Campbell Scientific 4WFBS120, 4WFBS350, 4WFBS1K 4 Wire Full Bridge Terminal Input Modules - Manual (Page 8)

4WFBS120, 4WFBS350, 4WFBS1K 4 Wire Full Bridge Terminal Input Modules (TIM)
FIGURE 2-1. Schematic
3. Measurement Concepts
Measuring strain is measuring a change in length. Specifically, the unit strain
( ) ε
is the change in length divided by the unstrained length
( )
L L / Δ = ε
,
and thus is dimensionless.
L + Δ L
L
L
T
L
T
+ Δ L
T
P P
FIGURE 3-1. Strain definition
As the subject is elongated in the longitudinal direction, the material will be
narrowed or thinned down in the transverse direction. The ratio of the
transverse strain to the longitudinal strain is known as the Poisson ratio (
ν).
L
L
L
L
T
T
Δ
Δ
= ν
3.1
This Poisson ratio is a known property for most materials and is used in some
half bridge strain and full bridge strain circuits.
Strain is typically reported in microstrain ( ) με
. Microstrain is strain
expressed in parts per million, i.e.: a change in length divided by one millionth
of the length.
A metal foil strain gage is a resistive element that changes resistance as it is
stretched or compressed. The strain gage is bonded to the object in which
strain is measured. The gage factor,
GF
, is the ratio of the relative change in
resistance to the change in strain: GF R R l l = Δ Δ / / . For example, a
2