Ultrasound flowmeters

This method is based on the relationship between the flowrate of the stream and the velocity of ultrasound introduced in this stream. There are several modifications of this method, such as Doppler-effect method and transit-time method. The first one is based on the Doppler effect, saying that frequencies of received waves are dependent on the motion of the source or receiver (observer) relative to the propagating medium. We will describe the second method, which is shown schematically in Fig 6.6.

Figure 6.6. Transit-time flowmeter.

A source of ultrasound 1 is attached outside to the pipe 2 with a flowing fluid 3 inside it. A sonic beam is propagating the flowing fluid at a specific velocity, proportional to the properties of the fluid (temperature, pressure, and density). An ultrasound beam 4 will travel faster in the direction of flow, and slower in the opposite direction. This beam arrives in to the receiver 5 faster than an ultrasound beam 6 from the transmitter 7 to the receiver 8.


Transit time of ultrasound beam from transducer the 1 to the receiver 5 can be evaluated as follows (from Bentley J. P. Principles of Measurement Systems, Longman, 1995, p. 411-412):

(6.46)

Transit time of ultrasound beam from the transducer 7 to the receiver 8 can be evaluated as follows:(6.47)

Let’s evaluate the time difference:

(6.48)

The ratio , therefore,(6.49)


Using (6.49) we can reduce (6.48) to the following form:

(6.50)

where, 

These devices can not be used for flow measurements of fluids with air bubbles or solid particles, since they will interfere with the transmission and receipt of ultrasound radiation. These particles serve as reflectors of ultrasound radiation.

Article Source:: Dr. Alexander Badalyan, University of South Australia