World Patents Pending ©AirSense Technology Ltd. 1999
ISSUE 2.4
Page 27
T E C H N I C A L
.
M A N U A L
the detector is in a state of low sensitivity. This is one of the major errors the system
designer is trying to avoid by using a reference system. The exact duration of this period
and the extent of the low sensitivity will be assessed later from a mathematical analysis.
In Fig. 4 the same items are illustrated as in Fig. 3. but which correspond to the time
when the external source of pollution ceases.
0
Transport
delay through
duct work
In Alarm
Time
D
Smoke Density
0
Pollution at Outlet
Pollution at Source
Time
D (1-E)
Smoke Density
0
CALCULATED POLLUTION ADDED
Time
Density required for alarm
Smoke Density
D (1-E)
It is a common error made by many people, that the time scale for pollution increasing
in a room is different from that of pollution decreasing. This is not true but may be
subjectively enforced due to the more rapid change seen at the start of inverse
exponential function. Exactly the same mechanism is working in both increasing and
decreasing pollution. The only difference is that in one the added air is polluted and
in the other the added air is clean. As can be seen from Fig. 4. there is a period
immediately following the end of the input of pollution when the detector is in alarm.
This is exactly what the system designer is trying to avoid by using a reference system.
Fig. 4. Smoke levels at
the end of an external
source of smoke.
