Det-Tronics U7602B-P Unitized UV Flame Detector/Controller - Manual (Page 10)

NOTE
Ultraviolet detectors are very sensitive to arc
welding, and if this type of radiation can be
expected, it must be controlled through proper
application. Successful application techniques
include careful positioning and shielding of the
detector/controllers.
INSTALLATION
U7602 DETECTOR/CONTROLLER POSITIONING
AND DENSITY
The detector has a nominal 90° cone of vision. What
this means in practical terms can be understood by
reference to a typical installation. Consider an appli-
cation such as a loading rack with a ceiling height of
25 feet (7.5 meters), and assume it is desired to have
complete detector coverage at floor level. If a detec-
tor is mounted 2 feet (0.6 meter) from the ceiling and
pointed straight down, the distance from the detector
to the designated level would be 23 feet (7 meters).
Because of its 90° cone of vision, the detector would
cover a circular area with a diameter of 46 feet (14
meters) at the designated level. A simple layout of
the area to be covered will easily reveal the number of
detectors required to completely supervise the desig-
nated area. In general, detectors should be placed
as close as practical to the probable hazard.
NOTE
Do not mount UV detectors close to the ceiling in
enclosed areas if dense smoke can be expected
to accumulate at the onset of a fire. Mounting
the detector on side walls a few feet (or about 1
meter) down from the ceiling will normally allow
time for the detectors to respond before they are
affected by smoke rising to the ceiling. It is also
advisable to shorten any time delay settings for
applications where smoke can accumulate dur-
ing a fire. If dense smoke can be expected to
accumulate prior to the presence of flame (as in
an electrical fire), do not use UV detectors alone.
A 25 count per second sensitivity setting is standard
on the U7602 and is appropriate for most indoor and
outdoor applications. Other sensitivity settings are
available, however, sensitivity requirements must be
carefully determined. The more sensitive the setting,
the more likely extraneous UV sources such as arc
welding in the area can affect the system. Therefore,
the sensitivity setting selected should be the least
sensitive that is practical for the application involved
without introducing undue risk of not “seeing” an
actual fire. Figure 9 shows the UV detector response
in counts per second to gasoline fires of different
sizes and at varying distances from the detector.
Refer to Figure 9 and consider the factors described
above (distance to potential fire, probable fire size,
extraneous UV presence) to establish the correct sys-
tem sensitivity requirements.
NOTE
The curves of Figure 9 represent typical gasoline
fires. The fire size refers to the surface area of
exposed fuel. For fire involving other materials,
the values obtained from the curve must be mod-
ified.
The time delay on the U7602 is adjustable, but is typi-
cally set for 5 seconds. For most outdoor applica-
tions a minimum time delay of 2 seconds is required
in order to ignore UV generated from lightning strikes.
Detectors should be located for the best unobstruct-
ed view of the area to be protected. Detectors must
be accessible for cleaning the viewing window and
reflector rings. Care must be taken so that dirt or
other foreign material will not accumulate and
obscure the detector viewing window. For outdoor
applications, the detectors should be pointed down-
ward to prevent the cone of vision from scanning the
horizon, since the detectors can be affected by long
duration lightning flashes or distant arc welding.
When practical, mount the detectors so that the UV
test lamp is on top, since dirt accumulation between
the window and the reflector ring can interfere with
the Automatic oi function. See Figure 7 for mounting
dimensions.
7 95-8239
260
240
220
200
180
160
140
120
100
80
60
40
20
0
0 10 20 30 40 50 60 70 80 90 100
0 5 10 15 20 25 30
1 (0.09) 2 (0.18) 6 (0.56) 8 (0.74) 10 (0.93) 12 (1.11) 14 (1.3) 36 (3.3) 4 (0.37)
DETECTOR OUTPUT VERSUS DISTANCE
FROM A GASOLINE FIRE AS A FUNCTION
0F SURFACE OR AREA OF THE SOURCE
DETECTOR OUTPUT – COUNTS PER SECOND
DISTANCE
FEET
METERS
A1487
SQUARE FEET (SQUARE METERS)
Figure 9—Sensitivity to a Gasoline Reference Fire