Campbell Scientific 4WFB120, 4WFB350, 4WFB1K 4-Wire Full Bridge Terminal Input Module - Manual (Page 20)

4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules
Subtracting the unstrained (zero) result from the strained result gives V
r
:
V
Vout
Vin
Vout
Vin
R R
R R R
R
R R
R R
R R R R R
r
strained unstrained
g g
g g
g
g
g
g g g
=
⎛
⎝
⎜
⎞
⎠
⎟ −
⎛
⎝
⎜
⎞
⎠
⎟ =
+
+ +
−
+
=
⋅
+ + ⋅ +
∆
∆
∆
∆
3
3
3 3
( ) ( )
3
6.6.
The terminal input module is selected so that R R
g 3
=
. Substituting R for
:
g
R
3
V
R R
R R R R R
R R
R R R
R
R R r
g g
g g g g g
g g
g g g
g
g g
=
⋅
+ + ⋅ +
=
⋅
+
=
+
∆
∆
∆
∆
∆
∆
( ) ( ) 4 2 4 2 2
6.7.
Solving for strain:
( )
( )
4 2
4 2
4 2
4 1 2
R R V
R V R V R
R V R R V
R V R V
g g r
g r g r g
g r g g r
g r g r
+ =
+ =
= −
= −
∆ ∆
∆ ∆
∆ ∆
∆
R
g
4
1 2
V
V
R
R
r
r
g
g
−
=
∆
6.8.
Strain is calculated by dividing equation 6.8 by the gage factor. The units are
converted to microstrain by multiplying by 10
6 uS/S.
( )
µε =
⋅
−
=
⋅
4 10
1 2
10 6 6
V
GF V
R
GF R
r
r
g
g
∆
6.9.
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