Biomedical Engineering Reference
In-Depth Information
into electrical signals. At higher frequencies,
microphones
, commonly known as ultra-
sound transducers, are used in sonar devices for sensory substitution systems discussed in
Chapter 7 and in medical Doppler and imaging systems.
A large variety of physical mechanisms can be harnessed to perform the microphone
function. These include electromagnetic induction discussed in Chapter 3 as well as optical
interferometry and the piezoelectric effect discussed earlier in this chapter. However, since
the advent of MEMSs, millions of low-cost microphones have been produced that use
changes in the capacitance between a fixed plate and a vibrating diaphragm to produce a
varying electrical output.
2.4.13 Flow
The most common medical applications of flow measurement include blood flow through
arteries and veins and airflow into and out of the lungs. It also includes the measurement
of drug dispensing through catheters and gas flow during anesthetics, among others. A
great number of different sensors can be used to measure flow by determining the rate
of displacement of either mass or volume. However, whichever sensor is used, inherent
difficulties in the measurement make the process complicated, and it is necessary to
consider the natural characteristics of both the environment and the medium (e.g., pipe
shapes and materials; temperature, pressure, and viscosity of the fluid; Fraden, 1996).
2.4.13.1 Differential Pressure Flowmeter
Flow is commonly measured using a differential pressure flowmeter across an orifice plate,
a Venturi tube, or a nozzle. The operation of these flowmeters is based on an observation
by Bernoulli that if an annular restriction was placed in a pipe, then the velocity of the
fluid through the restriction was increased. The increase in velocity at the restriction then
causes the static pressure to decrease, and a pressure difference is built up. The difference
between the pressure upstream and the pressure downstream of the restriction is related to
the rate of flow through it and therefore through the pipe. A differential pressure-based flow
transducer consists of an obstruction to cause a pressure drop and a method of measuring
the differential pressure across the obstruction (Webster, 1999).
Bernoulli's equation, which defines the relationship between fluid velocity,
v
, fluid
pressure,
p
, and height,
h
, above some fixed point for fluid flowing through a pipe of varying
cross section is the fundamental to quantifying the differential pressure measurement
technique.
For the inclined, tapered pipe shown in Figure 2-58, Bernoulli's equation states that
2
1
2
2
p
1
ρ
v
p
2
ρ
v
g
+
2
g
+
h
1
=
g
+
2
g
+
h
2
(2.59)
The sum of the pressure head, the velocity head, and the potential head is constant along
a flow streamline. It is assumed that the flow is frictionless (no viscosity) and of constant
density (incompressible).
If there is a restriction in a piece of pipe as shown in Figure 2-59, then since
h
1
=
h
2
Bernoulli's equation can be rewritten as
2
1
2
2
p
1
−
p
2
=
v
−
v
(2.60)
ρ
2
