World Patents Pending ©AirSense Technology Ltd. 1999
ISSUE 2.4
Page 15
T E C H N I C A L
.
M A N U A L
Absolute Scaling
The output of the smoke detector in the conventional aspirating smoke detector system
is assumed to be (essentially) zero for a clean air sample and a fixed known level for a
given amount of smoke pollution. It is, in effect, a meter giving an absolute measure-
ment of ‘smoke-like’ pollution. The full scale reading may be set to a minimum of 0.05%
obscuration per metre which makes it very sensitive. A usual full scale reading is between
0.1 - 0.2% obscuration per metre. The zero of the scale corresponds to zero pollution.
There are only a few applications where the air is totally free of pollution. There is
normally a constantly varying amount of background pollution. With the
‘Absolute’’
type of detector the alarm levels must be individually set to trigger at the desired level.
The correct level is one that will be reached by as low an amount of pollution as is
possible without being reached by the normally occurring background variations.
Alarms triggered by the normal background amounts of pollution will be unwanted
alarms which, as previously explained, must be avoided. The background levels will vary
according to the time of day and the particular activities in the protected area. The
requirement then, is to find what are the appropriate levels to set the pre-alarm and
alarm triggers points. In order to do this the detector output is used in the following way:
Smoke is sampled during the normal usage of the system for a period of 10 to 14 days,
and a chart recording of the levels against time is generated. This chart is assumed to
represent reasonably the future variations that will occur. The chart is examined for high
smoke levels during both the day and the night periods, and the pre-alarm and alarm
levels for day and night usage are both set to be higher than these respective peak levels.
The importance of avoiding unwanted alarms is so great that the trigger levels are
routinely set, as a matter of judgement on the part of the installer, to be
well
above the
peak levels, in case the chart recording is not truly representative of the future readings.
The aim of the exercise is to set the trigger levels at a point where the
probability
of an
unwanted alarm is very small and where there is still a high probability of detecting
unusually large signals.
There are a number of problems with the absolute scaling system;
n
The background level does not always vary as it did during the first two weeks after
installation.
Absolute v. relative scaling
