Campbell Scientific CR200/CR200X-series Dataloggers - Manual (Page 54)

Section 4. Sensor Support
4.3.1 Measurements Requiring AC Excitation
Some resistive sensors require AC Excitation. These include electrolytic tilt
sensors, soil moisture blocks, water conductivity sensors, and wetness sensing
grids. The use of DC excitation in these sensors can result in polarization, which
will cause erroneous measurement, shift calibration, or lead to rapid sensor
decay.
Other sensors, e.g., LVDTs (Linear Variable Differential Transformer), require
and AC excitation because they rely on inductive coupling to provide a signal.
DC excitation will provide no output.
CR200(X) bridge measurements cannot reverse excitation polarity to provide
AC excitation and avoid ion polarization. Sensors requiring AC excitation
should not be used with the CR200(X).
Other Campbell Scientific dataloggers (e.g. CR800 series, CR1000, CR3000)
are compatible with sensors that require AC excitation.
4.4 Pulse Count Measurement
FIGURE. Switch Closure Pulse Sensor p. 42 is a generalized schematic showing
connection of a pulse sensor to the CR200(X). The CR200(X) features two
dedicated pulse input channels, P_SW and P_LL, and two digital I/O channels,
C1and C2, for measuring pulse output sensors. Activated by the PulseCount ()
instruction, dedicated 16-bit counters on P_SW, P_LL, C1 and C2 are used to
accumulate all counts over the user specified scan interval. The value which is
output for each scan is the difference in the last known counter value and the
new counter value. Since the last count is maintained for each scan, even if the
counter rolls over between scans the correct count will be recorded. If the time
between scans is such that the counter exceeds 65,536 pulses during a scan, then
the counter will roll over twice resulting is an erroneous measurement.
PulseCount () instruction parameters specify the pulse input type, channel used,
and pulse output option.
Note: The PulseCount instruction must be executed once before the pulse or
control port is ready for input. This may be of particular concern for programs
with long scan intervals. For example, the PulseCount () instruction will not
yield a valid output until the turn of the second hour if the PulseCount ()
instruction is used within a program with a scan interval of 1 hour.
Figure 29: Switch Closure Pulse Sensor
42