Double signal filters out background noise

Sue Fielder, publicity manager for Peek Measurement, makes a case for Dual Frequency Doppler (DFD) technology. Fourier analysis is used to remove background noise from the actual flow data.

Over the years, the marketplace has been very sceptical about the use of Doppler technology in flow measurement.

With this, the choice of flowmeter techniques suitable for the ‘dirtier’ liquid applications has broadened.

End users are no longer restricted to the traditional electromagnetic or doppler solutions. Users of today’s modern Doppler flowmeters are now benefiting from higher product specifications, increased functionality, easier installation, maintenance and user friendly software.

For the last 25 years, Peek Measurement has been a leading developer, manufacturer and supplier of ultrasonic, non-invasive, clamp-on flowmeters.

As a specialist in Doppler technology, the company also has a policy of continuous development. Dual Frequency Doppler* (DFD) is the latest concept put into practice by Peek’s Polysonics Development Team.

The main drive behind the development of the DFD concept was the desire to further minimise the erratic effects of ‘site noise’ on flow measurements, to give end users consistent, repeatable and reliable readings in process plant and field use.

DFD range meters can now offer markedly improved performance in real-time measurement of homogeneous liquids that contain entrained gas or suspended particles.

Traditional market areas for Polysonics flowmeters include primary and secondary sewage processing as well as industrial processing such as process monitoring and effluent treatment.

With the established clamp-on design, advantages include:

  • simple installation and maintenance
  • no disruption to pipeline or process and no head loss
  • no-contact with process liquid
  • dedicated or portable units available
  • multi-channel options
How DFD Works

Two Piezo transducers (PZTs) are bolted to the outside of the pipe to transmit two independent ultrasonic signals into the pipeline flow. The signals remain unaffected by typical pipeline materials namely, concrete, metal or plastic.

The primary signal is of one frequency and the secondary of a different frequency, for example 500kHz and 400kHz. The two ultrasonic signals are both reflected, or ‘scattered’ by the solids or aeration contained within the flowing liquid and the scattering data are collected and sent to the flowmeter electronics where it is processed through a Discrete Fourier Transform (DFT).

The data collected from each transmission contains both Doppler and noise information

The two sets of Doppler information are shifted in frequency by a ratio of the transmission frequencies, however any noise that may be present at the application is not shifted.

One set of collected data is then stretched by a ratio of the two transmission frequencies. This process aligns the Doppler information in the same frequency domain and misaligns the noise (see Fig.2.).

Once the Doppler information is aligned, the two sets of data are multiplied together. Then the Doppler information is squared and the noise subtracted. This method makes tracking and identifying Doppler information much easier (see Fig.3.).

In line with the advances in Doppler technology that have improved the hardware, the internal diagnostics have become more sophisticated but are even simpler to use by the operator.

Advanced Diagnostics

The DFD flowmeters have a ‘learn mode’ for locking on range and reporting statistical information about signal strength and quality, even at extremely low velocities.

The internal expert system also features a unique ratings table to qualify the signal quality and strength as valid, marginal, risky or none. This facility requires no additional input from the operator but gives vital reassurance about the validity of the readings.

An automatic diagnostic expert system is also used, to characterise individual applications and compare readings against site data.

Alterations to the transmission power and flow range are made in direct response to these measurements, according to the conditions.


The Peek Measurement Polysonics range of ultrasonic, clamp-on flowmeters employ both transit-time and Doppler techniques, ensuring that the most appropriate solution is selected for each application.

Applications from ultra-pure water through to waste sewerage can be satisfied with the use of dedicated, portable or multi-channel models.

*patent pending

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