©
National Instruments Corporation
41
SCC-SG Series Strain-Gauge Modules User Guide
Formulas and Variable Definitions
In the equations in this document, the following formulas and variable
definitions are used:
ε
S
is the simulated strain.
GF
is the Gauge Factor, which should be specified by the gauge
manufacturer.
R
g
is the nominal gauge resistance, which should be specified by the gauge
manufacturer.
R
L
is the lead resistance. If lead lengths are long,
R
L
can significantly
impact measurement accuracy.
Note
You can neglect the
R
L
of the wiring if shunt calibration is performed or if lead
length is very short (~ <10 ft), depending on the wire gauge. For example 10 ft of 24-AWG
copper wire has a lead resistance of 0.25 W.
R
s
is the shunt calibration resistor value.
U
is the ratio of expected signal voltage to excitation voltage with the shunt
calibration circuit engaged. Parameter
U
appears in the equations for
simulated strain and is defined by the following equation:
V
r
is the voltage ratio that is used in the voltage to strain conversion
equations and is defined by the following equation:
Quarter-Bridge Type I
To convert voltage readings to strain units use the following equation:
To simulate the effect on strain of applying a shunt resistor across R
4
, use
the following equation:
U
R
g
–
4
R
s
2
R
g
–
------------------------
=
V
r
V
CH
strained
(
)
V
CH
(unstrained)
–
V
EX
-------------------------------------------------------------------------------
=
strain
ε
( )
4
–
V
r
GF
1 2
V
r
+
(
)
-------------------------------
1
R
L
Rg
-------
+
×
=
ε
s
4
U
GF
1 4
U
+
(
)
------------------------------
=
