World Patents Pending ©AirSense Technology Ltd. 1999
ISSUE 2.4
Page 19
T E C H N I C A L
.
M A N U A L
If the comparison of fast histogram and the current standard histogram shows no
change of use within an hour of the programmed time, then no switch to the
alternative standard histogram is made. This is particularly beneficial if the detector
remains in a quiet period for a day or more, such as for a weekend or holiday period,
when it will remain with the quiet period histogram for the whole period, as, after all,
this type of system is usually used where the optimum in sensitivity is required, and this
feature provides a unique level of sensitivity with this
artificial intelligence
sensitivity
switching.
An area that requires special care with the light scattering principle, is the exclusion of
abnormal levels of dust from the detector chamber. It is as well to mention that perfect
protection against dust particles is an ideal which is unlikely to ever be fully achieved
although a method of discriminating between dust particles and smoke particles of the
same size is always being sought. It is difficult to obtain, because many dust particles
exist in the same size region as smoke particles. As such, the combination of fully
monitored and compensated (see below) dust particle separation and dust discrimina-
tion used in Stratos provides a very adequate solution.
For reasons of power economy and reliability a low power air impeller is required to
draw air down the sampling pipes. This leads to the problem that the bore of the
sampling pipes must be large in order to obtain the high flow rates required (usually
15 to 25 mm.). High flow rates are required to obtain short transit times between the
sampling holes and the detector head. Even with a well designed system these may
often be as great as 60 seconds for the furthest holes. The result of fast air flow down
large bore pipes is a large volume of air being moved. In order to overcome long term
problems, the air which passes through the detector must be filtered to some degree
to remove the majority of dust particles. The nature of this filter is critical though,
because the matter it filters from the air remains in the filter medium, and as it builds
up will increase the efficiency of the filter. This manifests itself by the filter removing
progressively smaller and smaller particles the more it becomes loaded with dust, until
it is removing the majority of the smoke particles. At this time it will still be passing air
satisfactorily and there will, unless compensated for, be no external indication that it
is blocking smoke until such time as it fails to detect a fire. One indication that occurs
for a
relatively scaled
detector (Stratos) is that the spread and level of readings will
gradually decrease. Stratos will automatically decrease the scale range to compensate
Dust
Dust
Compensation
