9. OPERATING PRINCIPLE
9.1. Doppler Flow Meter
An ultrasonic beam is transmitted into the flowing fluid at an oblique angle to the flow. The receiver collects scattered
ultrasound from a core group of flowing particles travelling down the central axis of the open channel or pipe. This received
ultrasound is shifted in frequency from the transmitted ultrasound in accordance with the general Doppler equation:
ƒ
D
is the Doppler shifted frequency
ƒ
0
is the transmit frequency
V is the average velocity of the scatterers
c
0
is the speed of sound of the fluid
α
0
is the beam angle relative to the scatterer velocity
The figure below depicts a simplified, two-dimensional diagram of the ultrasound beam injected into the flow stream with
the center of the beam at angle. To simplify this explanation, the velocities along the centerline of flow are only considered.
If all of the particles along the center flow line travel at the same velocity, there is a distribution of Doppler frequencies
corresponding to the change in cos(α), due to the divergence of the ultrasound beam. The received Doppler shifted
frequencies mix with the transmit frequency, resulting in a group of beat frequencies. The time domain, sinusoidal beat
signals are digitized and converted to the frequency domain via a fast Fourier transform (FFT). The frequency spectrum
generated by the FFT is scanned to find the dominant peak frequency. The point of highest intensity, the frequency spectrum
peak, indicates the center flow velocity.
Figure 13: Doppler Principle
Doppler flow meters are not generally promoted for use in clean fluids. In real applications, strong Doppler shifted signals are
required for reliable flow measurements.
The amplitude of the Doppler shifted signals from any application is largely related to the suitability of the sonic scatterers in
the flow for producing detectable Doppler signals.
Identifying suitable sonic scatterers for Doppler flow meters is a complex combination of four basic criteria. How well the
criteria are met determines the reliability and accuracy of a Doppler flow application.
The criteria are:
• The scattering material must have a sonic impedance different from the fluid
• There must be some particles large enough to cause longitudinal scattering
• For a given channel size, the longitudinal scattering must have sufficient energy to overcome the Rayleigh (energy
wasting) scattering caused by smaller particles
• The scattering material must travel at the same velocity as the fluid for good accuracy
Operating Principle
Page 105 March 2021 HYB-UM-03155-EN-03
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