Ultrasonic flowmeters pass high-frequency sound wave to fluid path to measure the fluid velocity. The ultrasonic method utilises the change in the propagation time in the flowing fluid. There are two major types of ultrasonic flowmeters: Doppler and transit time type
Doppler flowmeter:
The Doppler flowmeter works based on the principle of Doppler’s effect, which says that the frequency of the sound waves shifts when reflected by a moving object.
This type of flowmeter requires objects in the flow stream. The incident wave hit the object or bubbles and reflects back as the reflected wave, the difference in the frequencies of the incident and reflected waves are measured which is proportional to the function of fluid flow in the stream.
If the reflected wave returns from a bubble advancing toward the ultrasonic transducer, the reflected frequency will be greater than the incident frequency. If the flow reverses direction and the reflected wave returns from a bubble travelling away from the transducer, the reflected frequency will be less than the incident frequency.
The relation between fluid flow and frequency shit:
^f = frequency shift f = incident frequency v = velocity of fluid c = speed of wave in process fluid
Transit-time:
Also known as counter propagation flowmeter. Transit time flow meter uses a pair of transducers both are capable of sending and sensing the sound signals. Usually used transducers are piezoelectric transducers.
The transducers are mounted, one opposite to another with an elevation which forms an upstream and downstream. The incident wave is received at the other end with a time span. The time lag in receiving the signal is proportional to the flow of the fluid. The signal sends in both directions upstream and downstream.
The relationship between flow speed and transit time:
Q = Calculated volumetric flow K = constant of proportionality t up = Time for sound pulse to travel from downstream location to upstream location t down = Time for sound pulse to travel from upstream location to downstream location
Advantages of ultrasonic flowmeter:
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No moving parts and allow unabstracted flow
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Can operate in both direction
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Unaffected by temperature, density and concentration
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Low maintenance
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No additional pressure drop
Disadvantages of ultrasonic flowmeter:
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Transit time required clean liquid and doppler meter need objects in the liquid.
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Sound beam must transverse a representative cross section, therefore flow profile dependent
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Long inlet and outlet section required.