Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1998-10-28
2001-02-06
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C340S630000
Reexamination Certificate
active
06184537
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a device for the detection of airborne pollutants. More particularly the invention relates to a detector for smoke and other airborne pollutants as may be generated in the event of a fire or in circumstances which can lead to a fire.
BACKGROUND OF THE INVENTION
Fire protection and suppressant systems which operate by detecting the presence of smoke and other airborne pollutants are well known. Upon a threshold level of smoke being detected, an alarm may be activated and operation of a fire suppressant system may be initiated. While the fire itself will cause damage, considerable damage can also be caused by operation of the fire suppression system and subsequent removal of the suppressant can be quite hazardous. Many traditional suppressants, such as halon, are also ozone depleting whereby this use is environmentally undesirable. A detection system which is sufficiently sensitive to detect an abnormal condition prior to the onset of a fire is very advantageous as it enables action to be taken at a very early stage before the onset of actual fire conditions. For example, when most substances are heated, even before heating occurs to a point at which a fire commences, emissions will be generated and if these can be detected by a very sensitive system, a warning provided at that very early stage may allow the problem to be detected and rectified, or the equipment turned off, before the fire actually starts.
It is also desirable for the detection system to have a wide dynamic range of operation whereby it is effective not only at low levels of smoke and other airborne pollutants as may be generated prior to the onset of actual fire conditions as discussed above, but also is able to detect a range of higher threshold levels of smoke and other pollutants. High levels of smoke will indicate a greater likelihood of there being a fire and the higher thresholds can trigger alarms to shut down air conditioning, close fire doors, call a fire fighting service, and eventually trigger a suppression system if the smoke level becomes sufficiently high.
It is known for detection systems to incorporate a sampling pipe network consisting of one or more sampling pipes with sampling holes installed at positions where smoke or pre-fire emissions can be collected. Air is drawn in through the sampling holes and along the pipe by means of an aspirator or fan and is directed through a detector at a remote location.
Although there are a number of different types of smoke detectors which can be used as the detector in a system as outlined above, one particularly suitable form of detector for use in such a system is an optical scatter detector which is able to provide good sensitivity at reasonable cost. Optical scatter detectors operate on the principle that smoke particles or other airborne pollutants of small size when introduced into a detection chamber having a high intensity light beam will cause light scatter. The scattered light is sensed by a scattered light detector. The greater the amount of smoke particles within the sample introduced into the detector chamber the greater will be the amount of light scatter. The scatter detector detects the amount of scattered light and hence is able to provide an output signal indicative of the amount of smoke particles or other pollutant particles within the sample flow.
SUMMARY OF THE INVENTION
An objective of the present invention is to provide a smoke detector of optical scatter type having improved sensitivity to the presence of smaller amounts of smoke and other pollutants, and in particular to the presence of relatively small quantities of such pollutants as may be generated prior to the onset of actual fire conditions.
A significant factor affecting the sensitivity of a detector of optical scatter type is the amount of background light within the detector chamber. In particular, if very low levels of light scatter are to be detected as will occur when very small amounts of smoke or other pollutant particles are within the sample airflow through the chamber, the background light level must be very low. Although various electronic techniques can be used to improve the signal to noise ratio of the output signal from the detector, these techniques are all limited by the background light level within the detector.
The present invention relates to various aspects of detailed design of a detector which operates on the optical scatter principle, in order to reduce the background light level within the detector and thereby improve the sensitivity of the detector and the signal to noise ratio of the output.
According to one aspect of the present invention there is provided a smoke detector comprising a detector chamber, an inlet for introducing an airflow to be sampled into the chamber, an outlet for said airflow from the chamber, means for generating a beam of light within the chamber, a scattered light detector responsive to the presence of scattered light within a zone of the chamber consequent on the presence of smoke particles within the sampled airflow in said zone, a light absorber at an end of the chamber remote from the light source for receiving and absorbing said beam after passage through the chamber, first collimator means for collimating the beam, and second collimator means beyond the first collimator means, said second collimator means comprising a collimator disc having an aperture of a size such that the beam collimated by the first collimator means passes through the aperture in the disc without contacting the edge of the aperture, said disc serving to trap glints of light arising from the first collimator means to thereby reduce stray light in the said zone at which detection of light scatter occurs, said zone being beyond the second collimator means.
Advantageously, the first collimator means comprises a collimator disc (a first collimator disc) the aperture of which is preferably smaller than the aperture of the disc of the second collimator means (the second collimator disc), the second disc serving to trap glints of light arising from the edge of the aperture of the first disc.
Advantageously a third collimator disk is positioned beyond the second disk and with an aperture size such that the collimated beam passing through the aperture in the second disk also passes through the aperture in the third disk without touching the sides of the aperture, the third disk serving to trap at least the significant majority of glints of light which might happen to pass beyond the second disk whereby to prevent such glints from reaching the zone at which detection of light scatter takes place.
Advantageously the first and second disks are located as close to the light source as practicable and the third disk is placed as close to the detection zone as practicable, for example closely adjacent to the inlet for the sampled air.
Further according to the present invention, there is provided a smoke detector having a detector chamber, an inlet for introducing an airflow to be sampled into the chamber, an outlet for said airflow from the chamber, means for generating a beam of light within the chamber, a scattered light detector responsive to the presence of scattered light within the chamber consequent on the presence of smoke particles within the sampled airflow, and a light absorber at an end of the chamber remote from the light source for receiving and absorbing the beam after passage through the chamber, said detector further comprising a series of collimator discs with apertures of progressively increasing size associated with the light source to prevent glints of light from entering the zone of the chamber at which detection occurs as a result of the presence of scattered light induced by the presence of smoke particles.
According to another aspect of the invention there is provided a smoke detector having a detector chamber, an inlet for introducing an airflow to be sampled into the chamber, an outlet for said airflow from the chamber, means for generating a beam of light within the chamber, a
Knox Ronald
Ryan Christopher T.
Lee John R.
Pyo Kevin
Vision Products Pty Ltd.
Wolf Greenfield & Sacks P.C.
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