Chapter 4
Theory of Operation
4-4
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•
Temperature variation in specimen decreases the accuracy of the
measurements.
•
Sensitivity at 1000
µε
, where GF = 2.0, is
∼
0.5 mV
out
/
V
EX
input.
Figure 4-3.
Quarter-Bridge Type I Circuit Diagram
The following symbols apply to the circuit diagram and equations:
R
1
and R
2
are half-bridge completion resistors.
R
3
is the quarter-bridge completion resistor (dummy resistor).
R
4
is the active strain-gauge element measuring tensile strain (+
ε
).
V
EX
is the excitation voltage.
R
L
is the lead resistance.
V
CH
is the measured voltage.
V
r
is the voltage ratio that is used in the voltage to strain conversion
equations and is defined by the following equation:
To convert voltage readings to strain units use, the following equation:
where
R
g
is the nominal gauge resistance of the sensor.
R
L
is the lead resistance.
GF
is the Gauge Factor.
+
–
–
+
V
EX
R
1
R
L
R
2
R
3
V
CH
R
4
(+ )
R
L
R
L
V
r
V
CH
strained
(
)
V
CH
(unstrained)
–
V
EX
-------------------------------------------------------------------------------
=
strain
ε
( )
4
–
V
r
GF
1 2
V
r
+
(
)
-------------------------------
1
R
L
R
g
------
+
×
=