Smoke Particle detector

Abstract: A high sensitivity smoke detector comprises a housing (10) incorporating a sampling chamber (16) through which air to be sampled is forced through the inlet (18) and outlet (20) in the direction of the arrows. A modulated light source (22), such as a laser, directs modulated light through the chamber (16) to a beam dump (26). The light beam (24) is offset from the axis of the housing (10) by an acute angle of between 15 and 50 degrees. If any smoke particles are positioned within a sampling volume (42), the light is scattered along a path (30) defined by baffles (32 to 40) to a light sensor (28) whose electrical output is passed through a phase-sensitive detection circuit which is referenced by the frequency at which the light source (22) is modulated, so as to produce an output dependent on the light scattered by the smoke particles.

Description: The invention relates to a high sensitivity detector of suspended particles in a gaseous medium and capable of detecting the presence of a concentration of particles in the medium which attenuate light by less than one per cent per metre of the medium, comprising: an elongate hollow chamber; means for forcing a sample of the gaseous medium along a predetermined flow path extending through the interior of the chamber between an entrance and an exit in the chamber wall, a portion of the flow path constituting a sampling region; light source means fixedly mounted in relation to the chamber and operative when energised to direct a light beam along an input path which extends across the interior of the chamber in a direction transverse to the elongation of the chamber and also transverse to the said flow path and passing to and through the sampling region, such that light in the beam is scattered by particles in the medium in the sampling region; light sensor means fixedly mounted in relation to the chamber and producing an output signal dependent on the light received and positioned to receive the scattered light along a predetermined output path from the said sampling region; beam dump means fixedly mounted in relation to the chamber for receiving unscattered light directly from the light source means and arranged to prevent substantially any reflection of such received light into the output path; and aperture defining means extending across the interior of the chamber transverse to its elongation and defining a plurality of apertures spaced apart along the length of the output path and each for defining the cross-sectional shape of the output path. Such a detector is shown in AU-B-31841/84, (family member of EP-A-140,502). In this detector, the gaseous medium enters the elongate chamber through the side wall thereof and passes along part of the elongation of the chamber and then exits through the side wall via an exit axially displaced along the chamber from the inlet. The light source is positioned in the wall of the chamber to direct the light input beam across the chamber so as to intersect the flow path of the gaseous medium along the chamber. The light is directed in a variety of directions. The light sensing means is positioned at one axial end of the chamber and the beam dump means is positioned at the opposite axial end. The power of the light is relatively great, with the aim of obtaining a high signal to noise ratio. Because the flow path for the sampled medium is right-angled and there is a change in its cross-sectional size, there is a tendency for the particulates to drop out of the medium and deposit onto the chamber wall where they may build up. Because of the powerful multi-directional light source, these deposited particulates will cause the output signal, measured by the light sensing means to increase gradually, even though there is no actual increase in the concentration of the particulates in the gaseous medium. Furthermore, the beam dump means, being positioned at one axial end of the chamber, does not collect light which passes unscattered through the particles and strikes the opposite side wall of the interior of the chamber. The invention aims to avoid these disadvantages. According to the invention, therefore, the detector as first set forth above is characterised in that the flow path extends across the interior of the chamber transversely to its elongation without substantial change of direction; the light source means is positioned so that the input path is inclined towards the output path and is offset from the output path by an angle between 15 and 50 angular degrees; and in that the beam dump means is positioned in rectilinear alignment with the light source and the sampling region and outside the flow path to receive unscattered light directly from the light source means. Reference is also made to AU-B-31,843/84 which shows a construction generally similar to that shown in AU-B-3 1841/84 referred to above. In this second reference, however, the light source means is arranged to produce a conically shaped beam whose generatrix is perpendicular to the axis of the chamber. This arrangement has the same disadvantages as set forth above. A smoke detector is also known from WO 84/02790. This detector comprises an air-permeable housing through which air containing the smoke to be detected diffuses. In this detector, a light source directs light obliquely across the interior of the housing towards beam dump means. Any smoke through which the light beam passes may cause some of the light to be scattered into a light detector offset from the light beam by a relatively small angle. This detector