Biomedical Engineering Reference
In-Depth Information
2.4.13.2 Temperature Flowmeters
An improved method of measuring flow would be to mark the fluid in some way and
then to detect the movement of the mark. Marking techniques could be as simple as
mechanical floats or as complicated as radioactive elements or a dye that changes the
optical properties of the medium. In medicine, the dye dilution method is used for studies
in hemodynamics. Under most circumstances, however, placing any foreign material into
the fluid is impractical or forbidden.
A noninvasive marker is temperature, and sensors called hot-wire or thermo-
anemometers use this principle. One sensor type often used to measure blood flow consists
of a small isolated heating element immersed in the fluid between two similarly immersed
temperature probes. If the medium is not flowing, then diffusion will result in the two
probes reading the same temperature, but if there is flow, the downstream probe will be
warmer than the upstream one. In general, the heating element is placed closer to the
downstream element to improve sensitivity, and it must always be heated to above the
highest temperature reached by the fluid under normal conditions. In another method,
a thermistor placed in the blood stream is kept at a constant temperature using current
feedback. The amount of energy required to maintain the temperature is proportional to
the flow rate because the temperature of the blood and its thermal conductivity remain
constant (Cromwell, Weibell et al., 1973).
2.4.13.3 Ultrasound Flowmeters
Another nonintrusive method to measure flow is ultrasound. Sensors can measure changes
in the effective velocity in the medium, or they can use the Doppler shift. In Figure 2-61,
two transducers are placed on either side of a pipe directed through the fluid toward each
other. If a pulse is transmitted from one, the time,
T
(s), taken to reach the other will be
T
=
D
/
c
(2.64)
where
D
(m) is the distance between the transducers, and
c
(m/s) is the speed of sound in
the medium if the fluid is still.
If the fluid is flowing, then the propagation speed is altered by the flow, and equa-
tion (2.64) becomes
D
c
±
v
c
cos
T
=
(2.65)
θ
FIGURE 2-61
Ultrasonic
flowmeter.
