Precision Flow's competence is the design, manufacture and application of "Clamp on" non-invasive and non-intrusive ultrasonic liquid flow meters.  

What is ultrasound?

Sound generated above the human hearing range (typically 20KHz) is called ultrasound. However, the frequency range normally employed in clamp on ultrasonic non invasive transit time flow meters and Doppler flowmeters is 100Khz to 10Mhz. Although ultrasound behaves similarly to sound, it has a much shorter wavelength.

What is a non intrusive clamp on ultrasonic flow meter?

Many flow meters rely on mechanical contact with the fluid and operate by mechanically altering the fluids flow path or by partially obstructing the fluids path to induce a measurable pressure drop or using the energy in the flowing liquid  to move some mechanical device. Precision Flow's clamp on ultrasonic flow meters operate with out mechanical obstructions and with out entering the pipe. As such they are non intrusive. Non intrusive ultrasonic flow meters are energy savers and are suitable for use with any sonically conductive liquid. As Precision flows clamp on ultrasonic flowmeters clamp to the outside of  a pipe they are also non intrusive. Intrusive ultrasonic probes are available to special order if required. These require holes to be drilled into the pipe wall. We mainly sell Clamp on sensors as these are more flexible and installation is obviously simple. The larger the pipe size, the more economical the clamp-on ultrasonic flowmeter becomes.

What are the different types of sonic flow meter?

The ultrasonic flow meter class is subdivided into four basic types:

Time of flight, Doppler, cross correlation and swept beam meters.

The most common types in general use are the Time of flight meter and  the Doppler ultrasonic flowmeter both of which Precision Flow offer. The cross correlation meter is not to be confused with a correlation detection system used in our clamp on ultrasonic flow meter such as the precision flow 190P.

In both types a piezoelectric crystal is excited by electrical energy at its mechanical resonance, thus emitting a sound wave, which, travelling at the speed of sound in the fluid, is used to infer the flow rate. The crystal is mounted in a transducer body this is either placed in contact with the fluid ( wetted or inserted transducers) or else mounted outside the piping (clamp on ultrasonic transducers)

How does a transit time time of flight flow meter work?

In the time of flight type transit time flow meter (in its most basic form), a transmitter beams a high frequency (approximately 1.5Mhz) pressure wave so as to form fixed cross angle with the pipe axis. The transit time employed by the wave to reach a receiver placed on the opposite pipe wall. The opposite measurement is then made. The upstream transit times are subtracted from the down stream times.  The resulting time difference is directly proportional to the flow velocity. As the internal cross sectional area of the pipe is easily measured  so the volumetric flow rate is easily calculated. The difficult part is accurately timing the upstream and down stream transit times the Precision Flow 190 P clamp on ultrasonic flow meter for example uses a special correlation detection system with a resolution of better than 20 pico seconds.

How does a Doppler flowmeter work?

Doppler flowmeters utilise the well known Doppler effect,  this is named after Christian Doppler, who documented  the effect in 1842. In general terms it is the change in frequency and wavelength of a wave as perceived by an observer moving relative to the source of the waves. In the Doppler  flow meter (in its most basic form), a transmitter beams a high frequency (approximately 1Mhz) pressure wave so as to form fixed cross angle with the pipe axis. As the beam travels into the non-homogeneous fluid, some energy is scattered back by solid particles or gas bubbles entrained in the flow. The relative motion of these discontinuities produces a frequency shift of the scattered wave, which is received and analysed by the ultrasonic flow meter. The different frequency  is known as the Doppler shift. this is linearly proportional to the fluid velocity. As the internal cross sectional area of the pipe is easily measured  so the volumetric flow rate is easily calculated.

Where are transit time ultrasonic flowmeters used?

Precision Flow Time of flight transit time ultrasonic flowmeters are generally used in clean liquid applications, where the ultrasonic beam is not attenuated or continually interrupted by fluid particles.

Where are Doppler ultrasonic flow meters used?

Precision Flow Doppler flow meters are used on applications where the liquid contains at least 100 PPM or larger suspended particles or bubbles sufficient to scatter signals toward the sensor. The minimum quantity of these sonic discontinuities needed depends on the fluid and the size of the particles because of the effect on velocity profile, accuracy depends on particle concentration. Doppler meters are less accurate than transit time meters but have good repeatability.

How important is pipe material?

Both transit time ultrasonic flow meters and Doppler flowmeters require  a non-porous sonically conductive pipe wall. Most pipes are sonically conductive there are a few exceptions and then you can not use a clamp on flow meter. If you would like to discuss your application with one of our engineers please do not hesitate to contact us.

Contact Information

Please phone or email us with any enquires you may have.

Telephone:  +44 (0) 1494 439127   Fax: +44(0) 8451 274255

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Last modified: December 05, 2008