is of a different type from the detector of the invention; it does not have a forced and predetermined flow path for the gaseous medium under test but relies purely on diffusion of ambient air into the housing. Smoke detecting apparatus embodying the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic axial cross-section through one of the smoke detectors; and Figure 2 is a block circuit diagram of the smoke detector of Figure 1. The smoke detector shown in Figure 1, comprises a housing 10 of circular cross-section, such as formed in plastics and closed off at each axial end. As shown in the Figure, it is closed off at one end by a plug 12 and at the other end by an end cap 14. The closed-off housing provides an air sampling chamber 16 having diametrically opposed inlet/outlet apertures 18 and 20. Purely by way of example, the internal diameter of the housing 10 may be 58 mm. and each of the apertures 18,20 may have a diameter of 25 mm. Air to be tested for the presence of any smoke particles is drawn into the sampling chamber 16 from outside by means of a fan or suitable pump (not shown), and any necessary associated pipe work, and passes through the chamber in the direction (in this example) of the arrows shown. The housing 10 supports a light source 22. One example of a suitable light source is a laser. The light source 22 produces a light beam shown diagrammatically at 24 which passes obliquely across the chamber 10 to a beam dump arrangement 26. The design of the beam dump arrangement 26 is such as to prevent reflection of the light back into the sampling chamber 16. Reflection is minimised by so designing the beam dump arrangement that any reflection which does take place is multiple reflection which is confined within the beam dump arrangement and does not pass back into the sampling chamber. In addition, all internal surfaces of the housing 10 are painted matt black to minimise such reflection. The housing 10 also incorporates a light sensor 28 which is mounted close to the end cap 14 and supported axially of the housing. Its field of view is indicated diagrammatically at 30 and is defined by five (in this example) annular baffle plates 32,34,36,38 and 40. The light sensor 28 may be of any suitable form, such as an avalanche photo-diode. The apertures defined by the baffles 32 to 40 may be (taken from left to right in the Figure) 8 mm., 7 mm., 6 mm., 5 mm. and 4 mm. In use, air to be sampled is fed across the sampling chamber 16 in the direction of the arrows shown via the inlet and outlet apertures 18,20. The light beam 24 passes obliquely across the sampling chamber. In the absence of any particles within the air, the light beam will simply pass across the sampling chamber and enter the beam dump 26, and essentially none of the light will reach the light sensor 28. However, if any smoke particles are present in the sampled air, these, when present within a sampling volume indicated diagrammatically at 42, will cause some of the light to be scattered and to pass along the path 30 so as to be received and sensed by the light sensor 28 - thus producing a corresponding electrical output as will be more specifically described below. The plug 12 carries a conical light stop 44 having a right angle at its vertex for preventing the reflection of any stray light along the path 30. The direction of the light beam 24 is offset from the axis of the housing 10, and thus offset from the light path 30, by an acute angle lying between 15 and 50 degrees. Figure 2 illustrates the circuit diagram of the detector in block diagram form. The circuit includes a power supply unit 50 producing an output voltage on a line 54 of suitable type and magnitude. Advantageously, the power supply is provided with a re-chargeable battery back-up (not shown). The power supply 54 energises an oscillator circuit 56 which provides an output power supply at a predetermined frequency on line 58 to energise the light source 22 correspondingly. In addition, the power supply feeds an amplifier 60 and a phase-sensitive detector circuit 62 and also energises the light sensor 28 if it is of a type requiring such energisation. In response to receipt of light, the light sensor 28 produces an electrical output on line 66 whose magnitude varies in frequency corresponding with the frequency of the amplitude variations of the received light. The electrical output is amplified by amplifier 60 and supplied to the phase-sensitive detector circuit 62. The phase sensitive detector 62 is supplied on a line 68 with a reference signal at the frequency of and in phase with the output of oscillator 56, this reference providing a frequency and phase reference for the phase-sensitive detector circuit 62. The latter therefore discriminates between output signals produced by the light sensor 28 as a result of receipt of scattered light from the light source 22 and output signals produced by the light sensor 28 as a result of light received from any other sources. The detector circuit 62 thus produces an output signal on a line 70 which is substantially only dependent on light scattered from the light source 22 and is substantially independent of stray light from any other sources. The signal on line 70 may be fed to a central monitoring point. The phase-sensitive detector 62 may be replaced by a narrow bandwidth filter tuned to the frequency of the oscillator 56. Alarm circuits may be provided, if required, to indicate failure of the light source and/or the fan or pump supplying air to the housing 10 and/or other components in the smoke detector.

