handbook of modern sensors

11.4 Ultrasonic Sensors 369
Fig. 11.8. Block diagram of an ultrasonic flowmeter with alternating transmitter and receiver.
sinusoidal ultrasonic waves (about 3 MHz) are transmitted as bursts with the same
slow clock rate (400 Hz). A received sinusoidal burst is delayed from the transmitted
one by time T , which is modulated by the flow (Fig. 11.7B). This time is detected
by a transit-time detector; then, the time difference in both directions is recovered by
a synchronous detector. Such a system can achieve quite good accuracy, with a zero
drift as small as 5 × 11 −3 m/s 2 over the 4-h period.
An alternative way of measuring flow with ultrasonic sensors is to detect a phase
difference in transmitted and received pulses in the upstream and downstream directions.
The phase differential can be derived from Eq. (11.18):
f = 4πf Dv c cos
c 2 , (11.19)
where f is the ultrasonic frequency. It is clear that the sensitivity is better with the
increase in the frequency; however, at higher frequencies, one should expect stronger
sound attenuation in the system, which may cause a reduction in the signal-to-noise
ratio.
For the Doppler flow measurements, continuous ultrasonic waves can be used.
Figure 11.9 shows a flowmeter with a transmitter–receiver assembly positioned inside
the flowing stream. As in a Doppler radio receiver, transmitted and received frequen-
Fig. 11.9. Ultrasonic Doppler flowmeter.