Frequently Asked Questions - Ultrasonic Flow Meters

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1. How Do Ultrasonic Flow Meters Work?

The Ultrasonic technologies in general use for closed piping applications are doppler and transit time. The non-invasive flow sensing capability of ultrasonic flow meters is provided by their "clamp-on" transducer offers a variety of advantages, including:

  • Quick, easy setup
  • Permanent or temporary flow measurement
  • No need to break into piping
  • No production downtime
  • No wetted parts to plug up flow passage
  • Complete chemical compatibility
  • Measures a wide range of flowing media, from clean liquids to slurries
  • No pressure drop
Applications for Ultrasonic Flow Meters:
  • Water & Wastewater
  • Condensate
  • Flow Surveys
  • Remote Field Site Tests
  • Acids & Caustics
  • Pulp & Paper Slurries
  • Chilled Water
  • Potable Water
  • Mining Recirculate
  • Toxic Liquids

2. What is the Doppler Effect?

The most common example of the "doppler effect" is the change, or shift, in sonic frequency that occurs when a moving vehicle, such as a car or train passes a specific (listening) point.

The approaching train projects a high-pitched sound, due to the compressed spacing (frequency) of the sonic waves in front of the approaching train. As the train passes the listener, the tone drops to a lower pitch, due to the expanded spacing of the sonic waves between the listener and the departing train.

The ultrasonic doppler flow meter employs a non-invasive piezoelectric transducer, hand-held or clamped on the outside surface of a full pipe, to transmit a continuous single frequency ultrasound through the pipe wall, into the flowing liquid. This signal is reflected from suspended particles, bubbles, or eddies and relayed to second receiver transducer (or a second receiving piezoelectric crystal within the same transducer). The ultrasonic doppler flow meter's microprocessor compares frequency shift (Doppler Effect) between the transmitted and returned signal. The degree of the Doppler Shift is proportional to the forward velocity of the flowing liquid.

Enhanced Operational Reliability

Conventional ultrasonic doppler technology can effectively measure flowing liquids containing suspended particles or entrained gases (air) larger than 30 microns size, in concentrations greater than 25 PPM. Some enhanced doppler designs can measure cleaner liquids (with fewer and/or smaller discontinuities) by sensing the turbulent swirls and eddies in the flow stream induced by non-symmetrical pipe configuration (i.e. elbows).

The non-invasive transducers can effectively transmit signals through PVC, steel, iron, and glass pipe walls. However, lined pipes and concrete pipes block the ultrasonic signal.

Single Hand-Held Transducer:
Single Hand-Held Transducer
Set of 2 Clamp-on Transducers:

3. What are Transit Time Flow Meters?

Transit time flow meters feature the most advanced non-invasive flow measurement technology available. Transit time flow meters provide a low cost system with unsurpassed accuracy and versatility.

Operating Principle

The transit time flow meter utilizes two transducers which function as both ultrasonic transmitters and receivers. The transit time flow meter operates by alternately transmitting and receiving a frequency modulated burst of sound energy between the two transducers. The burst is first transmitted in the direction of fluid flow and then against fluid flow. Since sound energy in a moving liquid is carried faster when it travels in the direction of fluid flow (downstream) than it does when it travels against fluid flow (upstream), a differential in the times of flight will occur. The sound's time of flight is accurately measured in both directions and the difference in time of flight calculated. The liquid velocity (V) inside the pipe can be related to the difference in time of flight (dt) through the following equation: V = K*D*dt, where K is a constant and D is the distance between the transducers.

Fuji Portaflow-X Ultrasonic Flow Meter The most attractive advantage of the Fuji Portaflow-X ultrasonic flow meter is the non-invasive setup, enabling multiple flow measurements of different running systems, and offering a significant cost advantage over time.

Fuji Electric Time Delta-C Ultrasonic Flow Meter The Fuji Electrid Time Delta C-Flow ultrasonic flow meter features high reliability and accuracy (1.0% of rate), and long-term maintenance-free operation. The non-intrusive, clamp-on sensors are quickly installed without interrupting process flows or incurring pressure loss.

Fuji Electric Time Delta S Ultrasonic Flow Meter The Fuji Electric Time Delta S ultrasonic flow meter is a fixed transit time flow meter for measuring flow rates of relatively clean homogeneous liquids utilizing clamp-on sensors. This flow meter is ideally suited for liquid flow measurement for pipe diameters from 0.50 to 235 inches.