GB2114286A - Combination optical scattering and ionization type smoke detectors - Google Patents

Abstract

A combination type optical scattering and ionisation type smoke detector comprises a housing formed with a plurality of lamina members 7, 7', 7'' which allow smoke but not light to enter the dark chamber 70. Inside the chamber is arranged an intermediate electrode 11 and an inner electrode (not shown) provided with a fine adjustment screw 4. The outer electrode of the ionisation smoke detector is provided by the housing itself which is made of electroconductive plastics material. Also within the housing is a light-scattering smoke detector 200 consisting of a light emitter 1 and sensor 6 so arranged that stray light cannot be reflected to the sensor via the lamina members 7, 7', 7'' and the adjustment screw 4 or supporting members. <IMAGE>

Description

SPECIFICATION Improvements in smoke detectors This invention relates to a smoke detector typically used for fire alarm purposes, particularly to a combination type smoke detector in which a light-scattering smoke detector and an ionization smoke detector are used in combination. More particularly this invention relates to such a combination type smoke detector in which a light-scattering smoke detector and an ionization smoke detector are contained in a dark chamber of the so-called labyrinth structure having a light barrier comprising a plurality of angled lamina members which allows passage of smoke but prevents transmission of light therethrough.

As the smoke detector, light-scattering type detector and ionization type detector are known. The two detectors are different in sensitivity to smoke. Therefore it is desirable to use these two detectors in combination in order to detect smokes of various natures.

In a combination type smoke detector, a light-scattering smoke detector and an ionization smoke detector are provided in a housing. Therefore this type of smoke detector is inevitably of large size. In order to eliminate this inconvenience, it was considered to incorporate the above-mentioned two kinds of smoke detectors in a common smoke detection chamber (dark chamber) wherein said chamber is used as an outer electrode of the ionization smoke detector. However, the realization of this idea has been hindered by the difficulty stated below.

The most efficacious light-scattering smoke detector available today is provided with a dark chamber of so-called labyrinth structure as disclosed in U. S. Patent No. 4,216,377.

The labyrinth dark chamber comprises a circular bottom plate and a lateral wall which formed by a plurality of angled lamina members as the means for allowing passage of smoke into the chamber while preventing penetration of light. Each lamina member has a cross section comprising a first straight or slightly arcuate portion along the circumference of the bottom plate of the dark chamber, a second straight portion extending from one end of the first portion at an obtuse angle and a third portion extending from the free end of the second portion at an angle to the direction opposite to that of the first portion. The lateral wall of the dark chamber is formed by arranging a plurality of such laminae in an interlocking relation with serpentine paths left therebetween.Such a complicated structure cannot be economically manufactured of a metallic material, and labyrinth dark chambers are usually made by molding a plastic material.

However, a dark chamber of a plastic material cannot be used as an electrode.

We tried to use electroconductive plastic materials for molding the labyrinth dark chamber and have found that labyrinth dark chambers made of an electroconductive plastic well function as the outer electrode for the ionization smoke detector.

On the other hand, in a light-scattering smoke detector, it is essential to prevent unwanted light which produces noise to the wanted signals from reaching the light-sensing element. Therefore, careful consideration is paid in order to insure that the inside wall and members and parts in the dark chamber do not reflect light. Installation of an additional ionization smoke sensing unit add to the members and parts which may reflect light.

Typical members of the ionization smoke detector which may reflect light are an intermediate electrode, a sensitivity-adjusting screw and supporting members.

In the light-scattering smoke detector, it is believed that the inside surface of the dark chamber should be finished at fineness of 800 mesh or more so as to prevent diffused reflection rather than giving it a light-absorbing finish such as matted finish or lusterless black coating. Because it is rather better to reflect light in the direction away from the light-sensing element than causing diffused light reflection, and the light-absorbing finish is generally expensive. In this sense, the most desirable light-scattering smoke sensor known so far is provided with fine inside finish and a conical projection (reflecting cone) on the bottom plate in order to deflect the reflected light away from the direction of the light-sensing element. In case where the dark chamber is sufficiently tall, the reflecting cone is unnecessary.

