World Patents Pending ©AirSense Technology Ltd. 1999
ISSUE 2.4
Page 20
T E C H N I C A L
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M A N U A L
for this, but the indication will consist of a marked decrease in the spread of readings.
The essence of the problem is not so much the filtering out of dust, which is a straight
forward design problem, but the life of the filter, and giving a reliable indication that
it is becoming fully loaded. The Stratos detector tackles the problem of dust in four
ways;
n
Only a small part of the total air sample is taken through the detector head. While
a large air flow down the sampling pipes is an advantage, the detector head itself
can measure the pollution density equally well for a small sample as a big sample.
As a consequence, only the small part of the total air sample flowing through the
detector head requires filtering and the filter and detector chamber life is
consequently greatly extended. Additionally the frictional resistance that would
otherwise restrict air flow is greatly reduced.
n
Progressive decreases in the variance and mean of readings in the histogram give
a warning that the filter is becoming fully loaded. This progressive reduction in
signal spread causes the detector to compensate, thereby automatically maintain-
ing the sensitivity of the Stratos detector.
n
The flow rate through the detector sampling chamber is relatively fast and an
individual dust particle which does get into the chamber will give just one
abnormally high reading from paired light pulses from the laser. Stratos looks for
such individual, abnormally high readings and ignores them, only accepting a
signal from the lower of the paired pulses. This process is known as Dust
Discrimination.
n
The filter itself is specifically designed to have a very high capacity for holding dust,
which further extends the filter life.
The Stratos scale is represented on the front panel of the Master Stratos detector
by a ten segment bargraph display, with the segments numbered 1 to 10. The
bargraph illuminates continuously from 1 upwards, to the measured amount of
pollution. The alarm trigger level is always pre-set at bargraph level eight.
If
there
were a segment numbered zero, then this is where the measured mean level of
background signal would be indicated. The main purpose of the bargraph is to
Bargraph Scale
