How to measure low flow rates of liquids using ultrasonic waves?

More and more companies in varying industries are trying to make the transition to low flow solutions. Especially in the chemical industry and food & pharma market the trend is to focus on continuous manufacturing, waste reduction, lower downtime and more flexibility.

In these industries the availability of ultrasonic flow meters for liquids suitable for 1” pipe lines or larger are enormous, but it is much harder finding solutions for smaller line sizes. Conventional ultrasonic flow meters use either the Doppler Effect or Transit Time measurement. These techniques are practically suitable for large bore sizes.

But what about ultrasonic flow meters for flow rates lower than 1500 ml/min or even 200 ml/min?

Due to the complexity of physics and technology there are not many measurement principles present in this particular flow area, especially ultrasonic flow meters. Therefore the big challenge was to find a solution to use ultrasound in tubes with very small diameters. In close collaboration with TNO (Netherlands organization for applied scientific research) Bronkhorst managed to develop an innovative instrument using Ultrasonic Wave Technology. This technology is applied in the new ES-FLOW series for measuring liquid volume flows between 4 to 1500 ml/min independent of liquid density, temperature and viscosity with an accuracy of 1% of rate ± 1 ml/min.

Transducer discs and sensor tube

How does Ultrasonic Wave Technology work?

The ES-FLOW ultrasonic flow meter is based on ultrasonic wave technology. Measuring is done in a straight stainless steel tube with an inner diameter of 1.3 mm, without obstructions or dead spaces. At the outer surface of the sensor tube multiple transducer discs are located which create ultrasonic waves by radially oscillation. Every transducer can send and receive, therefore all up- and down-stream combinations are recorded and processed. By accurately measuring the time difference between the recordings (nanosecond range) the flow velocity and speed of sound is calculated.

Knowing these parameters and the exact tube crosssection, the ES-FLOW ultrasonic flow meter is able to measure liquid volume flows. The distinctive character of this flow meter is that it’s capable to measure the actual speed of sound, meaning that the technology is liquid independent and calibration per fluid is not necessary. Next to that the speed of sound can be used as an indicator of the type of fluid present in the flow meter.

Four reasons why to use the ES-FLOW Ultrasonic flow meter

One sensor for multiple liquids: Many companies have changing process conditions and make use of various liquids like additives or solvents. As the ES-FLOW technology is fluid independent, recalibration is not needed with liquid changes. Also non-conductive liquids as demi water can be measured.
Easy to clean and reduced risk of clogging: Cleaning processes are often time consuming. Due to the straight sensor tube design with no dead volume, particles have reduced chance of clogging the instrument. Cleaning can be done in a few minutes therefore the amount of down time will be limited.
Vibration insensitive: Ultrasonic measurement is not sensitive for vibrations as it doesn’t rely on frequencies or rotations. It is also irrelevant if the flow is laminar or turbulent.
Integrated PID controller and fast response: The on-board PID controller can be used to drive a control valve or pump, enabling users to establish a complete, compact control loop with fast response time.

Watch the principle of operation of the ES-FLOW