World Patents Pending ©AirSense Technology Ltd. 1999
ISSUE 2.4
Page 24
T E C H N I C A L
.
M A N U A L
cannot be solved by the designer of the detector but must be solved by the system
designer. This is the transient problem of the changes which occur when pollution starts
to be detected at the inlet and when it ceases to be detected.
In order to examine what occurs, it is easiest to examine a simple case and then examine
the effect of variations. The following examinations make 2 assumptions. The first is that
a constant level of external pollution occurs. The second is that perfect mixing of
incoming air occurs instantaneously. Although this will not be realised in any practical
case, it will be closely approached. An area is taken which either has filtered air fed into
it from outside or has air being extracted from it and air entering it where possible.
Figures 1 & 2 depict such areas. The graph in Fig. 3 shows how the pollution rises at
the outlet of the area with time and the graph in Fig. 4 shows how the pollution falls
at the outlet of the area with time. Both show the result of the basic referencing equation
assuming theoretically perfect detectors.
Fig. 1. Simple case of
filtered air input and
natural venting.
Plant Room
Filter
Outlet
Fresh Air
Source
Pump
Protected Room
Inlet
Plant Room
Outlet
Exhaust
Pump
Protected Room
Fresh Air
Source & Inlet
Fig. 2. Simple case of
forced air venting and
natural inlet of air.
