Campbell Scientific CR3000 CR3000 Micrologger - Page 303

Section 8. Operation 'Main Program BeginProg R2 = 1000 R3 = 1000 R4 = 1000 Scan(500,mSec,1,0) 'Resistance of R2 'Resistance of R3 'Resistance of R4 '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 303