379
Measuring Strain
Correcting for wiring resistance
When using long strain gage wires, measurement is aected by their wiring resistance.
True strain values and strain values measured by the instrument are related as shown below.
You can use the scaling function to correct strain values. Set the conversion ratio slope to (
R
+
rL
) /
R
.
ε
i
=
R
R + rL
× ε
ε
i
: True strain value
ε
: Strain value measured by instrument
R
: Strain gage resistance value (
Ω
)
r
: Round-trip resistance value per 1 m of wire (
Ω
/m)
For 3-wire setup, one-way resistance value (
Ω
/m)
L
: Wire length (m)
Example settings
If using a strain gage with a wiring length of 5 m and a round-trip resistance value of 0.07
Ω
per
meter (with a gage resistance of 120
Ω
and a gage ratio of 2.00) with a 1-gage method/2-wire
connection
ε
i
=
120 + 0.07 × 5
120
=
120.35
120
≈
1.003
× ε
× ε × ε
ε
i
: True strain value
ε
: Strain value measured by instrument
Set the scaling conversion ratio’s slope to 1.003.
See “1.7 Using the Scaling Function” (p. 58).
If the instrument (LR8450, -01) and measurement target are located far apart, necessitating long
wiring, you can reduce error caused by wiring resistance by placing a Wireless Strain Module
(LR8534) close to the measurement target to minimize wiring length.
Correcting for gage factor
If using a strain gage with a gage factor other than 2.0
The U8554 and LR8534 measure strain with a gage factor of 2.0.
If using a strain gage with a gage factor other than 2.0, perform the following conversion to set the
scaling function’s conversion ratio slope:
ε
i
=
K
a
2.00
× ε
ε
i
: True strain value
ε
: Strain value measured by instrument
K
a
: Gage factor of strain gage being used
Example settings
If using a strain gage with a gage factor of 2.10
ε
i
=
K
a
2.00
× ε
=
2.10
2.00
× ε
=
0.952
× ε
Set the scaling conversion ratio’s slope to 0.952.
Knowledge and Information
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