A Flow Measurement Primer - Doppler Flow Meters     

Doppler Flow Meters monitor the flow rate of “difficult” liquids
including: wastewater, chemicals, acids, slurries, abrasives and viscous
liquids. Recommended for full pipes and any fluid that contains solids or
bubbles. The DFM-IV Doppler flow meter works best in applications that would defeat regular contacting flow meters. Because the Sensor is mounted on the outside of the pipe, it is unaffected by abrasives or harsh chemicals. There is no obstruction to the flow and no pressure drop.


Greyline - DFM IV

Construction/Design Principles:

Doppler flow meters measure flow from outside a pipe with a strap-on sensor. Doppler meters continuously transmit high frequency sound (640 kHz) that travels through the pipe wall and into the flowing liquid. Sound is reflected back to the sensor from solids or bubbles in the fluid. If the fluid is in motion, the echoes return at an altered frequency proportionate to flow velocity. Doppler flow meters continuously measure this frequency shift to calculate flow.

Applications: 

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Sewage

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Viscous Liquids

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Acids
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Slurries

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Solvents

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Treated Waste Water
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Sludge

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Pulp Stock

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Food Products
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Lubricating Oils

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Crude Oils

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Chemicals
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Aerated Water

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Abrasives

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Aggressive/Toxic Media
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Difficult liquids - Any liquid containing gas bubbles or solids larger than 100 Microns in concentrations larger than 75 PPM
   

Additional Information:

  

The Doppler effect was first documented in 1842 by Christian Doppler, an Austrian physicist. We hear everyday examples of Doppler: the sound of a train whistle changing pitch as it passes by, or the exhaust noise from a race car as it speeds past our location.

 
The Doppler technique only works on liquids which contain solids or gas bubbles to reflect its signal. These are "difficult" liquids that may damage regular flow meters: slurries, sludge, wastewater, abrasives, viscous and corrosive chemicals. Because the sensor mounts on the outside of the pipe, there is no pressure drop and no obstruction to flow.
 
For best performance Doppler sensors should be mounted away from turbulence creating devices like pipe elbows and tees, and away from velocity increasing devices like controlling valves and pumps. Typical accuracy is ±2% of full scale.
 
Doppler instruments include a strap-on sensor, connecting cable and an electronics enclosure which can be mounted at a convenient location nearby (within 500 ft / 152 m). Sensors can be rated intrinsically safe for mounting in hazardous-rated locations.
 

For complete product literature and applications information on Ultrasonic Level, Open Channel Flow and Doppler Flow please contact Cross Instrumentation - Phone: 800-332-3418
 

Information provided above, Compliments Of Greyline Instruments, Inc.
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Copyright © 2002 by Cross Instrumentation 
Specifications subject to change without notice - 10/01/02
 NOTE: The information provided above  has been compiled and published as a convenience to our customers. All technical designs, advice, and recommendations, including but not limited to, that appearing on this document are rendered free of charge. As a result, Cross Instrumentation disclaims any responsibility or liability to the buyer or user of this information for any result obtained or damages incurred by reason of the use or application of the design or other information contained herein.