Ultrasonic Doppler and Time of Flight Flowmeters

An basic introduction to the ultrasonic Doppler and Time of Flight Flowmeters

Ultrasonic flowmeters works in general after two different principles:

• The Doppler Effect Ultrasonic Flowmeter
• The Time of Flight Ultrasonic Flowmeter

The Doppler Effect Ultrasonic Flowmeter

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 (f_r - f_t) / 2 f_t cosΦ (1)

where

f_r = received frequency

f_t = 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.

Advantages with the Doppler Effect Ultrasonic Flowmeter

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:

• Obstruct less flow
• Can be installed outside the pipes
• The pressure drop is equal to the equivalent length of a straight pipe
• Low flow cut off
• Corrosion resistant
• Relative low power consumption

Limitations with Doppler Effect Ultrasonic Flowmeters

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.

The Time of Flight Ultrasonic Flowmeter

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

t_d = L / (c + v cosΦ) (2)

where

t_d = downstream pulse transmission time

L = distance between transceivers

Downstream pulse transmit time can be expressed as

t_u = L / (c - v cosΦ) (3)

where

t_u = upstream pulse transmission time

Since the sound travels faster downstream than upstream, the difference can be expressed as

t = t_d - t_u

    = 2 v L cosΦ / ( c² - v² cos²Φ)

    = 2 v L cosΦ / c² (4) (since v is very small compared to c)

Benefits with Ultrasonic Flowmeters

• Obstruction less flow
• Pressure drop equal to an equivalent length of straight pipe
• Unaffected by changes in temperature, density or viscosity
• Bi-directional flow capability
• Low flow cutoff
• Corrosion-resistant
• Accuracy about 1% of flow rate
• Relative low power consumption

Limitations with Ultrasonic Flowmeters

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.

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