Campbell Scientific AM416 Multiplexer - Manual (Page 24)

AM416 RELAY MULTIPLEXER
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7. GENERAL MEASUREMENT
CONSIDERATIONS
1. Long lead lengths - long lead lengths
contribute to the formation of induced and
capacitive voltages within the sensor and
AM416 lead wires. To minimize this
phenomenon, CSI recommends use of
Teflon, polyethylene, or polypropylene
insulation around individual conductors. Do
not use PVC insulation as conductor
insulation, although it may be used as a
cable jacket. It may also be necessary to
program a delay within the measurement
instruction in order to allow the capacitance
of the lead wires to discharge before
measurement. Please consult the theory of
operation section of your datalogger manual
for more information.
2. Common Earth Ground - A connection to
earth ground should be made at the
datalogger. The lead wire that connects the
datalogger power ground to the AM416
power ground establishes a common
ground. The Installation/Maintenance
Section of your datalogger manual has
more for information on grounding
procedures.
3. Completion resistors - In some applications
it may be advisable to place completion
resistors at the datalogger terminal strips.
In some cases, sensors specific to the use
of multiplexers are available from CSI.
Examples include soil moisture probes and
thermistors. Please consult CSI for
ordering and pricing information.
4. Contact degradation - Once excitation in
excess of 30 mA has been multiplexed, that
set of contacts may be rendered unsuitable
for later low voltage measurement. To
prevent undue degradation, it is advisable
to reserve certain channels for sensor
excitations and other channels for sensor
signals.
8. INSTALLATION
The standard AM416 may be operated in an
indoor, non-condensing environment. If
condensing humidity is a problem or if the
multiplexer might be exposed to liquids, a
water-resistant enclosure is required.
Several enclosures may be purchased through
CSI which offer a degree of protection against
dust, spraying water, oil, falling dirt, or dripping
noncorrosive liquids (Models AM-ENC, AM-
ENCT, ENC-24, ENC-30). All the enclosures
contain mounting plates for the multiplexer and
conduit bushings for cable entry. These
standard enclosures are rain-tight, but not
water-proof.
The AM416 is attached to the mounting plate
inside the enclosure with two screws. To
expose these screws, the top plate of the
multiplexer (four #1 phillips screws at the
corners) and the printed circuit board (two
straight-slot screws near the center of the
board) must be removed. Care must be taken
when removing the upper plate of the
multiplexer. It is generally easiest to lift the
edge opposite the strain relief flange up first,
then slide the upper plate out. Make sure to
clear the terminal strips.
The enclosure lids are gasketed. The screws
on the outside of the enclosure should be
tightened to form a restrictive seal. In high
humidity environments, user supplied foam or
putty (or a similar substance) helps to reduce
the passage of moisture into the enclosure via
the cable conduits. [CAUTION: Air movement
should not be restricted into any enclosure
containing batteries that may produce explosive
or noxious gases (e.g. lead-acid cells)]. U-bolts
are provided to attach the enclosure to a 1.25"
diameter pipe. The enclosure may also be lag-
bolted to a wall or similar flat surface.
8.1 ENVIRONMENTAL CONSTRAINTS
The AM416 has an operable temperature range
of -40oC. to +65oC. The multiplexer is
susceptible to corrosion at high relative
humidity. Desiccant packs are available from
CSI and they should be used inside the
enclosure to remove water vapor.