Scattered light fire detector

Details Scattered light fire detector Scattered light fire detector

2000-01-19

2001-04-17

Tong, Nina (Department: 2736)

Communications: electrical

Condition responsive indicating system

Specific condition

C340S628000, C340S619000

Reexamination Certificate

active

06218950

ABSTRACT:

CROSS-REFERENCES TO RELATED APPLICATIONS
This application claims the priority of German Patent Application, Serial No. 199 02 319.0, filed Jan. 21, 1999, the subject matter of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
The invention is directed to a method for evaluating scattering signals measured with a scattered light system of a fire detector which may include a microprocessor. The scattering signals are measured at two scattering angles to determine an alarm value which is compared with an alarm threshold. The invention is also directed to a fire detector for carrying out the method.
Scattered light fire detectors typically operate with infrared light emitted by a transmitter diode at a wavelength between 800 nm and 1 &mgr;m. The fire produces an aerosol which enters a measurement volume of the fire detector. The light scattered by the aerosol is measured at a backscatter angle, i.e., at an angle between 0° and 90°, and/or a forward scattering angle, i.e., a scattering angle between 90° and 180°. These angles are in relation to an axis connecting the transmitter with the receiver.
The measurement of light aerosols at a forward scattering angle produces a relatively large measurement signal. Conversely, the measurement of dark aerosols at a forward scattering region produces a measurement signal which is smaller by approximately a factor 10. The magnitude of the measurement signals increases with increasing forward scattering angle. The signal produced in the backscattering regime is independent of the type of smoke and smaller than in the forward scattering regime. The difference between the scattering signals of light and dark aerosols in the backscattering regime is noticeably smaller than in the forward scattering regime.
Conventional scattered light fire detectors operating in the forward scattering regime recognize different types of dark smoke less reliably than different types of light smoke. Accordingly, the sensitivity of the fire detectors has to be adjusted to the dark smoke to safely trigger an alarm. Such a sensitivity setting, however, tends to cause a high incidence of false alarms, since the detector is too sensitive to the light smoke. In particular, a false alarm can be triggered by water vapor, cigarette smoke, vapors or fumes produced by hot grease. Conventional scattered light fire detectors are therefore not suitable for use, for example, in large kitchens or in saw mills, since the intensive vapors and dust produced in these places can be easily mistaken for light smoke.
Fire detectors operating in the backscattering regime, however, are adversely affected by particles and dust or by salt crystals which can enter the measurement volume of the fire detector and produce a significant backscatter signal, thereby producing a significant risk of false alarms.
German Pat. No. DE 42 31 088 A1 discloses a method wherein scattering signals of an aerosol which may be present in the measurement volume of a scattered light fire detector, are measured under at least two scattering angles and compared with reference data for various types of smoke which are stored in a memory. The method determines the type of smoke present in the measurement volume and sets an alarm value depending on the type of smoke. However, this method is suitable mainly for analyzing known types of smoke using the reference data stored in the memory, and may produce erroneous results for the more frequently occurring mixed fires, since such mixed fires cannot be adequately classified.
SUMMARY OF THE INVENTION
It is therefore desirable to provide a method which reliably recognizes the most common types of smoke and which can in particular evaluate mixed fires, without setting off a disproportionate number of false alarms.
According to one aspect of the invention, a method is provided which determines the alarm value as a function of the ratio of the scattering signals.
This approach takes into account that most common fires are mixed fires which produce aerosols which cannot always be unambiguously classified. The ratio of the scattering signals, also referred to as the light-dark-quotient, produces a continuous rating of the aerosols which may be present in the measurement volume of the fire detector, thereby obviating the need to store predetermined smoke patterns for comparison with the measurement result. For example, if a small light-dark-quotient is the determined, then it can be concluded that a light aerosol is present. Likewise, a large light-dark-quotient is indicative of dark aerosols. Accordingly, the alarm value is determined as a function of the brightness of the aerosol. The type of smoke which is actually present need not be determined. As a result, the sensitivity of a scattered light detector operating according to the method of the invention can be maintained at an approximately constant value for all aerosols, i.e., independent of the brightness of an aerosol, thereby significantly reducing the risk of a false alarm. The two optical paths for the scattered light should be arranged in such a way that one of the paths responds predominantly to light aerosols, whereas the other path responds predominantly to dark aerosols.
According to one embodiment of the invention, the backscatter angle is approximately 70°. The signals produced by scattering IR radiation from an aerosol have a minimal value at approximately this scattering angle. The measurement values can be calibrated in this way and the light-dark-quotient reliably determined. The forward scattering angle may be approximately twice the backscatter angle.
Advantageously, the ratio of the scattering signals for at least one “fraudulent” value, which are determined at these measurement angles, may be stored in a memory. A “fraudulent” value is referred to as a value of a scattering ratio which is known to produce a false alarm. These fraudulent values may originate from, for example, water vapor, dust and/or vapors from manufacturing processes. In this way, fraudulent values can be recognized as such and positively distinguished from smoke, so that a false alarm is not triggered. Accordingly, a scattered light fire detector operating according to the method of the invention can also be used in environments where conventional fire detectors cannot be employed due to their high susceptibility to false alarms. The susceptibility of the detector to false alarms can thus be adapted to the actual requirements.
The light-dark-quotient quotient S
R
/S
V
(S
R
: backscattering signal, S
V
: forward scattering signal) is typically in the range between 0.2 and 0.8 and can be further processed by determining a factor F, F′ defined as
F=
((
S
R
/S
V
)−0.2)/0.6
for (S
R
/S
V
) between 0.2 and 0.8, and
F′=
2−((
S
R
/S
V
)−0.2)/0.2
for (S
R
/S
V
) greater than 0.8.
The factors F, F′ can then be used to determine the brightness of the aerosol.
A light-dark-quotient of the 0.2, i.e., F=0, indicates the presence of a very light smoke, wherein a light-dark-quotient of 0.8, i.e., F=1, indicates the presence of a very dark smoke. Water vapor produces a ratio S
R
/S
V
of approximately 0.20, which is not produced by any other known type of aerosol, making it possible to identify water vapor uniquely as a fraudulent value. If fraudulent values, such as dust and the like, are present in the measurement volume of the fire detector, then the quotient S
R
/S
V
can be greater than 1. In this case, the backscattering signal is greater than the forward scattering signal, so that the factor F′ should be determined. Such large values suggest that most probably no combustion aerosols are present in the measurement volume of the fire detector and only fraudulent values are indicated. This can be taken into consideration when the measurement signal is evaluated.
The alarm value may be a weighted sum of the values corresponding to the scattering signals. This summation takes into consideration the different weight of the measurement values determined at th

Affiliated with

Also associated with

Rating

Scattered light fire detector does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Scattered light fire detector, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Scattered light fire detector will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2504874