Ultrasonic Flowmeter
● Measuring Principle
An ultrasonic flowmeter is an instrument that measures flow rate by detecting the effect of fluid flow on an ultrasonic beam (or ultrasonic pulse). Ultrasonic flowmeters typically adopt the
time-difference measurement principle.
One transducer transmits a signal through the pipe wall, the medium, and the opposite pipe wall, where it is received by a second transducer. At the same time, the second transducer transmits a signal that is received by the first transducer. Due to the influence of the fluid velocity, a time difference Δt exists between the two signals.
From derivation, the relationship between flow velocity
V and time difference Δt is:
V=2LC2⋅Δt
From velocity
V, the volumetric flow rate
Q can be further calculated.
When ultrasonic waves propagate in flowing fluid, they carry information about the fluid velocity. By analyzing the received ultrasonic signals, the fluid velocity can be detected and converted into flow rate.
According to detection methods, ultrasonic flowmeters are classified into several types:
time-difference method,
Doppler method,
beam deflection method,
noise method,
correlation method, etc.
Ultrasonic flowmeters have been widely applied only in recent decades with the rapid development of integrated circuit technology.
An ultrasonic flowmeter consists of three parts:
- ultrasonic transducers
- electronic circuitry
- flow display and totalizing system
Ultrasonic flow measurement accuracy is almost unaffected by fluid temperature, pressure, viscosity, density, etc. They can also be designed as non-contact and portable instruments.
(1) Phase-Difference Method
If the ultrasonic transmitter emits continuous pulses or long pulse trains, a phase difference ΔΦ arises between the downstream and upstream received signals:
ΔΦ=ωΔt=c22ωLu
where
ω = angular frequency of ultrasound.
Measuring ΔΦ allows calculation of velocity
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