13
VDC. In this configuration, the NXP must
be set to Source mode (see Figure 2.5).
2.2 Cautions
The NXP MUST be
installed in a non-hazardous area. Only
the XPR explosion-proof sensor
housing, if used, may be installed in a
hazardous location.
Disconnect the NXP Gas Detector Head
from any external power source and
from any battery backup power supply
before installing, replacing, or handling
system components.
Both the Gas Detector head and any
connected controller unit should be
calibrated and alarm levels tested at
the time of initial installation before the
system is first put into service.
2.3
NXP Detector Head
Environmental Ratings
The NXP housing has an environmental
rating of NEMA 4X and an ingress
protection rating of IP66. These ratings
apply to the NXP only, and do not extend
to the wall cube power supply. If the NXP
is to be installed in an area where it could
be exposed to pressurized jets of water,
wind-blown sand or dust, or other adverse
environmental conditions, it is
recommended that power be connected
through weather-proof cabling or conduit.
See section 2.7 for instructions on
removing the wall cube and connecting
external wiring.
2.4
Siting of detector heads
The specific placement of gas detector
heads should be determined by the type
of area being monitored, the type and
source of the atmospheric hazard being
measured, prevailing wind patterns, and
other information.
Call Biosystems Technical Service
Department at (860) 344-1079 or 800-
711-6776 for additional advice.
In general, for gases lighter than air the
detector heads should be placed at a level
slightly above the area where leaks are
likely to occur. For gases that are heavier
than air the detectors should be located
close to floor level or in inspection pits or
ducts in which gases that are heavier than
air may collect.
There are many circumstances that may
modify this general advice. It is beyond
the scope of this manual to attempt to
describe all of the potential situations that
could cause this advice to be modified,
but a single example follows:
The molecular weight of nitrogen (MW
≅
28) is very close to that of air (MW
≅
29).
When nitrogen is at the same temperature
as the air into which it is introduced, it
mixes readily, and tends to spread evenly
throughout the affected atmosphere. On
the other hand, if the nitrogen is under
pressure, and then suddenly released into
the atmosphere, as the gas expands
(going from higher pressure to a lower
pressure) it cools. The cooler gas is
denser than the air into which it is being
introduced, so it no longer mixes as
readily. Instead, the nitrogen tends to sink
to floor level and spread laterally. In this
case remote detectors being used to
monitor for oxygen deficiency should be
located near floor level in order to detect
the deficiency as quickly as possible.
The nearer in density to air a gas is, the
more easily it will flow with air due to
drafts, ventilation etc. A compromise
approach for placement of detectors used
to measure gases which are only slighter
lighter (such as carbon monoxide) or
heavier (such as hydrogen sulfide) than
air is to mount the detectors at a height as
close as possible to the breathing area of
personnel being protected.
When installing detector heads it is
important to ensure that the sensor is not
exposed to liquid or dust contamination
that would interfere with the passage of
gas through the protective filter into the
sensor. Detector assemblies should be
placed so that the sensor points straight
downward. A splash deflector should be
used when water or other liquids are
chronically present in the area where the
detector is located.
2.5 Wiring
requirements
Recommendations for wire used to
connect NXP detector assemblies to a
standard controller unit with a 24 VDC
power supply are listed in Table 2.5.
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