Specifications

Section 3. CR3000 Measurement Details
FIGURE 3.6-1. Model of Resistive Sensor with Ground Loop
In Figure 3.6-1, V
x
is the excitation voltage, R
f
is a fixed resistor, R
s
is the
sensor resistance, and R
G
is the resistance between the excited electrode and
CR3000 earth ground. With R
G
in the network, the measured signal is:
()
VV
R
RR RRR
x
s
sf sfG
1
=
++ /
[3.6-1]
R
s
R
f
/R
G
is the source of error due to the ground loop. When R
G
is large the
equation reduces to the ideal. The geometry of the electrodes has a great effect
on the magnitude of this error. The Delmhorst gypsum block used in the 227
probe has two concentric cylindrical electrodes. The center electrode is used
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
CR3000 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
3-20