Biomedical Engineering Reference
In-Depth Information
Nylon
Polyropylene 7823
High density polyethylene
Hytrel 7241
Teflon TFE I
Teflon TFE II
Hard U.V. curable
Soft U.V. curable
6
5
4
3
2
1
0.1
1
Frequency (kHz)
10
FIGURE 4.27
Frequency dependence of pressure sensitivity of fibers with a 0.7-mm outer diameter coated
with various elastomers at 27°C.
stress decreased, the pressure interaction area per fiber strand will decrease
accordingly.
Giallorenzi et al. [55] published various phase versus pressure sensitivi-
ties for 0.700-mm-diameter fiber jackets of various materials. These data also
show the acoustic frequency dependence. Of the several materials shown,
Teflon TFE has the greatest sensitivity, but nylon exhibits the smallest fre-
quency dependence. Since they differ only by a factor of 1.4, nylon appears
to be the better choice. From the published graph (Figure 4.27), the phase
response
12
P = 1.45 10
Δ
Δ
3 10
×
rad/rad
lb/ n.
9
2
=
214 10
×
rad/rad/#/in.
(4.15)
5
2
×
i
Here ΔØ is the optical phase change for the pressure change of 1.45 × 10 −5
lb in. −2 over the length of a fiber whose end-to-end transmission phase
delay is Ø. We now calculate the fractional phase change for a pressure
change of 0.01 in. of water.
P =
Δ
9
2
11
rad/rad
(4.16)
=
214
×
10
(
10
)
=
214
×
10
Δ
0 01
.
We want ΔØ for 0.01 in. of water to be 20,000 × 10 −6 rad, so we need to multi-
ply Ø by the ratio (20,000 × 10 −6 )/(214 × 10 −11 ) = 9.35 × 10 6 , that is, Ø = 9.35 × 10 6
rad. If we simplify the problem by assuming that the fiber length is the phase
 
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