Operator`s manual

SECTION 14. INSTALLATION AND MAINTENANCE
14-6
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 terminal to 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. If a safety
ground does exist, it is good practice to verify
that it carries no current. If the integrity of the
AC power ground cannot be verified, it is better
to ground the system to a massive metal object
such as a steel water 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 earth 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
±5.0V. 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. If the 21X ground and the sensor
ground are at sufficiently different potentials, the
signal will exceed 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
power grounds are at the same potential. To be
safe, the ground of all the AC sockets in use
should be tied together with a 12 AWG wire.
14.9 MAINTENANCE
The 21X requires a minimum of routine
maintenance. A few preventative maintenance
steps will optimize battery life and decrease the
chances of datalogger failure.
The 21X alkaline supply should not drop below
9.6V before 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
120
o
C. 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 front
of this manual. If needed, the calibration
procedures described in this section can be
performed 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.