Detector for scattered light

Details Detector for scattered light Detector for scattered light

2002-08-26

2004-12-14

Allen, Stephone B. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C356S436000, C356S441000

Reexamination Certificate

active

06831289

ABSTRACT:

The present invention relates to a detector for scattered light as part of a hazard detector, particularly for detecting particles in a carrier medium, with a housing, with an inlet and an outlet in the housing, between which the carrier medium flows through the housing on a flow path, with a light source, which directs light to a scattered light centre, which lies on the flow path, with a receiver for a part of the light which is scattered onto particles in the scattered light centre, and with a light trap for light which is not scattered in the scattered light centre.
Such types of detectors for scattered light are known and serve, especially in aspiration fire alarm systems, to detect solid matter or liquid particles, in which the carrier medium consists of a representative partial quantity of the air of a room to be observed or of the device cooling air of a device to be observed. In an aspiration alarm system, this representative quantity of air is actively suctioned by means of a ventilator and fed into the inlet of the detector for scattered light. In devices to be monitored, such as for instance, EDP equipment or individual components thereof, as well as in similar electronic devices, such as for example, measuring, control and regulating devices, relaying equipment, and PBX devices, it is basically also possible to use the internal flow of the device-cooling air to feed a representative partial quantity of the device cooling air as carrier medium into the inlet of the detector for scattered light. An active suctioning ventilator is then unnecessary.
While the carrier medium flows through the scattered light centre on its flow path through the housing of the detector for scattered light, the light of the light source traverses the scattered light centre, and consequently, the carrier medium flowing through it, and, provided that it is not scattered onto particles in the carrier medium, is absorbed in the light trap opposite. The detector for scattered light is predominantly in this operating state. If the ray of light meets a particle, which could be, for example, a smoke particle or smoke aerosol, which provides the first indication of a fire in the initial stages, this particle diverts a fraction of the light as scattered light from its original direction, which is then absorbed by a highly light-sensitive receiver and whose intensity is measured by means of a subsequent evaluation circuit. If a certain threshold value of the light intensity is exceeded, an alarm is triggered.
Detectors for scattered light for detecting particles in a carrier medium are known from EP 0 756 703 B1 and EP 0 729 024 A2, in which the carrier medium flows through the housing in a longitudinal direction and either several light sources facing each other (EP '703) or a receiver (EP'024) are arranged on the longitudinal wall of the housing. These known detectors for scattered light are disadvantageous in that, for one thing, in light sources opposite each other, there is a risk that a majority of the light of a light source sent is reflected on the glass body of an opposite light source and a part of this reflected light then falls unintentionally on the light-sensitive receiver, consequently making it more difficult to determine the scattered light portion. On the other hand, as far as the arrangement of the receiver on the longitudinal wall of the housing goes, it is disadvantageous that this is easily dirtied, since it is placed in the flow path, which could lead to reduced responsiveness or else to an increased error rate.
Detectors for scattered light of the type mentioned at the start are known from EP 0 463 795 B1 and WO 97/42485, in which the flow path of the carrier medium runs crosswise to the longitudinal direction of the housing, and consequently, crosswise to the receiver axis. The disadvantages of these known detectors for scattered light, in particular, are that the inlets and outlets placed crosswise to the housing with the feeding pipes for the carrier medium to be connected thereto do not facilitate either a compact construction of the detector for scattered light itself or its compact arrangement within a larger detector housing, in which, for example, an air current sensor and the evaluation circuit are also accommodated.
Finally, a scattered light measuring device of the type mentioned in the beginning is known from EP 0 257 248 A2, which exhibits a funnel or paraboloid-shaped light trap for light which is not scattered in the scattered light centre, with said light trap opening towards the light source.
The purpose of the present invention is to develop a detector for scattered light, of the type mentioned at the start, i.e., with a housing, with an inlet and an outlet in the housing, between which the carrier medium flows through the housing on a flow path, with a light source, which directs light on a scattered light centre, which lies on the flow path, with a receiver for a part of the light scattered in the scattered light centre onto particles, and with a light trap for light not scattered in the scattered light centre, in such a way as to ensure a compact structural shape and yet maintain high responsiveness.
This purpose is solved in a detector for scattered light of the previously described type with two alternative and highly advantageous embodiments of the light trap, as described in patent claims
1
and
2
. According to a first alternative, it is provided for the light source to be placed outside the flow path, furthermore, for the centre axis of the light cone of the light source to run, at least partially, parallel in relation to or on the centre line of the flow path, and finally, for the light trap allocated to the light source to be part of the flow channel guiding the flow path. According to a second alternative, which can also be chosen cumulatively, the receiver is arranged outside the flow path, and the receiver axis runs, at least partially, in parallel in relation to or on the centre line of the flow path, and the light trap allocated to the receiver is part of the flow channel that guides the flow path.
The two embodiments according to the invention of the detector for scattered light lie are advantageous in that the light trap allocated to the light source, as well as the light trap allocated to the receiver, is at the same time a part of the flow channel that conducts the carrier medium, for example, the representative partial quantity of the device cooling air of an EDP device, on the flow path through the detector for scattered light. In the process, it is advantageous when—as provided in an embodiment of the detector for scattered light according to the invention—the flow channel exhibits a bend where it functions as a light trap, so that the flow path of the carrier medium is diverted, and consequently, the light source “looks into empty space” towards the centre axis of its light cone and/or the receiver towards the receiver axis, as a result of which interfering reflections are excluded.
Advantageous embodiments of the invention are specified in the sub-claims.
First, two alternative embodiments of the shape of the light trap, which is allocated to the light source, are provided. According to a first alternative, this light trap is designed in such a way that, when seen from a cross sectional plane, which is vertically positioned on the receiver axis level formed by the receiver axis and the centre axis of the light cone of the light source, it exhibits the shape of a funnel, which opens towards the light source, and, cf.
FIGS. 10 and 11
, towards the receiver respectively. According to a second alternative, the light source is designed in such a way that—again as seen in the previously described cross-sectional plane—it approximately exhibits the shape of a parabola, whose opening points to the light source and, cf.
FIGS. 10 and 11
, towards the receiver. The advantages of the embodiment of the light trap according to the invention in both cases lie in the fact that light sent by the light source and not scattered in the sc

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