GB2158573A

GB2158573A - Scattered-light type smoke detector

Info

Publication number: GB2158573A
Application number: GB08506035A
Authority: GB
Inventors: Tetsuya Nagashima
Original assignee: Hochiki Corp
Current assignee: Hochiki Corp
Priority date: 1984-03-09
Filing date: 1985-03-08
Publication date: 1985-11-13
Also published as: JPS60148688U; DE3507996A1; NO850914L; JPH0435990Y2; CH662196A5; GB8506035D0

Abstract

In a scattered-light type smoke detector which is provided with peripheral smoke inlets 2 for introducing smoke into an internal dark chamber, a light emitter 6 which radiates light into the dark chamber and a light receiver 7 which receives light radiated from the light emitter and scattered by smoke in the chamber are disposed in such a manner that the optic axes thereof intersect at an angle theta of substantially 145 DEG C. <IMAGE>

Description

SPECIFICATION Scattered-light type smoke detector This invention relates to a scattered-light type smoke detector which is capable of detecting smoke by radiating light into a smoke detection area from a light emitting section and letting light scattered by smoke which enters the smoke detection area be incident upon a light receiving section.

A smoke detector of this type is required to provide a photo-output which has a predetermined detection sensitivity for smoke having a certain extinction rate.For example, according to the most strict UL standard, it is required that a predetermined fire detection sensitivity be obtained for an extinction rate of 4%ifs or less with respect to grey smoke generated by burning of a cotton wick and for an extinction rate of 10%/fit or less with respect to black smoke generated by burning of an oil such as kerosine.

It is an object of the present invention to provide a scattered-type smoke detector having a smoke detection characteristic which satisfies the strict UL standard, and which is capable of maximizing the amount of light scattered by smoke and capable of minimizing light noise when no smoke exists.

In accordance with the present invention, there is provided a scattered-light type smoke detector comprising a dark chamber having, at the periphery thereof, inlets for enabling smoke to enter the chamber, a light emitting section for radiating light into the dark chamber; and a light receiving section for receiving light radiated from said light emitting section and scattered by smoke; said light emitting section and said light receiving section being so disposed that the optic axes thereof intersect at an angle of substantially 145".

In a detector in accordance with the present invention, a photo-output which satisfies the detection sensitivity required by the UL standard can be obtained both for grey smoke having an extinction rate of 4%/ft or less and for black smoke having an extinction rate of 10%/ft of less and light noise can be minimized.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an explanatory view of one embodiment in accordance with the present invention; Figure 2 is a graph showing the relationship between photo-outputs of a scattered-light type smoke detector according to the present invention and smoke density with respect to grey smoke and black smoke; Figure 3 is a graph showing the relationship between photo-outputs of a scattered-type smoke detector according to the present invention and smoke density with respect to grey smoke and black smoke, when the intersection angle of the optic axes is small; and Figure 4 is a graph showing the relative outputs when a photo-output in the case of an intersection angle of 145" is assumed to be 1.

A preferred embodiment of the present invention is now described.

Referring to Figure 1, a detector casing 1 having a detector circuitry incorporated therein is fitted to a detector base which is adapted to be fixed to a ceiling etc. An outer cover 3 having, at the periph ery thereof, a plurality of smoke inlets 2 is provided at a lower portion of the detector casing 1. In the outer cover 3, there is provided an inner cover 4 having a labyrinth structure which prevents external light from entering through the inlets 2 and constitutes a passage for introducing smoke. The outer cover 3 and the inner cover 4 are fitted to the lower portion of the detector casing 1 to form a dark chamber therein.

A holder 5 is fitted to an upper inner wall of the dark chamber, i.e. a bottom of the detector casing 1. A light emitting element 6 is mounted on the right-hand side of the holder 5 (as viewed in Figure 1) and a photodetector 7 is mounted on the lefthand side thereof behind a lens 8. The light emitting element 6 and the photodetector 7 are so disposed that the optic axes 9 and 10 thereof may be directed slantingly downwardly so as to intersect at a smoke detection area 11 located at a central portion of the dark chamber.The optic axis 9 of the light emitting element 6 and the optic axis 10 of the photodetector 7 intersect at an included angle of substantially 145". The positions of the light emitting element 6 and the photodetector 7 are adjusted as required when they are mounted, but there may be an allowance of the intersection angle 6 within a range of 145 + 2.5 due to structural limitations when mounting the light emitting element 6 and the photodetector 7.

