handbook of modern sensors

11.4 Ultrasonic Sensors 367
A sensor’s design determines its operating limits. At a certain velocity, the
molecules of a moving medium while passing near a heater do not have sufficient
time to absorb enough thermal energy for developing a temperature differential between
two detectors. Because the differential is in the denominator of Eq. (11.13), at
high velocities computational error becomes unacceptably large and accuracy drops
dramatically. The upper operating limits for the thermal transport sensors usually
are determined experimentally. For instance, under normal atmospheric pressure and
room temperature (about 20 ◦ C), the maximum air velocity that can be detected by a
thermal transport sensor is in the range of 60 m/s (200 ft/s).
While designing thermal flow sensors, it is important to assure that the medium
moves through the detectors without turbulence in a nonlaminar well-mixed flow. The
sensor is often supplied with mixing grids or turbulence breakers which sometimes
are called mass equalizers (Fig. 11.4A).
The pressure and temperature of a moving medium, especially of gases, make a
strong contribution to the accuracy of a volume rate calculation. It is interesting to note
that for the mass flow meters, pressure makes very little effect on the measurement
as the increase in pressure results in a proportional increase in mass.
A data processing system for the thermal transport sensing must receive at least
three variable input signals: a flowing medium temperature, a temperature differential,
and a heating power signal. These signals are multiplexed, converted into digital form,
and processed by a computer to calculate characteristics of flow. Data are usually displayed
as velocity (m/s or ft/s), volume rate (m 3 /s or ft 3 /s), or mass rate (kg/s or lb/s).
Thermal transport flowmeters are far more sensitive than other types and have a
broad dynamic range. They can be employed to measure very minute gas or liquid
displacements as well as fast and strong currents. Major advantages of these sensors
are the absence of moving components and an ability to measure very low flow
rates. "Paddle wheel," hinged vane, and pressure differential sensors have low and
inaccurate outputs at low rates. If a small-diameter tubing is required, as in automotive,
aeronautic, medical, and biological applications, sensors with moving components
become mechanically impractical. In these applications, thermal transport sensors are
indispensable.
11.4 Ultrasonic Sensors
Flow can be measured by employing ultrasonic waves. The main idea behind the
principle is the detection of frequency or phase shift caused by flowing medium. One
possible implementation is based on the Doppler effect (see Section 6.2 of Chapter 6
for the description of the Doppler effect), whereas the other relies on the detection of
the increase or decrease in effective ultrasound velocity in the medium. The effective
velocity of sound in a moving medium is equal to the velocity of sound relative to
the medium plus the velocity of the medium with respect to the source of the sound.
Thus, a sound wave propagating upstream will have a smaller effective velocity, and
the sound propagating downstream will have a higher effective velocity. Because the
difference between the two velocities is exactly twice the velocity of the medium,