There are two main types of Doppler flow meter systems in common use today, Continuous Wave and Pulsed Wave. They
differ in transducer design and operating features, signal processing procedures and in the types of information provided.
Continuous wave (CW) Doppler is the older and electronically more simple of the two kinds. As the name implies, CW Doppler
involves continuous generation of ultrasound waves coupled with continuous ultrasound reception. A two crystal transducer
accomplishes this dual function. The main disadvantage of CW Doppler is its lack of selectivity or depth discrimination. CW
Doppler measurements are a spot velocity measurement. The sensor is not able to determinate at which level the velocity
has been detected. Due to this reason the flow profile cannot be represented. To correct this deficiency, the average flow
velocity is computed using a calibration factor. This factor must be determined previously such as by implementing a grid
measurement which is time consuming and expensive.
9.2. Pulse Wave Doppler (PW)
Pulsed wave (PW) Doppler systems use a transducer that alternates transmission and reception of ultrasound. One main
advantage of pulsed Doppler is its ability to provide Doppler shift data selectively from a small segment along the ultrasound
beam, referred to as the “SCAN Window” or cell. The location of the SCAN Window is operator controlled. The ultrasonic pulsed
wave Doppler is a new development that supersedes all the older Doppler principles. In contrast to CW Doppler principle,
with the Pulse Doppler a shorter ultrasonic frequency bundle of defined length is transmitted. This makes it possible to
assign a defined measurement window for received signals, step by step over the entire flow profile. The frequency shift
of the transmitted ultrasonic signal into a defined measurement window is the measurement of the flow velocity in that
measurement window. Reflections of particles in other areas do not have any influence on the velocity measurement.
9.3. Ultrasonic Signal Diagnostics
The flow information is derived from ultrasonic signals travelling through a moving fluid. One of the most critical aspects
of the ultrasonic flow meter diagnostic software is to evaluate the individual ultrasonic signals to determine if the signal
is acceptable for an accurate velocity measurement. If there is a problem with the ultrasonic signals themselves, the flow
measurement will be incorrect. It is impossible to state what an ultrasonic signal should look like in general, but essentially
the signal needs to appear as expected by the receiver and processing software.
The amplitude or strength of the ultrasonic signal (dB) depends on just about every aspect of the measurement system.
Depending on the distance of the SCAN Window from the sensor, the lower is the received signal because of spreading and
signal scattering within the fluid. High suspended loads weaken the signal as well.
In general, signal strength readings between 50 dB and 80 dB are expected with a properly mounted sensor. Signal strength
readings higher than 90 dB may indicate that the sensor need to be cleaned, as dirt may weaken the signal or sediment covers
the sensor. Signal strengths lower than 35 dB are considered to be no signal at all.
Operating Principle
Page 106 March 2021HYB-UM-03155-EN-03
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