Introduction : Ultrasonic Flow Measurement

An ultrasonic flow meter is a type of flow meter that measures the velocity of a liquid or gas (fluid) by using the principle of ultrasound. Using ultrasonic transducers, the flow meter can measure the average velocity along the path of an emitted beam of ultrasound, by averaging the difference in measured transit time between the pulses of ultrasound propagating into and against the direction of the flow. Ultrasonic flow meters are affected by the temperature, density and viscosity of the flowing medium. They are inexpensive to use and maintain because they do not use moving parts, unlike mechanical flow meters.

There are three different types of ultrasonic flow meters. Transmission (or contrapropagating transit-time) flow meters can be distinguished into in-line (intrusive, wetted) and clamp-on (non-intrusive) varieties. Ultrasonic flow meters that use the Doppler shift are called Reflection or Doppler flow meters. The third type is the Open-Channel flow meter.

The basic principle of operation employs the frequency shift (Doppler Effect) of an ultrasonic signal when it is reflected by suspended particles or gas bubbles (discontinuities) in motion. This metering technique utilizes the physical phenomenon of a sound wave that changes frequency when it is reflected by moving discontinuities in a flowing liquid. Ultrasonic sound is transmitted into a pipe with flowing liquids, and the discontinuities reflect the ultrasonic wave with a slightly different frequency that is directly proportional to the rate of flow of the liquid.

To detect flow through a pipe, ultrasonic flowmeters use acoustic waves or vibrations of a frequency >20 kHz. Depending on the design, they use either wetted or nonwetted transducers on the pipe perimeter to couple ultrasonic energy with the fluid flowing in the pipe.

Doppler Flowmeters. Doppler flowmeters are named for the Austrian physicist and mathematician Christian Johann Doppler (1803–1853), who in 1842 predicted that the frequencies of received sound waves depended on the motion of the source or observer relative to the propagating medium. To use the Doppler effect to measure flow in a pipe, one transducer transmits an ultrasonic beam of ~0.5 MHz into the flow stream.

Liquid flowing through the pipe must contain sonically reflective materials such as solid particles or entrained air bubbles. The movement of these materials alters the frequency of the beam reflected onto a second, receiving transducer. The frequency shift is linearly proportional to the rate of flow of materials in the pipe and therefore can be used to develop an analog or digital signal proportional to flow rate.

The key questions which need to be answered before selecting an ultrasonic flow meter or doppler flowmeter are:

1.    Does the liquid have particulates of 100 ppm of 100 microns in size?

2.    Do you require a handheld or continuous process monitor?

3.    Do you require an analog output?

4.    What is the minimum and maximum flow rate for the flow meter? 

5.    What is the minimum and maximum process temperature? 

6.    What is the minimum and maximum process pressure? 

7.    What is the size of the pipe? 

8.    Is the pipe always full?