World Patents Pending ©AirSense Technology Ltd. 1999
ISSUE 2.4
Page 11
T E C H N I C A L
.
M A N U A L
Thus the air sampling system can overcome most of the disadvantages of other
detection methods, but tend to be economically designed to the limit of the market.
(i.e. The design maximises the possible performance advantages on the assumption
that the customer will pay more for them.)
There are many
aspirating systems
available around the globe, although there are only
three generally available
High Sensitivity
aspirating Detection systems in common
usage. It is known that there are a few products under development at this time which
may become available in this rapidly expanding market sector. In definition of the term
High Sensitivity, this is taken to mean that the system is capable of detecting the
electronic fire simulations described in the appendix of BS 6266 1992. All three
detectors have essentially similar levels of sensitivity, with no single product having any
great advantage over the other in this aspect. Interestingly, all three detectors use the
Light Scattering principle of operation, but each in a different manner; One uses a
Xenon flash bulb as its light source, whilst two take advantage of the very reliable
semiconductor laser (in this application, Mean Time Between Failure can be in excess
of 1000 years). Of the two laser based products, one is a particle counter, whilst the
other is responsive to the Mass of airborne material over a wide size range rather than
the number (count) of particles.
A potential problem with High Sensitivity Smoke Detectors is the possibility of alarm
signals being given by dust particles. This problem is dealt with in various ways
depending upon the actual system and its detection principle, but it is usually at least
partly dealt by an air filter of some description on the assumption that dust particles are
physically larger than smoke particles. The two laser based systems also use dust
discrimination to enhance this effect, although both do so in differing manners. It is also
useful to understand that there is an unfortunate effect whereby the more a filter
becomes clogged with dust, the smaller the particles are which it will remove out of the
air-stream. This is can be a very serious shortcoming, because air will still flow easily
through the filter when the size of particle being stopped is the size of smoke particles.
Specific types of
Aspirating Systems.
