Some questions we are frequently asked :
What is ultrasound?
Ultrasound refers to acoustic waves or vibrations of a frequency beyond the range of human hearing (generally above 20,000 Hz).
At what frequencies do ultrasonic flow meters operate?
The frequency generally utilized is 0.4 MHz (400 kHz).
For small diameter (D < 300mm) pipes, 1 MHz (1,000 kHz) is used.
Why are such high frequencies used?
Frequencies in the normal range (i.e. in the audible range) are apt to mix with and become lost in the ambient noise.
As acoustic wave frequencies become higher, they share properties similar to light such as
(1) Rectilinear propagation
And are in accordance with the laws of physics which lends itself to analysis.
Is ultrasound harmful to humans or animals?
No. Ultrasound is increasingly being used in the medical field in place of X-rays.
Is the time difference method the only method used in ultrasonic flow measurement?
Presently there are three measurement methods in practical use.
(1) Frequency difference method
(2) Time difference method
(3) Doppler method
Methods (1) and (2) are mainly used for measurement of relatively clean water.
This is because fluctuations in propagation level of ultrasonic signals (i.e. sensitivity of ultrasonic transmissions and reception) become problematic with dirty water which reduces signal strength.
Measurement method (3) involves detection of frequency shifts (changes) in acoustic signals.
As changes in signal propagation level is not directly a factor, this method is suitable for measurement of dirty water such as sewage.
What kind of pipe material enables ultrasonic measurement?
We have had numerous experiences with measurements through steel, stainless steel, cast iron, ductile iron, and resins.
(1) Although we have measured through RC steel pipes, transmission of acoustic signals are difficult, and this type of pipe material is not conducive to ultrasonic measurement.
(2) Measurement through pipes of asbestos material is possible on rare occasions, but for all practical purposes, measurement is not possible with this material.
(3) Hume pipes cannot be measured with sensors mounted on the outside of the pipe. Special measurement methods incorporating sensors on the inside of the pipe are
employed for this type of application.