concave reflector, Conveniently a further reflecting means is placed at the centre, and in front of said reflecting means, to reflect said light beam in a desired direction, such as back towards said source or towards a light absorbing means within or beyond said chamber.

Said detector cell is mounted at the rear of said further reflecting means, and positioned to receive scattered light which has been collected and focussed by said concave reflector. Thus the detector cell is prevented from detecting direct light from said laser beam but will receive a large proportion of the scattered light which is received by said further reflecting means.

This alternative arrangement does not require a reflective wall chamber, as previously described, and does not require a chamber of special shape such as an elliptical shape. Therefore, it will be appreciated that any dust buildup on chamber walls would not necessarily affect the sensitivity of the monitor apparatus. By suitable deflection of the air stream and with careful attention to chamber orientation or air turbulence and velocity the concave reflector can be made practically resistant to dust buildup.

A combination of a reflective elliptical wall chamber and concave reflector to achieve a resultant combination of all sources of reflected light is also possible according to the present invention.

Two detector cells may be used to indicate differing proportions of light scattered either at small angles or angles of up to 90° to the light beam axis respectively in response to incidence of particles of differing size.

By comparison with the said prior art, first mentioned version of the present invention utilizes a highly light-reflective chamber with a collimated coherent laser beam instead of a highly light-absorbing chamber with a wide-angle incoherent Xenon lamp. This improvement provides an opportunity for substantial improvement in the ratio of scattered light to remnant incident light, improving the "optical signal-to-noise ratio". According to this invention, the incident light intensity (required to produce a given scattered light intensity appropriately matched to the sensitivity of said detector cell), may be reduced. Therefore the energy consumption of the light source may be reduced. Use of a Xenon lamp in the prior art accounted for the majority of the power consumption of the entire instrument. Thus it is possible in the present invention to provide for a substantial reduction in power consumption. Moreover there is provided an opportunity for increased sensitivity of the overall instrument, such that lower levels of particle concentration can be detected. In order to improve the electrical signal-to-noise ratio of the detector cell, the laser beam may be pulsed or modulated. The modulation frequency may be synchronised with a digital filter means provided by a phase-locked loop or microprocessor circuit (not shown), to optimise the signal-to-noise ratio. This modification further reduces the required intensity of the laser beam, reducing the energy consumption of the overall instrument. If the laser lamp is pulsed (at say ImS mark to lOOOmS space) rather than modulated (at say lOOOHz), maximum opportunity for reduced energy consumption and increased operational life may be achieved at the expense of signal-to-noise ratio (affecting ultimate sensitivity and sampling rate).

In taking advantage of the reducing cost and improving availability of miniature lasers such as solid state or Helium-Neon lasers in the performance of the present invention, either type may be used however, solid state lasers are not readily available for operation above the infra-red spectrum, so use of Helium-Neon lasers having a shorter wavelength in the visible spectrum would provide greater sensitivity. When using the instrument to detect very small particles or molecules, or for the purposes of calibration, because the narrow optical bandwidth of a laser may bias the detector sensitivity in favour of certain particle sizes (thus rendering comparatively lower sensitivity to other size particles); conveniently two colinear laser beams operating at different wavelengths may be provided. BRIEF DESCRIPTION OF DRAWINGS

Figure 1 shows a sectional view of a sampling chamber for a light sensing pollution detection device.

10

Figure 2 shows a further embodiment of a sampling chamber.

The two laser beams may be rendered colinear by use of a prism 12 or mirror system. One embodiment of such a m - system would utilize the differing angle of refraction applicable to each wavelength. By use of a single prism 12, each laser beam would be projected at a differing incident angle such that the two beams emerge as a pair of colinear beams into the collimator 13. This novel employment of

20 differing angle of incidence allows for the necessary physical separation of the two lasers 14, 15_. It would be possible to operate both laser beams at the same time, and/or to alternate each beam to provide additional information about the nature of the particles or fluid.

25 If the laser beams are directed back towards said source, they strike said prism at an angle greater than the critical angle and may be totally reflected toward a light absorbing means. Alternatively, a light absorbing means may be employed within or without the chamber and thus allow

30 less critical alignment of said reflecting means.

Naturally the said detector cell 16 must be responsive to both laser wavelengths. If said detector cell has differing sensitivity at the two said laser wavelengths, , each said laser could be pulsed alternately at a rate _ synchronised with a switched-gain amplifier means to compensate for said differing sensitivity. It is known that the light-scattering coefficient for any gas or aerosol is dependent upon the wavelength of the incident light. Rayleigh has found that for gases, this coefficient varies inversely with the fourth power of the wavelength employed. Larger airborne particules have a coefficient which varies inversely with an exponent less than four (possibly as low as zero). Therefore, for a given span of airborne particle sizes, said compensation means could also be used to compensate for the differing light-scattering coefficients at the two said laser wavelengths. Alternatively the whole instrument could be used to determine the said exponent for a known air/gas/particulate sample.

