Ultrasonic flowmeter is an instrument that measures liquid or gas flow through ultrasonic technology. It works on the basis that the speed at which sound waves travel through a fluid changes depending on the direction and speed of the fluid flow. Ultrasonic flowmeter is widely used in industry, petrochemical, water supply system and environmental engineering and other fields.
Working principle
Ultrasonic flowmeters usually use the following two main working principles:
1. Time difference method (also known as propagation time method) : This method relies on the time difference of ultrasonic signal propagation in the fluid to measure the flow rate. Assume that there are two pairs of ultrasonic sensors, installed in the upstream and downstream positions of the pipeline, forming a symmetrical measurement path. Ultrasonic signals travel at different times in both upstream and downstream directions:
a.Downstream direction: The ultrasonic signal travels in the direction of the fluid flow, and its propagation speed will be accelerated.
b.Countercurrent direction: The ultrasonic signal travels against the direction of the fluid flow, and its propagation speed will be slowed down.
DOWN
By measuring the travel time in these two directions, the flow rate of the fluid can be calculated. The difference in travel time is proportional to the velocity of the fluid.
Advantages:
• High accuracy: Especially suitable for single, clean liquids, the best results when the fluid does not contain impurities or bubbles.
• Wide application: Suitable for measuring various pipe diameters.
Cons:
• Depends on the acoustic properties of the fluid: it is greatly affected by impurities or bubbles in the fluid.
• Accuracy degrades in the case of fluid turbulence or uneven flow velocity distribution.
2. Doppler effect method: This method uses Doppler effect to measure flow. The Doppler effect method uses changes in the frequency of sound waves to measure velocity. Reflections occur when ultrasonic waves travel through the fluid and meet suspended particles or bubbles. If the fluid is in motion, the reflected ultrasonic frequency will be different from the emitted frequency, and this change in frequency is the Doppler effect.
• When the fluid moves towards the sensor, the frequency of the reflected wave increases.
• When the fluid moves away from the sensor, the frequency of the reflected wave is reduced.
By measuring the difference in frequency between the transmitted and received waves, the flow rate v can be calculated.
Advantages:
• Ideal for measuring fluids containing suspended particles or bubbles: not limited by fluid purity.
• Wide range of application: can be used to measure dirty liquid or high bubble content of fluids.
Cons:
• Dependent on scattered particles or bubbles in the fluid: Sufficient reflective particles are required in the fluid to make measurements.
• Low relative accuracy: The measurement results are more sensitive to noise and flow conditions.
Channel concept
In ultrasonic flowmeters, channels refer to the number of paths through which ultrasonic signals propagate. Each channel consists of a pair of transmit and receive sensors that measure flow. The use of multiple channels can improve the accuracy and stability of measurement. Common channel configurations include single-channel, dual-channel, and four-channel configurations.
Single channel (1 channel) : The flowmeter uses only a pair of sensors to form a measurement path. It has the advantages of low cost, simple installation, but relatively low measurement accuracy, especially in the case of uneven fluid flow distribution.
Dual channel (2-channel) : two pairs of sensors are used to form two measurement paths. The two-channel configuration significantly improves measurement accuracy because it allows the flow rate of the fluid to be sampled at different locations, reducing the impact of uneven flow distribution on measurement results.
• Four channels (4 channels) : Four pairs of sensors are used to form four measurement paths. This configuration provides higher measurement accuracy and stability for applications that require high precision measurements, such as large pipelines or environments with complex measurement conditions. The four-channel configuration can more fully reflect the flow velocity distribution of the fluid and reduce errors.
Thank you
