With the drive of efficient process utilization and the need for pollution-free detection, the demand for non-invasive measurement technology is increasing. Here, we talk about how ultrasonic flow sensors work and what are the advantages of invasive measurement methods

Non-invasive sensor benefits

The non-invasive sensor has no physical effect on the flowing liquid and is simply clipped to the tube from the outside. The intrusive sensor will directly affect the liquid, and it will inevitably produce some pollution. In addition, the intrusive sensor "intrudes" into the liquid channel, which can actually cause fluid disturbance, asymmetry, or pressure loss, resulting in incorrect measurement results.

Ultrasonic flow sensor engineering principle

The transducer is the core of ultrasonic flow sensor. They consist of piezoelectric ceramics and composite materials. When a DC voltage is applied, they expand or contract depending on whether the voltage is positive or negative. When an alternating voltage is applied, the piezoelectric ceramics periodically expand and contract while emitting sound waves of the same frequency.

This sound wave is emitted in the form of a pulsed ultrasonic beam from the excitation transducer and detected by the receiving transducer. The electronic signal is transmitted and output through various signal ports.

Physical principle of ultrasonic flow sensor

There are several ways to calculate the flow rate from the ultrasonic signal. A typical method is the transmission time difference measurement method. This approach leads to neither pressure drop nor leakage risk. Suitable for almost all liquids, including fluids with different viscosity, color, density and electromagnetic properties.

How does the transmission time difference measurement method work?

The left figure shows the same direction of ultrasonic transmission and flow

The middle figure shows the opposite direction of ultrasonic transmission and flow

The picture on the right shows two sets of ultrasonic waves, one with the same direction of transmission and flow, and the other with the opposite direction.

Through the time-digital converter, the transmission time of ultrasonic wave in the direction of medium flow and the opposite direction can be measured with high precision. In the flow direction, the ultrasonic transmission time is faster than the opposite direction. The time difference between the two directions can calculate the flow rate.

In order to improve the measurement effect, two sets of ultrasonic transducers (a total of 4) are used, as shown in the figure below. 4 ultrasonic transducers are arranged in X form. One end of the transducer sends out pulsed ultrasonic waves, which are transmitted to the other end at a given frequency. The transmission time difference between the two directions is proportional to the average velocity. The average flow rate is multiplied by the cross-sectional area of the pipe to obtain the flow rate.