Campbell CR850, Operator'S Manual, Page: 296 / 566

Section 8. Operation
296
8.1.3.3 Strain Calculations
Read More! The FieldCalStrain() Demonstration Program
(p. 154)
section has
more information on strain calculations.
A principal use of the four-wire full bridge is the measurement of strain gages in
structural stress analysis.
StrainCalc() calculates microstrain,
μ
ε , from an
appropriate formula for the particular strain bridge configuration used. All strain
gages supported by
StrainCalc() use the full-bridge schematic. In strain-gage
parlance, "quarter bridge", "half bridge" and "full bridge" refer to the number of
active elements in the electronic full-bridge schematic: quarter-bridge strain gage
has one active element, half-bridge has two, full-bridge has four.
StrainCalc() requires a bridge configuration code. Table StrainCalc()
Instruction Equations
(p. 296)
shows the equation used by each configuration code.
Each code can be preceded by a negative sign (-). Use a positive code when the
bridge is configured so the output decreases with increasing strain. Use a
negative code when the bridge is configured so the output increases with
increasing strain. In the equations in table
StrainCalc() Instruction Equations
(p.
296),
a negative code sets the polarity of V
r
to negative (-).
Table 63. StrainCalc() Instruction Equations
StrainCalc()
BrConfig Code Configuration
1
Quarter-bridge strain gage:
2
Half-bridge strain gage. One gage parallel to strain, the other at 90° to
strain.
3
Half-bridge strain gage. One gage parallel to + , the other parallel to -
:
4
Full-bridge strain gage. Two gages parallel to + , the other two parallel
to - :
5
Full-bridge strain gage. Half the bridge has two gages parallel to + and
- , and the other half to +
and - :