Campbell Scientific CR3000 Micrologger - Manual (Page 303)

Section 8. Operation
303
'Main Program
BeginProg
R2 = 1000 'Resistance of R2
R3 = 1000 'Resistance of R3
R4 = 1000 'Resistance of R4
Scan (500,mSec,1,0)
'Full Bridge Measurement:
BrFull (X,1,mV2000,1,1,1,2500,True,True,0,_60Hz,1.0,0.0)
X1 = ((-1 * X) / 1000) + (R3 / (R3 + R4))
R1 = (R2 * (1 - X1)) / X1
NextScan
EndProg
8.1.3.1 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 transformers), require an
ac excitation because they rely on inductive coupling to provide a signal. dc
excitation will provide no output.
CR3000 bridge measurements can reverse excitation polarity to provide ac
excitation and avoid ion polarization.
Note Sensors requiring ac excitation require techniques to minimize or eliminate
ground loops. See Ground Looping in Ionic Measurements
(p. 96).
8.1.3.2 Accuracy of Ratiometric-Resistance Measurements
The ratiometric-accuracy specification for resistance measurements is:
±(0.02% * V1 + Offset), 0° to 40°C,
±(0.025% * V1 + Offset), ‐25° to 50°C,
±(0.03% * V1 + Offset), ‐40° to 85°C,
where V1 is the voltage measurement and Offset is equal to one of the
following, where the Basic Resolution is the resolution of a single A/D
(p.
449) conversion. Note that excitation reversal reduces offsets by a factor
of two:
• Offset = 1.5 x Basic Resolution + 1.0 µV if the measurement is made on a
differential input channel with input reversal
• Offset = 3 x Basic Resolution + 2.0 µV if the measurement is made on a
differential input channel without input reversal