2
Section I —
Installation and Startup
INSTALLATION
CAUTION
Always store and transport the sensor in the
original factory packaging to ensure proper
protection against contamination.
SENSOR LOCATION
Proper sensor positioning is essential to ensure
maximum gas detection capability. Optimum sensor
placement and density varies depending upon the
conditions at the job site. The system designer and
installer must examine the specific area to be protected
and identify the most likely leak sources and gas
accumulation areas to determine the best sensor
locations.
The following factors should be considered for every
installation:
1. What kind of gas is to be detected? If it is lighter
than air (Acetylene, Hydrogen, Methane, etc.),
place the sensor above the potential gas leak.
Place it close to the floor for gases that are heavier
than air (Benzene, Butane, Butylene, Propane,
Hexane, Pentane, etc.) or for vapors resulting from
flammable liquid spills. Careful analysis of both the
vapor hazard and the application is required — first
to determine the feasibility of detection and then to
ensure that proper sensor locations are selected.
2. How rapidly will the gas diffuse into the air? Locate
the sensor as close as practical to the anticipated
source of a gas leak.
3. Ventilation characteristics of the immediate area
must also be considered. Air movement can cause
the gas to accumulate more heavily in one area
than another. Smoke generator tests are useful in
identifying typical air current patterns as well as
“dead” air spots for both indoor and outdoor
applications. The sensors should be placed where
the most concentrated accumulation of gas is
anticipated.
4. The sensor should be located in an area where it is
safe from potential sources of contamination that
could poison the sensing element.
5. The sensor should be pointed down to prevent the
buildup of contaminants on the gas inlet.
6. The sensor must be accessible for testing and
calibration.
7. Exposure to excessive heat or vibration can result in
premature failure of electronic devices and should
be avoided if possible. Shielding the transmitter
from intense sunlight will reduce solar heating and
enhance the service life of the unit.
IMPORTANT
All diffusion-based gas sensors including the
catalytic gas sensor used with the Model 505 must
contact the target gas in order to provide an
accurate gas measurement and response. This
must always be remembered when selecting
locations for gas sensor installation.
WIRING REQUIREMENTS
Wire Size and Type
The transmitter is typically connected to the
controller/power source using a three conductor
shielded cable. Shielded cable is highly recommended
to protect against interference caused by EMI and RFI.
Cable shields should be connected to earth ground at
the transmitter end only for maximum noise immunity.
Shield termination to ground at the controller end only is
also acceptable, but may offer reduced noise immunity.
The maximum allowable distance between the Model
505 and the output signal receiver (controller) is
determined by the wire size used. For the Model 505
with signal loop impedance option “A,” the maximum
loop resistance is 125 ohms. For the Model 505 with
signal loop impedance option “B,” the maximum loop
resistance is 500 ohms. Calculate the total linear
distance and overall resistance of the signal cable. Do
not exceed the maximum loop resistance for the Model
505 version being installed.
Power cable size must be adequate to ensure that no
less than the minimum operating voltage is delivered to
the transmitter under all operating conditions. Minimum
operating voltage for the Model 505 with signal loop
impedance option “A” is 10 vdc, for the Model 505 with
signal loop impedance option “B” it is 17 vdc. A linear,
filtered and regulated 24 vdc power supply is
recommended. Maximum wire size accepted by the
Model 505 wire terminals is 12 AWG (2.5 mm
2
). It is
acceptable to power the Model 505 from a 24 vdc
power source that is remotely located from the output
signal receiver.
In applications where the wiring cable is installed in
conduit, the conduit should not be used for wiring to
other electrical equipment.
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