With specific reference to Figure 1, a novel circular light absorbing device 22 is shown for receiving the unscattered light beam travelling through the chamber. The device is circular as shown with reflective walls so that received light is continually reflected around the walls as shown and prevented from returning into the sampling chamber. Scattered light is received by reflector 17 (Figure

2) for focussing the large proportion of scattered light onto the detector cell 16. The cell preferably has a finite dimension to enable receipt of more light without the necessity to focus the light to a sharp point. A lens or lens system (not shown) may be used as an alternative to the reflector.

It is possible to achieve adequate sensitivity to utilise a light emitting diode (LED) as an alternative to a double or single laser beam. Port means 23 are provided for introducing sample air into and out of the chamber.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS;-

1. A fluid pollution monitoring apparatus of the type for detecting suspended particles and molecules, including a fluid sampling chamber, means for projecting a collimated . light beam into said sampling chamber, means introducing sample fluid from an area to be monitored into said chamber to be exposed to said light beam, a light detector cell positioned to avoid direct receipt of said light beam, and means in said sampling chamber for directing any scattered light produced by the presence of suspended particles and molecules in the chamber towards said detector.

2. A fluid pollution monitoring apparatus of the type for detecting suspended particles and molecules, including a fluid sampling chamber, means for providing or projecting a collimated light beam into said sampling chamber, means introducing sample fluid from an area to be monitored into said chamber to be exposed to said light beam, a light detector cell positioned at a location shaded from said light beam, focussing means in said chamber directing a substantial proportion of scattered light towards said detector cell.

3. A fluid pollution monitoring apparatus of the type for detecting suspended particles and molecules, including a fluid sampling chamber, means for providing or projecting a collimated light beam into said sampling chamber, wherein said light beam is generated by at least two lasers operating at different wavelengths adapted to be projected through a single prism into said sampling chamber as a pair of co-linear beams.

4. A fluid pollution monitoring apparatus as claimed in Claims 1 or 2, wherein said collimated light beam is a coherent light beam produced by a laser.

5. A fluid pollution monitoring apparatus of the type for detecting suspended particles and molecules, including a fluid sampling chamber, means for projecting a collimated light beam into said sampling chamber, means introducing sample fluid from an area to be monitored into said chamber to be exposed to said light beam, light detector means positioned to avoid direct receipt of said light beam, said detector means being positioned at a location shaded from said light beam, means in said sampling chamber directing scattered light towards said reflecting means onto said detector means.

6. A fluid pollution monitoring apparatus of the type for detecting suspended particles and molecules, including a fluid sampling chamber, means for providing or projecting a collimated light beam into said sampling chamber, means for introducing sample fluid from an area to be monitored into said chamber to be exposed to said light, a light detector cell positioned at a location shaded from the said light beam, reflecting means in said chamber focussing scattered light directed towards said reflecting means onto said detector cell wherein said light beam is generated by at least two laser beams operating at different wavelengths adapted to be projected through a single prism into said sampling chamber as a pair of co-linear beams.

7. A fluid pollution monitoring apparatus as claimed in Claims 1, 3 or 5, wherein the output of said light detector means varies in proportion to the level of scattered light received to provide a measure of the concentration of particles and therefore fluid pollution within said chamber.

8. A fluid pollution monitoring apparatus as claimed in Claims 1, 2 or 5, wherein said light beam is generated by a light emitting diode directing its light into a collimator.

9. A fluid pollution monitoring apparatus as claimed in Claims 1, 3 or 4, wherein the sampling chamber includes a tube with flat walls at each end to confine an air sample introduced therein, the light beam being projected along the tube between said walls wherein said tube is of elliptical cross-section and has reflective sidewalls to focus the scattered light onto said light detector means.

10. A fluid pollution monitoring apparatus as claimed in Claim 7, wherein said detector means comprises an optical fibre or lasing rod means, a detector cell located at or near one end to receive scattered light directed along the length of said optical fibre means.

11. A fluid pollution monitoring apparatus as claimed in Claim 8, wherein said sampling chamber is tapered along its length to assist in focussing of scattered light at a line or point for receipt by said light detector means.

12. A fluid pollution monitoring apparatus as claimed in Claims 1, 2 or 3, wherein unscattered light beam is absorbed by a light receiver having a substantially cylindrical reflective interior wall.