Biomedical Engineering Reference
In-Depth Information
EXAMPLE PROBLEM 10.8
Calculate the strain in a metal wire gauge for a fractional change in resistance of 10 percent.
Solution
Combine Eqs. (10.6) and (10.11) to obtain
D
R
R
¼
2
D
l
l
¼
2
S
0
1
R
¼
:
2
S
0
:
05
R
S
¼
Strain gauges typically fall into two categories: bonded or unbonded. A bonded strain
gauge has a folded thin wire cemented to a semiflexible backing material, as illustrated in
Figure 10.14.
An unbonded strain gauge consists of multiple resistive wires (typically four) stretched
between a fixed and a movable rigid frame. In this configuration, when a deforming force
is applied to the structure, two of the wires are stretched, and the other two are shortened
proportionally. This configuration is used in blood pressure transducers, as illustrated in
Figure 10.15. In this arrangement, a diaphragm is coupled directly by an armature to a mov-
able frame that is inside the transducer. Blood in a peripheral vessel is coupled through a
thin fluid-filled (saline) catheter to a disposable dome that is sealed by the flexible dia-
phragm. Changes in blood pressure during the pumping action of the heart apply a force
on the diaphragm that causes the movable frame to move from its resting position. This
movement causes the strain gauge wires to stretch or compress and results in a cyclical
change in resistance that is proportional to the pulsatile blood pressure measured by the
transducer.
In general, the change in resistance of a strain gauge is typically quite small. In addition,
changes in temperature can also cause thermal expansion of the wire and thus lead to large
changes in the resistance of a strain gauge. Therefore, very sensitive electronic amplifiers
FIGURE 10.14
A bonded-type strain gauge transducer.
