Campbell CR800 Series, Operator'S Manual, Page: 335 / 598

Section 8. Operation
335
Four-Wire Full-Bridge Measurement and Processing
'This program example demonstrates the measurement and processing of a four-wire resistive
'full bridge. In this example, the default measurement stored in variable X is
'deconstructed to determine the resistance of the R1 resistor, which is the variable
'resistor in most sensors that have a four-wire full-bridge as the active element.
'Declare Variables
Public X
Public X1
Public R1
Public R2 = 1000 'Resistance of fixed resistor R2
Public R3 = 1000 'Resistance of fixed resistor R2
Public R4 = 1000 'Resistance of fixed resistor R4
'Main Program
BeginProg
Scan (500,mSec,1,0)
'Full Bridge Measurement:
BrFull (X,1,mV2500,1,Vx1,1,2500,True,True,0,_60Hz,1.0,0.0)
X1 = ((-1 * X) / 1000) + (R3 / (R3 + R4))
R1 = (R2 * (1 - X1)) / X1
NextScan
EndProg
8.1.2.3.1 Ac Excitation
Some resistive sensors require ac excitation. Ac excitation is defined as
excitation with equal positive (+) and negative (–) duration and magnitude.
These include electrolytic tilt sensors, soil moisture blocks, water-conductivity
sensors, and wetness-sensing grids. The use of single polarity dc excitation with
these sensors can result in polarization of sensor materials and the substance
measured. Polarization may cause erroneous measurement, calibration changes,
or rapid sensor decay.
Other sensors, for example, LVDTs (linear variable differential transformers),
require ac excitation because they require inductive coupling to provide a signal.
Dc excitation in an LVDT will result in no measurement.
CRBasic bridge-measurement instructions have the option to reverse polarity to
provide ac excitation by setting the RevEx parameter to True .
Note Take precautions against ground loops when measuring sensors
that require ac excitation. See
Ground Looping in Ionic Measurements
(p.
101).
8.1.2.3.2 Accuracy — Resistance Measurements
Read More Consult the following technical papers at
www.campbellsci.com/app-notes
for in-depth treatments of several topics
addressing voltage measurement quality:
• Preventing and Attacking Measurement Noise Problems
• Benefits of Input Reversal and Excitation Reversal for Voltage