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SECTION
14.
INSTALLATION AND MAINTENANCE
damage the
datalogger. Campbell Scientific's
DC112 phone modem has spark gaps on
the
phone
lines. A
12
AWG wire should
be
run
from
the modem ground
terminalto
earth
ground.
In
laboratory applications, locating a stable
earth ground is
not
always
obvious.
In
older
buildings, new
cover plates on old AC sockets
may indicate that a safety ground exists
when
in
fact the socket is not
grounded.
lf a safety
ground does
exist, it
is
good practice to verify
that
it
carries
no
current.
lf
the integrity
of
the
AC power ground cannot be verified, it is better
to ground the system to a massive
metalobject
such as
a
steelwater
pipe.
In
the field, an earth ground may
be
created
through a
grounding
rod. A
12
AWG
or larger
wire should
be run between the 21X terminal
and the
eafth
ground.
Campbell Scientific's
CM10 and CM6 Tripods come complete with
ground and
lightning rods, grounding wires, and
appropriate ground wire clamps.
14.8.2
EFFECT OF GROUNDING ON
MEASUREMENTS: COMMON MODE RANGE
The
common mode range
is
the
voltage range,
relative to the
21X ground, within which
both
inputs of a
differential measurement must lie
in
order for the
differential measurement to
be
made. Common
mode range for the
21X
is
IS.OV.
For example,
if
the high side of a
differential input is at 4V and
the low side is
at
3V relative to 21X ground, a measurement
made on the
+5.0V range would indicate
a
signal
of
1V.
However,
if
the high input
changed to 6V, the
common mode range
is
exceeded and
the measurement cannot be
made.
Common mode range may be exceeded when
the
21X is measuring the output from a sensor
which has
its
own grounded power supply and
the low side
of
the signal is referenced to power
ground.
lf
the
21X ground and the sensor
ground are at sufficiently different potentials,
the
signalwillexceed
the
common mode
range. To
solve this problem,
the sensor power ground
and the
21X ground should
be
connected,
creating one ground for the system.
In
a laboratory application, where more than
one AC socket may
be used to power various
sensors, it is not always safe to assume that
the
14-6-
power grounds are
at
the same
potential.
To
safe, the ground of
allthe AC
sockets in use
should
be
tied together
with a
12
AWG wire.
14.9
MAINTENANCE
The
21X requires a minimum of routine
rnaintenance. A few preventative maintenance
steps
willoptimize battery
life
and decrease the
chances of datalogger failure.
The 21X alkaline supply should not drop below
9.6V betore
replacement. When not in
use,
remove the eight cells to eliminate potential
corrosion
of
contact points and store in
a
cool
dry place.
When not in use, the
21XL lead acid battery
should be stored in
a
cool, dry environment with
the AC charging circuit
activated.
Discharging
the
21XL lead acid batteries below 11.76V
causes permanent damage, or at least
drastically diminishes
the batteries' normal
life
expectancy.
When connecting an
external battery to the
21X, use care to avoid shorting the +12V and
ground from
the battery.
Four packets of active desiccant are provided
with each
21X. Two
packets are located
in
the
battery base and two spare packets are sealed
in
a plastic
bag.
Used desiccant may
be
reactivated by oven drying for
about 16 hours at
120oC. Routine desiccant replacement
is
required
for reliable datalogger service.
14.10 CALIBRATION
PROCEDURES
The
21X requires little maintenance or
calibration.
Measurements
are made in such
a
way that small errors
in
the calibration are
automatically
removed.
Measurements can
be
made
to determine whether the accuracy of the
21X
is
within the specifications given
at
the
of
this
manual. lf
needed, the calibration
procedures described
in
this section can
be
pedormed by an experienced
technician having
the suggested equipment.
NOTE: The
precision of the 21X exceeds
that
of most standard electronic equipment.
These procedures require that the test
equipment have
a precision and accuracy
equal to or better than the 21X.
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