Specifications

Section 4. Sensor Support
4-7
There are four delays per channel measured. The CR1000 processes the four
sub-measurements into a single reported value. In cases of excitation reversal,
excitation "on time" for each polarity is exactly the same to ensure that ionic
sensors do not polarize with repetitive measurements.
Read more! A white paper entitled “The Benefits of Input Reversal and
Excitation Reversal for Voltage Measurements” is available at
www.campbellsci.com.
4.2.3.2 Ground Reference Offset Voltage (MeasOff = True)
When MeasOff is enabled (= True), the CR1000 measures the offset voltage of
the ground reference prior to each VoltSe() or TCSe() measurement. This
offset voltage is subtracted from the subsequent measurement.
4.2.3.3 Background Calibration (RevDiff, RevEx, MeasOff = False)
If RevDiff, RevEx, or MeasOff is disabled (= False) in a measurement
instruction, offset voltage compensation is still performed, albeit less
effectively, by using measurements from automatic background calibration.
Disabling RevDiff, RevEx, or MeasOff speeds up measurement time; however,
the increase in speed comes at the cost of accuracy 1) because RevDiff, RevEx,
and MeasOff are more effective techniques, and 2) because background
calibrations are performed only periodically, so more time skew occurs
between the background calibration offsets and the measurements to which
they are applied.
Disable RevDiff, RevEx and MeasOff when CR1000 module
temperature and return currents are slow to change or when
measurement duration must be minimal to maximize
measurement frequency.
4.2.4 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 with 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
an AC excitation because they rely on inductive coupling to provide a signal.
DC excitation will provide no output.
CR1000 bridge measurements can reverse excitation polarity to provide AC
excitation and avoid ion polarization.
Sensors requiring AC excitation require techniques to minimize
or eliminate ground loops. See Section 7.5 Ground Looping in
Ionic Measurements.
NOTE
NOTE