World Patents Pending ©AirSense Technology Ltd. 1999
ISSUE 2.4
Page 9
T E C H N I C A L
.
M A N U A L
are protecting; to give a rapid and sensitive alarm. The price paid for this degree of
certainty is the time delay required for both detectors to trigger an alarm and the
effective sensitivity are reduced to that of the least suitable detector of the pair.
Where a large volume is to be covered or an area is to be covered at a high sensitivity;
instead of using many low cost, inherently low sensitivity point detectors, an Aspirating
system may be used. Such a system consists of a single, more sensitive and highly
developed detector and an air sampling system to cover the required volume. The air
sampling system consists of a length or lengths of pipe with strategically placed
sampling holes along its length. An air impeller is used to draw air along the pipe from
the sampling holes and through the detector measuring chamber. The transit time for
air to travel from the sampling holes to the detector head can be significant, so there
must be significant advantages to this system to offset this delay. The detector is
designed to be stable, reliable and highly sensitive (approximately 10 to 200 times the
sensitivity of a point detector). Such additional sensitivity allows the detection of smoke
at a very much earlier stage in the development of a fire, and as a consequence, this may
reduce damage levels considerably. This saving of time more than offsets the delay due
to the transit time.
Aspirating smoke detectors are susceptible to considerable dilution of smoke in the air
sample, and it is important that this effect is understood. For example, if it is assumed
that a detector chamber has a typical sensitivity of 0.1%/metre obscuration, and it is
drawing its sample through ten holes in the length of pipe, and that each sampling hole
contributes equally to the volume through the detector measuring chamber, then if
smoke only effects one of the sampling holes, then, due to dilution from the other 9
holes, the actual detector responsiveness will only be equivalent to a 1% detector. If
however, say 5 of the 10 holes are effected by smoke, the apparent sensitivity will be
0.5%/metre. The more widespread the distribution of smoke among the sampling
holes, then the higher will be the density of smoke appearing in the chamber, and the
apparent
sensitivity will be greater. Sampling pipe design should take this effect into
consideration.
The action of pulling air into the air sampling system gives a better smoke collection
method than the naturally occurring air convection relied upon by point detectors, and
the sampling pipe systems frequently allow the sampling holes to be strategically better
High Sensititivity
Aspirating Systems
(a.k.a. Air Sampling
Systems)
