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Engineering ToolBox > Ultrasonic Doppler and Time of Flight Flowmeters
Ultrasonic flowmeters works in general after two different principles:
The Doppler Effect Ultrasonic Flowmeter use reflected ultrasonic sound to measure the fluid velocity. By measuring the frequency shift between the ultrasonic frequency source, the receiver, and the fluid carrier, the relative motion are measured.
The resulting frequency shift is named the Doppler Effect.
The fluid velocity can be expressed as
v = c (fr - ft) / 2 ft cosΦ (1)
where
fr = received frequency
ft = transmission frequency
v = fluid flow velocity
Φ = the relative angle between the transmitted ultrasonic beam and the fluid flow
c = the velocity of sound in the fluid .. more about speed of sound
This method require there is some reflecting particles in the fluid. The method is not suitable for clear liquids.
Doppler meters may be used where other meters don't work. This might be liquid slurries, aerated liquids or liquids with some small or large amount on suspended solids. The advantages can be summarized to:
The Doppler flowmeters performance are highly dependent on physical properties of the fluid, such as the sonic conductivity, particle density, and flow profile.
Non uniformity of particle distribution in the pipe cross section may result in a incorrectly computed mean velocity. The flowmeter accuracy is sensitive to velocity profile variations and to the distribution of acoustic reflectors in the measurement section.
Unlike other acoustic flowmeters, Doppler meters are affected by changes in the liquid's sonic velocity. As a result, the meter is also sensitive to changes in density and temperature. These problems make Doppler flowmeters unsuitable for highly accurate measurement applications.
With the Time of Flight Ultrasonic Flowmeter the time for the sound to travel between a transmitter and a receiver is measured. This method is not dependable on the particles in the fluid.
Two transmitters / receivers (transceivers) are located on each side of the pipe. The transmitters sends pulsating ultrasonic waves in a predefined frequency from one side to the other. The difference in frequency is proportional to the average fluid velocity.
Downstream pulse transmit time can be expressed as
td = L / (c + v cosΦ) (2)
where
td = downstream pulse transmission time
L = distance between transceivers
Downstream pulse transmit time can be expressed as
tu = L / (c - v cosΦ) (3)
where
tu = upstream pulse transmission time
Since the sound travels faster downstream than upstream, the difference can be expressed as
t = td - tu
= 2 v L cosΦ / ( c2 - v2 cos2Φ)
= 2 v L cosΦ / c2 (4) (since v is very small compared to c)
The operating principle for the ultrasonic flowmeter requires reliability high frequency sound transmitted across the pipe. Liquid slurries with excess solids or with entrained gases may block the ultrasonic pulses.
Ultrasonic flowmeters are not recommended for primary sludge, mixed liquor, aerobically digested sludge, dissolved air flotation thickened sludge and its liquid phase, septic sludge and activated carbon sludge.
Liquids with entrained gases cannot be measured reliably.