Gain accuracy when contact with the measured flic is not possible.
by Cheryl Ades Anspach
September 19, 2018

Today’s business environment demands informed decision-making. It is essential to efficiently manage operations, achieve regulatory compliance and provide superior customer service—all while continuing to control revenue and resources. When it comes to liquid flow measurement, every drop counts.

Often, flow is the only parameter measured to understand what is happening inside pipes in industrial facilities. Flow measurement allows operators to know that the right process medium is in the right place, at the
right time.

Users of flow meters are faced with a multiplicity of instrument designs to choose from to fulfill measurement requirements.

Different meter technologies are needed because each type functions properly in applications where the characteristics are compatible with its particular sensing principle.

Why Choose Ultrasonic?

There are multiple industrial applications that call for accurate flow measurement. However, traditional methods are not always an option, hence the need for nondestructive, noninvasive technologies and methods. Ultrasonic flow meters are the equipment of choice whenever contact with the measured fluid is not possible.

Transit time ultrasonic flow meters are used in the majority of liquid flow applications because of their ability to obtain accurate measurements.

With this technology, ultrasonic waves transmit upstream and downstream through the pipe wall and liquid in the pipe. By measuring the difference in the travel time and knowing the pipe size, the meter determines the velocity and flow.

Transit time ultrasonic meters also have diagnostic capabilities that other flow technologies do not. By measuring the speed of sound of the fluid, they can report changes in fluid temperature and/or other properties.

Clamp-on ultrasonic meters are easily installed without cutting or tapping the pipe. They reside outside the process line and provide precise flow measurement with reduced installation costs, uninterrupted production, installation flexibility across a wide range of pipe sizes, no pressure head loss, no contact with internal liquid and no moving parts to maintain.

Clamp-on ultrasonic flow meters can be a preferable alternative to traditional mechanical meters, which are subject to deteriorating accuracy caused by wear and tear. Additionally, the need for mechanical meters to be periodically tested, recalibrated and repaired means they have to be removed, requiring users to either replace the meter with a temporary device or cease abstraction until the meter is refitted back into the line.

How They Operate

Transit time ultrasonic meters use two transducers. These function as both ultrasonic transmitters and receivers. They operate by alternately transmitting and receiving a frequency-modulated burst of sound energy between the two transducers. The burst is transmitted in the direction of fluid flow and then against fluid flow.

Since sound energy in a moving liquid is carried faster when it travels in the direction of fluid flow (downstream) than it does when it travels against fluid flow (upstream), a differential in the times of flight will occur. This phenomenon is accurately measured in both directions, and the difference in time of flight calculated. The liquid velocity (V) inside the pipe can be related to the difference in time of flight (dt) through the following equation:

Design Enhancements

Recent developments have enabled today’s clamp-on transit time ultrasonic flow meters to reach a level of applicability, functionality, economy and performance establishing it as the prime candidate for a host of applications.

Typical examples for ultrasonic flow meters include water systems, wastewater effluent, agricultural irrigation and industrial discharge.

New meters reside outside the pipe and are compatible with a wide range of pipe sizes. They can be retrofitted into existing installations without shutting down system operations—reducing both installation time and material costs.

V = K x D x dt

K = a constant
D = the distance between the transducers
Equation 1

The meter can provide the baseline and load profile information needed to effectively optimize system efficiency and reduce energy consumption.

Benefits to End Users

Selecting the flow meter best suited for an application will result in improved accuracy, reliability and lowered maintenance costs, and can promote a long life for the instrument. Ease of installation and low cost of ownership make clamp-on ultrasonic flow meters an appropriate choice for many end users.

Clamp-on meters can positively impact the overall cost of flow monitoring and efficiency. They offer ease of use so that operators can do their jobs without unnecessary disruptions.

In part because clamp-on meters sit outside of the pipe, they can be used to measure several types of fluids, ranging from ultra-pure water, clean liquids and high viscosity liquids to sewage and wastewater, corrosive and/or abrasive liquids and hydrocarbons.

Industrial organizations can sometimes better address key areas such as process planning, maintenance and resource deployment, and leakage detection and rectification with these flow meters.


Ultrasonic technologies find many applications in measurement. Transit time techniques find favor in flow measurement and are used in a variety of flow meters. The performance of these meters is now well understood, and their use in challenging applications continues to produce useful results.

Read more articles on Flow Meters here.