The Ultrasonic Flowmeter -  Utilizing Wave Behavior To Determine Flow Rate

An ultrasonic flowmeter works on the principle that the flow of a liquid can be determined if information about sound waves that are passed through the flowing liquid is known. There are different types of ultrasonic flowmeter but all have the same basic idea.

Transit time ultrasonic flow meters have two transmitter/receivers of ultrasonic sound pulses. These devices are normally placed at a set distance determined by the pipe material and  thickness and the type of liquid. One transmitter/receiver is positioned so that the sound wave it transmits moves with the flow of liquid.

The other transmitter/receiver is placed so that the sound wave it sends out is going against the flow. When these two sound waves are measured the difference is the time it takes the waves can be used to determine the flow rate of the liquid in the pipe.

These types of flow meters are useful for clean liquids and the accuracy of them is affected by bubbles or particulate matter in the liquid.

Another type of flowmeter that uses sound waves to determine flow rates is the Doppler shift flowmeter. This is more for liquids that contain bubbles or matter in the liquid.

The idea of this meter is based on the idea that the frequency and wavelength of a sound wave is changed when it reflects off a moving particle. This change in the sound wave from outgoing to incoming wave can be used to determine the velocity of the flow of liquid. This can help to determine the volumetric flow rate if other variables are known.    

How do the Designs Vary?

Ultrasonic flow meters are generally easy to install. The sensors are clamped onto a pipe where the flow rate is to be measured. There is no moving parts that can wear out or interfere with the flow of the liquid.

The clamp-on ultrasonic flowmeters come in a single or a dual-sensor version. In the single-sensor version, it transmits and can receive crystals that are ported into the same sensor body, which is clamped on to a single point of the pipe’s surface, and a coupling compound is used to ultrasonically connect the sensor to the pipe.

In the dual sensor version, the transmit crystal is in one sensor body, while the receiving crystal is in a different sensor body.

Clamp-on Doppler flowmeters are subject to intrusion from the pipe wall itself, also from any air space between the sensor and the wall, and if the pipe walls are made of stainless steel, it could conduct the transmit signal far enough so that the returning echo will be shifted enough to interfere with the reading.

There is also a built-in acoustic discontinuity in copper, concrete-lined, plastic-lined, and fiberglass-reinforced pipes, which are significant enough to either completely, scatter the transmitted signal or attenuate the return signal. This dramatically decreases flowmeter accuracy and most of the time clamp-on meters will not work at all if the pipe is lined.

Learn more about a Peak Flow meter.