Ultrasonic Flow Meters
The Ultrasonic technologies in general use for closed piping applications are Doppler and transit time. The non-invasive flow sensing capability of these meters 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
- Water & Wastewater
- Condensate
- Flow Surveys
- Remote Field Site Tests
- Acids & Caustics
- Pulp & Paper Slurries
- Chilled Water
- Potable Water
- Mining Recirculate
- Toxic Liquids
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.
Operating Principle
The Ultrasonic Doppler Shift 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 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.*
* Ultrasonic flow meters measure the average flow velocity of the media along the sonic path. For further details, refer to "Fluid Flow Profiles" in Insertion-style Turbine & Paddle Wheel Flow Metering Technology Explained, or call an Instrumart engineer.
| Single Hand-held Transducer | Set of 2 Clamp-on Transducers | |
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Enhanced Operational Reliability
The proprietary circuitry in the YD50-B Series units automatically adjusts the signal sensitivity, allowing measurement of cleaner flowing liquids as well as more stable, repeatable measurement of liquids with suspended solids and entrained gases.
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.
Transit Time
Transit Time Flow Meters feature the most advanced non-invasive flow measurement technology available. These meters provide a low cost system with unsurpassed accuracy and versatility.
The Transit Time Flow Meter utilizes two transducers which function as both ultrasonic transmitters and receivers. The 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.