When the labyrinth light-scattering smoke detector of U. S. Patent No. 4,216,377 was invented, nothing was noticed with respect to the relation between orientation of the angled laminae of the labyrinth and position of the light source. In designing combination type smoke detectors, we have found that when the relative position of the light source and the light-sensing unit is reversed, the signal to noise ratio is remarkably improved. Thus we have concluded that noise is remarkably reduced if the light source, the light-sensing unit and members and parts in the dark chamber are arranged so that the light-emitting head and members which may reflect the light from the light source are located on the same side with respect to any normal line to any surface of the lamina members which may receive the light from the light source.

According to this invention, there is provided a combination type smoke detector provided with a light-scattering smoke detector in which the light source and the light-sensing unit are located in a common vertical plane, and an ionization smoke detector, a dark chamber of labyrinth structure consisting of a generally cylindrically lateral wall comprised of a plurality of angled lamina members, a bottom plate and a base plate, said dark chamber being made of an electroconductive plastic material and functioning as the outer electrode.

According to a further aspect of this invention, there is provided a combination type smoke detector as described above, wherein the light source and the light-sensing unit of the light-scattering smoke detector and the members of the ionization smoke detector are arranged so that members which may reflect the light from the light source and the lightemitting head of the light'source are located on the same side with respect to any normal line to any surface of the lamina members which may receive the light from the light source.

The apparatus of this invention can be easily manufactured by those skilled in the art in accordance with the information provided in this specification, using known materials. It is not necessary to mention details of designing and manufacturing, except that the preferred electroconductive plastic materials are polymers which contains electroconductive carbon black powder, especially acetylene black. The usable polymer includes polyvinyl chloride, polystyrene, ABS resins etc. There is no limitation in carbon black content. But preferred range is 10-30%, and preferred plastic materials are polyvinyl chloride and polycarbonate resin containing about 15% acetylene black. The labyrinth dark chamber is made integrally by molding one of these electroconductive plastic materials.

Now the invention is described in detail with respect to a preferred embodiment with reference to the attached drawings.

Figure 1 is a schematic elevational crosssectional view showing the structure of an embodiment of the combination type smoke detector in accordance with the first aspect of this invention.

Figure 2 is a schematic plan view showing the relative location of a light-scattering smoke detector and an ionization smoke detector in a labyrinth dark chamber arranged in accordance with the second aspect of this invention, in said location undesirable reflection of light does not occur.

Figure 3 is a schematic plan view showing another relative location of a light-scattering smoke detector and an ionization smoke detector in a labyrinth dark chamber, in said location undesirable reflection of light occurring.

Figure 4 is an elevational cross-sectional view along line A-A in Fig. 2.

The conception of the arrangement of the members in the dark detection chamber is illustrated in Fig. 1 and 2 with respect to a preferred embodiment of this invention. Fig. 1 is an elevational cross-sectional view of a preferred embodiment approximately corresponding to line B-B in Fig. 2. The combination type smoke detector of this invention comprises a labyrinth dark chamber 70 which was made by molding polycarbonate resin ("Teijin Panlight L-1225") containing about 15% by weight of acetylene black, and functions as the outer electrode of the ionization smoke detector, a light-scattering smoke detector 200, an ionization smoke detector 100, a base plate 300 and a housing 400.

Of these elements and the arrangement thereof, those except the ionization smoke detector is well described in the above mentioned U. S. Patent No. 4,216,377, and therefore we do not explain them in detail thereon. Also the ionization smoke detector per se is well known among those skilled in the art, and thus we need not specifically explain thereon here. Only here it is pointed out that a sensitivity adjustment screw 4 is secured on the bottom plate 72 of the labyrinth dark chamber 70 and thereunder an intermediate electrode 11 and an inner electrode 1 2 are provided.

As disclosed in the above-mentioned U. S.

patent specification, the labyrinth dark chamber is composed of a bottom plate 72 and a plurality of angled lamina members 71 which are formed as planted on the periphery of the circular bottom plate 72, so that they form a cylindrical wall with a serpentine path provided between two neighboring members through which smoke can flow in. The angled lamina members form a barrier which prevents the penetration of light therethrough as is well explained in the above-mentioned patent. The dark chamber is secured on the base plate 300 and covered by a housing 400, which of course allows passage of smoke.

