Flow metering tutorial - Part 4: Ultrasonic flow meters

Mohit Arora and Prashant Bhargava, Freescale Semiconductor, Inc. -July 31, 2012

[Part 1 of this series covers the fundamental concepts and principles incorporated by flow meters along with various flow measurement methods used in mechanical flow meters. Part 2 covers the pulse based counting method and the various sensors that are used in industry and the way they generate different pulse waveforms to be used in variety of flow meters. Part 3 covers the methods used to perform the measurement of the pulse waveforms in these flow metering systems.]

Ultrasonic Flow Meters: Introduction
These are the type of flow meters that measure the velocity of liquid or gas by using the principle of ultrasound or sounds waves. Fast digital signal processors and sophisticated signal analysis guarantee reliable measuring results even under difficult conditions where ultrasonic flow meters had failed previously.

Advantages over traditional meters
Unlike the traditional meters that have moving parts, ultrasonic meters do not include any moving parts and thus are more reliable, accurate and provide a maintenance free operation. With the homogenous fluids, the principle is independent of pressure, temperature, conductivity and viscosity.

Ultrasonic meters offer completely non-invasive measurement and do not need any pipe construction. Unlike the traditional meters, they don't face the issue of pressure loss.

Since ultrasonic signals can also penetrate solid materials, the transducers can be mounted onto the outside of the pipe and are usable under wide range of nominal diameters (Figure 1).

Figure 1: An ultrasonic meter installed directly on the pipe (Reference: http://www.greyline.com/)

An important point to note is that ultrasonic meters are more suitable to measure non-conductive fluids.

Ultrasonic Meters: The Theory
Swimming against the flow requires more power and more time than swimming with the flow. Ultrasonic flow measurement is based on this elementary transit time difference effect.

Two sensors mounted on the pipe simultaneously to send and receive ultrasonic pulses. At zero flow, both sensors receive the transmitted ultrasonic wave at the same time, i.e., without transit time delay.

When the fluid is in motion, the waves of ultrasonic sound do not reach the two sensors at the same time. This measured "transit time difference" is directly proportional to the flow velocity and therefore to flow volume. By using the absolute transit times both the averaged fluid velocity and the speed of sound can be calculated.

The transducers can be mounted in reflection or in diagonal mode as shown in Figure 2.

Figure 2: Ultrasonic Transducers mounted in Diagonal Mode and Reflect Mode

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