CR200/CR200X Series Dataloggers - Campbell Scientific
CR200/CR200X Series Dataloggers - Campbell Scientific
CR200/CR200X Series Dataloggers - Campbell Scientific
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Section 4. Sensor Support<br />
4.3.1 Measurements Requiring AC Excitation<br />
Some resistive sensors require AC Excitation. These include electrolytic tilt<br />
sensors, soil moisture blocks, water conductivity sensors, and wetness sensing<br />
grids. The use of DC excitation in these sensors can result in polarization, which<br />
will cause erroneous measurement, shift calibration, or lead to rapid sensor<br />
decay.<br />
Other sensors, e.g., LVDTs (Linear Variable Differential Transformer), require<br />
and AC excitation because they rely on inductive coupling to provide a signal.<br />
DC excitation will provide no output.<br />
<strong>CR200</strong>(X) bridge measurements cannot reverse excitation polarity to provide<br />
AC excitation and avoid ion polarization. Sensors requiring AC excitation<br />
should not be used with the <strong>CR200</strong>(X).<br />
Other <strong>Campbell</strong> <strong>Scientific</strong> dataloggers (e.g. CR800 series, CR1000, CR3000)<br />
are compatible with sensors that require AC excitation.<br />
4.4 Pulse Count Measurement<br />
FIGURE. Switch Closure Pulse Sensor p. 42 is a generalized schematic showing<br />
connection of a pulse sensor to the <strong>CR200</strong>(X). The <strong>CR200</strong>(X) features two<br />
dedicated pulse input channels, P_SW and P_LL, and two digital I/O channels,<br />
C1and C2, for measuring pulse output sensors. Activated by the PulseCount ()<br />
instruction, dedicated 16-bit counters on P_SW, P_LL, C1 and C2 are used to<br />
accumulate all counts over the user specified scan interval. The value which is<br />
output for each scan is the difference in the last known counter value and the<br />
new counter value. Since the last count is maintained for each scan, even if the<br />
counter rolls over between scans the correct count will be recorded. If the time<br />
between scans is such that the counter exceeds 65,536 pulses during a scan, then<br />
the counter will roll over twice resulting is an erroneous measurement.<br />
PulseCount () instruction parameters specify the pulse input type, channel used,<br />
and pulse output option.<br />
Note: The PulseCount instruction must be executed once before the pulse or<br />
control port is ready for input. This may be of particular concern for programs<br />
with long scan intervals. For example, the PulseCount () instruction will not<br />
yield a valid output until the turn of the second hour if the PulseCount ()<br />
instruction is used within a program with a scan interval of 1 hour.<br />
Figure 29: Switch Closure Pulse Sensor<br />
42