In the case of the light-scattering smoke detector with a labyrinth dark chamber, it suffices if the angled lamina members and a light source are designed and arranged so that the light emitted from the light source is reflected by the surface of the angled lamina members so as not to enter the light-sensing element 6. But in the case of this invention, parts and members, especially the sensitivityadjusting screw 4 of the installed ionization smoke detector, reflects light and the reflected light enters the light-sensing unit, if the ar ranger ent is improper.

Reference should be made to Fig. 3. When the angled lamina members, the light-scattering smoke detector and the ionization smoke detector are arranged as shown in this drawing, if the light source 1 is placed on the left side as shown in this figure, a ray 2 emitted from the light source 1 may hit the surface of a lamina member. The ray may be reflected by the surface of the lamina member and the reflected ray then may hit the sensitivity adjustment screw 4 and be reflected 5 so as to enter the light-sensing unit 6.

In contrast, in this invention, as seen in Fig.

2, the light source 1 is located on the right side. In this arrangement, any ray emitted from the light source is reflected by the surface of a lamina member 7 in a direction away from the adjusting screw of the ionization smoke detector 100. Therefore, no rays hit the sensitivity adjustment screw. This relation can be expressed as follows. The light reflecting members such as sensitivity adjustment screw, intermediate electrode and the light-emitting head of the light source are placed on the same side with respect to any normal line 8 to the inside surfaces of the angled lamina members.

The rays from the light source are projected to the bottom surface of the dark chamber. In this embodiment, the light-scattering smoke detector is designed so that the rays from the light source 1 may hit the conical projection of the bottom plate with large incidental angles and thus may be reflected to the directions away from the light-sensing unit. In other words, in this embodiment, the sensitivity adjustment screw 4, the intermediate electrode 11 etc. and the light source are arranged so that they are located on the same side with respect to the normal line 10 to the bottom surface at the point where the outermost ray of the light flux from the light source 1 hits the conical surface.

Thus, as has been described above, this invention provides a combination type smoke detector which brings about a good sensitivity to smoke with a very high signal to noise ratio even if the inside surface of the dark detection chamber is not finished with light absorbing coating.

This invention provides a low cost and small size combination type smoke detector for the first time. According to this invention, the cost for finishing the inside surface of the dark detection chamber is reduced.

Claims (8)

1. A combination type smoke detector provided with a light-scattering smoke detector and an ionization smoke detector which comprises a dark chamber of labyrinth structure consisting of a generally cylindrical lateral wall comprised of a plurality of angled lamina members and a bottom plate, and a base plate, said dark chamber being made of an electroconductive plastics material and functioning as the outer electrode of the ionization smoke detector.

2. A combination type smoke detector as claimed in claim 1, in which the bottom plate of the dark chamber is provided with a reflecting cone.

3. A combination type smoke detector as claimed in claim 1 in 2, of which the dark chamber is integrally made by molding an electroconductive plastics material.

4. A combination type smoke detector as claimed in claim 1, 2 or 3 wherein the electroconductive plastics material is a carbon black containing polymer, said polymer being selected from a group consisting of polyvinyl chloride, polystyrene, acrylic resin, ABS resin, polyamide resin, polyacetal resin, polycarbonate resin, and polyolefin resin.

5. A combination type smoke detector as claimed in claim 4, wherein the polymer contains 10-30% by weight of acetylene black.

6. A combination type smoke detector as claimed in claim 5, wherein the electroconductive plastics material is polycarbonate resin containing 12- 18% by weight of acetylene black.

7. A combination type smoke detector according to any of the preceding claims wherein the light source and the light sensing unit of the light-scattering smoke detector and the members of the ionization smoke detector are arranged so that members which may reflect light from the light source and the light-emitting head of the light source are located on the same side with respect to any normal line to any surface of the lamina members which may receive the light from the light source.

8. A combination type smoke detector substantially as herein described with reference to the accompanying drawings.