Campbell CR800 Series, Operator'S Manual, Page: 333 / 598

Section 8. Operation
333
equations. In the diagrams, resistors labeled R
s
are normally the sensors and
those labeled R
f
are normally precision fixed (static) resistors. CRBasic example
Four-Wire Full-Bridge Measurement
(p. 334)
lists CRBasic code that measures and
processes four-wire full-bridge circuits.
Offset voltages compensation applies to bridge measurements. In addition to
RevDiff
and
MeasOff
parameters discussed in
Offset Voltage Compensation
(p.
323),
CRBasic bridge measurement instructions include the RevEx parameter that
provides the option to program a second set of measurements with the excitation
polarity reversed. Much of the offset error inherent in bridge measurements is
canceled out by setting RevDiff , MeasOff , and RevEx to True .
Measurement speed can be slowed when using RevDiff , MeasOff , and RevEx .
When more than one measurement per sensor are necessary, such as occur with
the BrHalf3W() , BrHalf4W() , and BrFull6W instructions, input and excitation
reversal are applied separately to each measurement. For example, in the four-
wire half-bridge (
BrHalf4W()
), when excitation is reversed, the differential
measurement of the voltage drop across the sensor is made with excitation at both
polarities and then excitation is again applied and reversed for the measurement of
the voltage drop across the fixed resistor. Further, the results of measurement
instructions (X) must be processed further to obtain the resistance value. This
processing requires additional program execution time.
Resistive-Bridge Circuits with Voltage Excitation
Resistive-Bridge Type and
Circuit Diagram
CRBasic Instruction and
Fundamental Relationship
Relational Formulas
Half-Bridge
1
CRBasic Instruction: BrHalf()
Fundamental Relationship
2
:
Three-Wire Half-Bridge
1,3
CRBasic Instruction: BrHalf3W()
Fundamental Relationship
2
: