World Patents Pending ©AirSense Technology Ltd. 1999
ISSUE 2.4
Page 12
T E C H N I C A L
.
M A N U A L
Particle counting works on the principle of counting the number of particles in a given
volume of air sampled. In order to do this, the rate of air-flow through the chamber must
be maintained within limits, or the effect of increased counting due to increasing the
air-flow must be allowed for.
The method used is to focus a laser beam into a very small volume in the air-stream and
measure the light scattered by a particle entering this volume. This will give a light
scatter pulse of a duration and size proportional to the size of particle. By its very nature
it is an absolute measuring instrument, i.e. its reading is the number of particles per unit
volume; it is not relative to anything. At first sight this appears to be a point in its favour,
and for measuring the impurity of air in a clean room it certainly is. It should be taken
into consideration though that the smoke level that is indicative of fire is not absolute,
it may be any level depending upon the normal amount of smoke present. The increase
in the count rate above normal is an indication of unusually large numbers of particles
being present. The size of the particle being counted is not a prime factor in the count,
since either a very large particle, or a very small particle will increase the count by one
unit.
It is assumed that dust particles are larger than smoke particles, and the particle counter
can give a signal indicative of particle size and the system may be set to ignore those
above a certain size. In relatively clean environments this may entirely obviate the need
for a filter. Unfortunately, the size of a fire can only be accurately measured either by the
weight or volume (mass) of smoke particles emitted and
not
the
number
of particles.
A particle counter will reach saturation when the rate at which it is required to count
particles is beyond its capability. This can be near the normal amount of smoke present
which includes dust particles when they are present.
These systems look for the amount of light scattered by a stream of air through an
optical chamber. With perfectly clean air there is a very small amount of scatter.
As the volume of particles below a given size increases, the amount of light scatter
increases. Measuring the amount of scatter thereby gives a measure of the volume
of impurity in the air. This has a direct relationship to the size of the source fire.
The ultimate limit to the sensitivity of such a detector is set by the amount of
electrical noise being generated by the sensor, the light source and their associ-
ated circuitry. This is because the system must be able to differentiate between the
Particle counters
Light scatter
(mass detection)
systems.
