Ultrasonic flow meters

Technology and Operating Principle of Ultrasonic Flow Meters

An ultrasonic flow meter detects fluid velocity using ultrasonic signals and uses this value to calculate the flow rate from the pipe cross-sectional area. Flow calculation can be based on different measurement principles, including transit time, which is particularly suitable for clean liquids in closed pipes.

With the transit-time principle, two sensors transmit and receive ultrasonic pulses along the pipe. The system measures the difference between the time taken by the signal to travel with the flow and against it. This difference, which is proportional to fluid velocity, is processed by the instrument to provide instantaneous flow rate, totalized volume, fluid velocity and, on models equipped with temperature probes, thermal energy and power.

The Doppler method uses a different principle: the ultrasonic signal is reflected by solid particles or bubbles in the fluid, and the frequency shift is used to estimate flow velocity. In general, transit-time measurement is associated with clean, homogeneous liquids, while Doppler technology may be considered when particles or gas are present, subject to instrument compatibility and process conditions.

In clamp-on systems, correct positioning of the external transducers is essential because the signal must pass through the pipe wall and propagate effectively through the liquid.

In addition to non-invasive clamp-on configurations, inline meters installed directly in the pipe are also available. Read our technical article on ultrasonic flow meters to learn how they work, the benefits they offer and the factors to consider for correct installation.

How to Select the Right Ultrasonic Flow Meter

The selection of an ultrasonic flow meter depends on the measurement objective and the conditions in which the instrument will operate. Within the range of flow meters, ultrasonic configurations meet these requirements without introducing mechanical components into the flow.

Portable Measurements, Continuous Monitoring and Thermal Energy

For field checks, diagnostics and multi-point measurement campaigns, a portable meter can be moved between different pipes and used to record data without a permanent installation. The RIF600P uses the transit-time principle and is intended for clean liquids. It includes a data logger for survey activities and, depending on the configuration, supports fluid velocities up to 10 m/s, process temperatures up to 160 °C, IP65 protection and accuracy of ±1% of the reading under test-bench conditions.

When measurement must remain active over time, continuous readout and connection to the control system become key requirements. The wall-mounted RIF600W can measure flow rate, volume and fluid velocity and, in configurations equipped with temperature probes, thermal energy and power. Its 4-20 mA and pulse outputs and Modbus RS485 communication enable integration with supervisory systems.

The RIF600S addresses the same requirement with a modular architecture designed for industrial and automation applications. Dedicated configurations are available for small-diameter pipes, while instruments for HVAC, heating and cooling circuits can combine flow measurement with supply and return temperatures.

When data must be collected or transmitted remotely, the RIF645 provides a non-invasive solution with RS485, LoRa and IoT connectivity. For audits, preliminary checks or limited-duration projects, ultrasonic flow and thermal energy meter rental is also available, enabling real operating data to be collected before committing to a permanent installation.

Difference Between Clamp-On and Inline Meters

Clamp-on meters use transducers mounted on the outside surface of the pipe. They do not require the pipe to be cut, do not create pressure loss and introduce no components in contact with the liquid. They are therefore suitable for existing lines, temporary surveys and processes where pipe modifications would cause downtime or be difficult to manage.

The non-invasive nature of the installation does not eliminate the need for proper application assessment. The ultrasonic signal must pass through the pipe wall and propagate through the liquid. Pipe material, wall thickness, surface condition, any internal lining and correct sensor positioning all affect reading quality.

Inline meters are installed directly in the pipe and become an integral part of the process. They can be considered when the project includes permanent measurement and a defined line configuration from the outset.

Fluid, Pipework and Installation Conditions

The nominal diameter, material and wall thickness of the pipe must be known, especially for clamp-on installations, because the instrument uses these parameters to process the signal and calculate flow rate.

Transit-time meters are generally suitable for clean, sufficiently homogeneous liquids. Water, process water, glycol, oils and certain chemical solutions may be measured when they are compatible with the selected model, sensors and operating conditions. Significant levels of air bubbles, dissolved gas, suspended solids or highly aerated fluids can attenuate the signal and may require a different technology assessment.

The installation point also affects measurement stability. Bends, valves, pumps, branches and changes in pipe section can generate turbulence or non-uniform velocity profiles. Where specified in the technical documentation, straight upstream and downstream pipe runs help identify a section that is more representative of the flow.

Industrial Applications of Ultrasonic Flow Meters

Ultrasonic flow meters are used in water systems, water treatment, pump testing, industrial circuits and consumption monitoring. Portable versions are suitable for maintenance and diagnostics, while fixed configurations provide continuous monitoring and data transfer to automation systems.

In HVAC, heating and cooling systems, flow measurement can be combined with circuit temperatures to support energy audits and energy monitoring. Small-diameter solutions are also used in water management, agriculture and aquaculture when the liquid and operating conditions are compatible.

For chemical, pharmaceutical or food applications, suitability must be verified against the fluid, pipework and process requirements. When pipe diameters are large, the liquid has specific characteristics, or particular accuracy and integration performance is required, Riels technical consulting supports the selection of the configuration best aligned with the system and the required measurement result.

Quality, Accuracy and Technical Support

Riels ultrasonic flow meters are selected to deliver high build quality, signal stability and accuracy in industrial applications. Models such as the RIF600P, RIF600W and RIF600S offer stated accuracy of up to ±1%, depending on the specific configuration and measurement conditions, and can include displays, data loggers, analog outputs, pulse outputs and communication protocols for plant control and monitoring.

Reliable, configurable products are complemented by technical consulting, functional checks and calibration services, supporting selection, installation and long-term performance.

Contact Riels to identify the most suitable flow meter for your application or to request technical support and calibration services.

FAQ

How does an ultrasonic flow meter work?

An ultrasonic flow meter uses high-frequency sound waves to detect fluid velocity in a pipe. With the transit-time principle, the system compares the time taken by the signal to travel with and against the flow direction. It uses this difference to calculate velocity and therefore flow rate.

What is the difference between clamp-on and inline ultrasonic flow meters?

A clamp-on meter uses external sensors mounted on the pipe and does not require cutting the pipe or direct contact with the fluid. An inline meter is installed directly in the line and becomes an integral part of the system. The selection depends on installation requirements, fluid, pipe diameter, required accuracy and measurement objective.

When should a transit-time flow meter be selected?

Transit-time measurement is often suitable for clean, homogeneous liquids such as water, glycol, oils or solutions compatible with the selected model. It is appropriate when the ultrasonic signal can propagate consistently through the fluid and pipe wall.

Can ultrasonic flow meters measure dirty or aerated liquids?

Compatibility with dirty or aerated liquids, or fluids containing suspended solids, depends on the technology and model. Bubbles, gas or particles can affect the measurement. In some cases, Doppler technology or a dedicated configuration may be considered, subject to the operating conditions.

What information is required to select the correct meter?

Required information includes the fluid, nominal pipe diameter, pipe material and wall thickness, flow range, fluid velocity, temperature, required accuracy, installation type, output signals, need for a data logger or totalizer, and any thermal energy measurement requirement. For complex applications, Riels technical consulting can support the analysis of process data.

Does an ultrasonic flow meter require maintenance or calibration?

Ultrasonic flow meters have no mechanical parts immersed in the flow, but performance depends on correct installation, sensor condition, signal quality and operating conditions. Periodic checks, functional verification and calibration may be required depending on the model, application and the plant's internal procedures.