Campbell Scientific CR5000 Measurement and Control Module - Manual (Page 74)

Section 3. CR5000 Measurement Details
3-20
for excitation; because it is encircled by the ground electrode, the path for a
ground loop through the soil is greatly reduced. Moisture blocks which consist
of two parallel plate electrodes are particularly susceptible to ground loop
problems. Similar considerations apply to the geometry of the electrodes in
water conductivity sensors.
The ground electrode of the conductivity or soil moisture probe and the
CR5000 earth ground form a galvanic cell, with the water/soil solution acting
as the electrolyte. If current was allowed to flow, the resulting oxidation or
reduction would soon damage the electrode, just as if DC excitation was used
to make the measurement. Campbell Scientific probes are built with series
capacitors in the leads to block this DC current. In addition to preventing
sensor deterioration, the capacitors block any DC component from affecting
the measurement.
3.7 Pulse Count Measurements
Many pulse output type sensors (e.g., anemometers and flow-meters) are
calibrated in terms of frequency (counts/second). For these measurements the
accuracy is related directly to the accuracy of the time interval over which the
pulses are accumulated. Frequency dependent measurements should have the
PulseCount instruction programmed to return frequency. If the number of
counts is primary interest, PulseCount should be programmed to return counts
(i.e., the number of times a door opens, the number of tips of a tipping bucket
rain gage).
The interval of the scan loop that PulseCount is in is not the sole determining
factor in the calculation of frequency. While normally the counters will be
read on the scan interval, if execution is delayed, for example by lengthy
output processing, the pulse counters are not read until the scan is
synchronized with real time and restarted. The CR5000 actually measures the
elapsed time since the last time the counters were read when determining
frequency so in the case of an overrun, the correct frequency will still be
output.
The resolution of the pulse counters is one count. The resolution of the
calculated frequency depends on the scan interval: frequency resolution =
1/scan interval (e.g., a pulse count in a 1 second scan has a frequency
resolution of 1 Hz, a 0.5 second scan gives a resolution of 2 Hz, and a 1 ms
scan gives a resolution of 1000 Hz). The resultant measurement will bounce
around by the resolution. For example, if you are scanning a 2.5 Hz input once
a second, in some intervals there will be 2 counts and in some 3 as shown in
figure 3.7-1. If the pulse measurement is averaged, the correct value will be
the result.
3 2 3 2
FIGURE 3.7-1. Varying counts within Pulse interval.