By setting the intersection angle 0 of the axes of the light emitting element 6 and the photodetector 7 in the smoke detection area 11 to be 145 , there can be obtained photo-outputs which can provide predetermined detection sensitivities with respect to grey smoke having an extinction rate of 4%/ft or less and black smoke having an extinction rate of 10%/ft or less, as required by the UL standard.

The reason why the intersection angle H of the optical axes of the light emitting element 6 and the photo- detector 7 at the smoke detection area 11 is set to 145 is now described in detail.

It has been confirmed experimentally that when the intersection angle 0 of the optic axes of the light emitting element 6 and the photodetector 7 is changed, a difference between the photo-output with respect to grey smoke and the photo-output with respect to black smoke becomes larger as the intersection angle 6 becomes smaller.

Figure 2 shows photo-outputs for grey smoke and for black smoke in relation to varying smoke densities when the intersection angle 6 is set as 145 according to the present invention. If a photooutput which provides a required detection sensitivity is assumed to be 1.8V, such a photo-output of 1.8V is obtained for a smoke density having an extinction rate of 4%ifs with respect to grey smoke which is generated by burning of a cotton wick and for a smoke density having an extinction rate of 10%/fit with respect to black smoke which is gener ated by burning of kerosine.

Figure 3 shows photo-outputs for varying smoke densities of grey smoke and black smoke when the intersection angle 6 is decreased to 90". While a required photo-output of 0.6V is obtained for grey smoke whose density has an extinction rate of 4%/ ft, 0.6V is obtained for black smoke only when its density is increased to have an extinction rate of 16%/fit.

Figure 4 shows relative photo-outputs in relation to intersection angles 6 when the photo-output in the case of an intersection angle 6 of 145 is assumed to be 1. As the intersection angle 6 becomes smaller, the relative outputs decrease. From Figure 4, it can be understood that the maximum amount of light scattered by smoke is obtained when the intersection angle 6 is 145".

As apparent from the experimental data shown in Figures 2, 3 and 4, the smaller the intersection angle 6 the larger is the relative difference between photo-outputs for grey smoke and black smoke. An intersection angle 6 at which the required sensitivities are obtained both for grey smoke having an extinction rate of 4%/fit or less and black smoke having an extinction rate of 10%ifs or less is 145 as can be seen from the characteristic graph of Figure 3.

As the intersection angle 6 becomes larger, the light emitting element 6 and the photodetector 7 are brought into positions where they confront each other more directly, which increases light noise as can be seen from Figure 1. For this reason, the intersection angle 6 is preferred to be within the range of 145 i 2.5 as described above.

With the arrangement of Figure 1 wherein the smoke detection area 11 is located under the smoke detecting section comprised by the holder 5, the light emitting element 6 and the photodetector 7, in the position where the smoke detector is installed onto the ceiling etc., a printed board for the circuitry of the detector within the detector casing 1 can be formed integrally with the smoke detector section and smoke can be introduced smoothly and quickly into the smoke detection area through the inlets 2.

Although in the embodiment described above the light emitting element 6 and the photodetector 7 are disposed so that the intersecting optic axes thereof are directed downwardly, in other embodiments the intersecting optic axes may be directed in other directions such as a horizontal direction, i.e. by mounting the casing 1 on a vertical wall surface.

Claims

1. A scattered-light type smoke detector comprising a dark chamber having, at the periphery thereof, inlets for enabling smoke to enter the chamber, a light emitting section for radiating light into the dark chamber; and a light receiving section for receiving light radiated from said light emitting section and scattered by smoke; said light emitting section and said light receiving section being so disposed that the optic axes thereof intersect an at angle of substantially 145".

2. A scattered-light type smoke detector as claimed in claim 1, wherein said light emitting section and said light receiving section are disposed within the dark chamber.

3. A scattered-light type smoke detector as claimed in claim 2, wherein said light emitting section and said light receiving section are mounted on a holder disposed within said dark chamber.

4. A scattered-light type smoke detector as claimed in claim 1, 2 or 3 wherein the intersection angle of the optic axes is within a range of 145 + 2.5 .

5. A scattered-light type smoke detector as claimed in any of claims 1 to 4, wherein said light emitting section and said light receiving section are housed in an upper portion of the dark chamber and are disposed so that the axes thereof are directed slantingly downwardly.

6. A scattered-light type smoke detector substantially as hereinbefore described with reference to and as illustrated in Figure 1 of the accompanying drawings.