Claims: A high sensitivity detector of suspended particles in a gaseous medium and capable of detecting the presence of a concentration of particles in the medium which attenuate light by less than one per cent per metre of the medium, comprising: an elongate hollow chamber (10); means for forcing a sample of the gaseous medium along a predetermined flow path (18,20) extending through the interior (16) of the chamber (10) between an entrance and an exit in the chamber wall, a portion of the flow path (18,20) constituting a sampling region (42); light source means (22) fixedly mounted in relation to the chamber (10) and operative when energised to direct a light beam along an input path which extends across the interior of the chamber (10) in a direction transverse to the elongation of the chamber (10) and also transverse to the said flow path (18,20) and passing to and through the sampling region (42), such that light in the beam is scattered by particles in the medium in the sampling region (42); light sensor means (28) fixedly mounted in relation to the chamber (10) and producing an output signal dependent on the light received and positioned to receive the scattered light along a predetermined output path (30) from the said sampling region (42); beam dump means (26) fixedly mounted in relation to the chamber (10) for receiving unscattered light directly from the light source means (22) and arranged to prevent substantially any reflection of such received light into the output path (30); and aperture defining means (32 - 40) extending across the interior of the chamber (10) transverse to its elongation and defining a plurality of apertures spaced apart along the length of the output path and each for defining the cross-sectional shape of the output path; characterised in that the flow path (18,20) extends across the interior of the chamber (10) transversely to its elongation without substantial change of direction; the light source means (22) is positioned so that the input path is inclined towards the output path and is offset from the output path by an angle between 15 and 50 angular degrees; and in that the beam dump means (26) is positioned in rectilinear alignment with the light source (22) and the sampling region (42) and outside the flow path (22) to receive unscattered light directly from the light source means (22). A detector according to claim 1, characterised by modulating means (56) for modulating the intensity of the light in the light beam in a predetermined manner, and output means (62) responsive only to corresponding modulation in the output signal of the sensor means (28) whereby to indicate detection of the said particles. A detector according to claim 2, characterised in that the modulating means (56) comprises means for electrically modulating the light source means with a predetermined modulation, and in that the output means (62) comprises a phase-sensitive output circuit to which the modulation signal is supplied as as reference. A detector according to claim 3, characterised by electronic circuitry utilising the phase-sensitive output circuit to achieve a high level of amplification. A detector according to claim 2, characterised in that the modulating means comprises means (56) for electrically modulating the light source means (28) with a predetermined modulation, and in that the output means comprises an amplifier (60) responsive to the output of the light sensor means (28) and having a narrow bandwidth filter to amplify only the component of the electrical output having the frequency of the predetermined modulation. A detector according to any preceding claim, characterised in that the beam dump means (26) comprises surfaces geometrically juxtaposed such that any of the light in the beam which they reflect is not directed towards the defined flow path (18,20), the surfaces being substantially non-light-reflective.

Inventors: BURGESS DOUGLAS EDWARD, DENNIS PETER NEIL JOHN, BURGESS, DOUGLAS EDWARD, DENNIS, PETER NEIL JOHN
Applicants: KIDDE FIRE PROTECTION LTD, KIDDE FIRE PROTECTION LIMITED
Application year: 1991
Main classification: G08B17
IPC codes: G08B17