Detailed description of the design principle of ultrasonic flowmeter and how to correctly select the probe position during use

The application of ultrasonic flowmeters is becoming more and more common. In many industrial control conditions that require dynamic measurement, ultrasonic flow is very useful. How does the ultrasonic flowmeter work? What kind of mechanism is its working principle? Let's share it with you today.
 
We know that there are all kinds of waves around us in production and life. Some waves can be felt, such as light waves and ordinary sound waves, waves of object vibration, the most typical of which are seismic waves, and some waves we can't feel, such as ultrasonic waves, radio waves, and electromagnetic waves. Waves are divided into two categories according to their properties: electromagnetic waves and mechanical waves. Electromagnetic waves are generated by the action of electromagnetic forces and are the propagation process of electromagnetic field changes in space. Electromagnetic waves can be divided into radio waves, infrared rays, visible light, ultraviolet light, X-rays, etc. according to frequency. Electromagnetic flowmeters often used in industrial production are this type of instrument. Mechanical waves are the propagation of mechanical fluctuations generated by mechanical forces in the medium. They propagate mechanical energy and can only propagate in the medium. Sound waves are a kind of mechanical waves, which are divided into infrasound waves, audible sound waves, and ultrasonic waves according to frequency. Waves with frequencies below 20Hz are infrasound waves; waves with frequencies between 20 and 20,000Hz are audible sound waves; waves with frequencies above 20kHz are called ultrasonic waves.

      Ultrasonic waves are mechanical vibration waves with frequencies above 20kHz. They have good directivity, concentrated energy, and strong penetrating ability. They can penetrate steel plates several meters thick, consume little energy, and produce obvious reflection and refraction when encountering the interface between two media.

      Ultrasonic waves have a certain attenuation phenomenon during propagation, which is caused by the medium absorbing the energy of ultrasonic waves. Ultrasonic waves decay quickly in the air, especially when the frequency is high, so when propagating in the air, ultrasonic waves with lower frequencies are used, generally in the tens of kHz (40kHz). However, they decay slowly in liquids and solids and propagate farther, so higher frequencies can be used, generally hundreds of kHz or higher.

      Ultrasonic sensors have a wide range of applications and can be used to measure flow (ultrasonic flowmeter), liquid level (ultrasonic level meter), material level (ultrasonic level meter), material thickness, and anti-theft alarms.
When installing the ultrasonic flowmeter, you must determine the installation location, so that you can better avoid problems with the installation of the ultrasonic flowmeter. If there are problems when installing the ultrasonic flowmeter, it will be a very troublesome process. So everyone must know what to avoid when installing.
　　
　　1. Avoid installing the machine at the water pump, high-power radio station, frequency conversion, that is, where there is a strong magnetic field and vibration interference;　　
　　2. Choose a pipe section that is uniform and dense, and easy for ultrasonic transmission;　　
　　3. There must be a long enough straight pipe section. The upstream straight pipe section of the installation point must be greater than 10D (Note: D=diameter), and the downstream must be greater than 5D;　　
　　4. The upstream of the installation point should be 30D away from the water pump;　　
　　5. The fluid should fill the pipeline.
　　
　　The installation requirements of the ultrasonic flowmeter are the simplest and most convenient among all flowmeter installations. Just select a suitable measuring point, enter the pipeline parameters at the measuring point into the flowmeter, and then fix the probe on the pipeline.
　　
　　The two sensors must be installed in the horizontal direction of the pipeline axis and installed within the range of ±45o of the horizontal position of the axis to prevent the upper part from being full of pipes, bubbles or precipitation in the lower part from affecting the normal measurement of the sensor. If the installation is not symmetrical due to the space limitation of the installation site, the sensor can be installed vertically or at an angle while ensuring that there are no bubbles in the upper part of the pipe.
　　
　　Before installing the probe, select the dense part of the pipe for probe installation. Clean the area outside the pipe where the probe is to be installed, remove all rust and paint, and use an angle grinder to polish it. Then use a clean rag dipped in acetone or alcohol to wipe off the oil and dust, and then apply enough coupling agent to the center of the probe and the pipe wall. During the installation of
　　
　　the ultrasonic flowmeter , be sure to pay attention to the absence of air bubbles and gravel between the probe and the pipe wall. On the horizontal pipe section, the probe should be installed on the horizontal axis of the pipe section to prevent bubbles from existing in the upper part of the pipe. If the installation site space is limited and the probe cannot be installed horizontally and symmetrically, the probe can be installed vertically or at an angle while ensuring that there are no bubbles in the upper part of the pipe. The
　　
　　probe spacing is based on the innermost distance edge of the two probes. After entering the required parameters, check the numbers displayed in the display window 25 and make the probe spacing consistent with the data in the window 25.
　　
　　There are four ways to install the probe. The four methods are v method, z method, n method and w method. They are explained below. Generally, the v method can be used first when the pipe diameter is small (dn15~200mm); the z method should be used when the v method cannot detect the signal or the signal quality is poor. The z method should be used first when the pipe diameter is above dn200mm or when measuring cast iron pipes. Generally, it is a standard installation method, which is easy to use and accurate in measurement. The measurable pipe diameter range is from 15mm to about 400mm; when installing the probe, pay attention to the horizontal alignment of the two probes, and their center lines are horizontal with the pipe axis.
　　
　　When the pipe is very thick or the v method installation signal is weak due to the presence of suspended matter in the liquid, too thick scaling on the inner wall of the pipe or too thick lining, and the machine cannot work normally, the z method should be used for installation. The reason is that when using the z method, the ultrasonic wave is directly transmitted in the pipe without refraction (called single sound path), and the signal attenuation is small. The z method can measure the pipe diameter range from 100mm to 6000mm. When actually installing the flow meter, it is recommended to use the z method for pipes above 200mm (so that the measured signal is the largest).
　　
　　During installation, the ultrasonic beam refracts twice in the pipe and passes through the fluid three times (three sound paths), which is suitable for measuring small diameter pipes. The n method improves the measurement accuracy by extending the ultrasonic transmission distance (uncommon method).
　　
　　Like the n method, the w method also improves the measurement accuracy of small pipes by extending the ultrasonic transmission distance. Suitable for measuring small pipes below 50mm. When using the w method, the ultrasonic beam refracts three times in the pipe and passes through the fluid four times (four sound paths).
　　
　　Checking the installation means checking whether the probe is properly installed and whether it can receive the correct and strong enough ultrasonic signal that can make the machine work normally, so as to ensure the long-term and reliable operation of the machine. By checking the received signal strength, total transmission time, time difference and transmission time ratio, it can be determined whether the installation is optimal.
　　
The quality of the installation is directly related to the accuracy of the flow value and whether the flow meter can operate reliably for a long time. Although in most cases, the measurement results can be obtained by simply applying a coupling agent to the probe and sticking it to the outside of the pipe wall, the following inspections should be performed to ensure the best measurement results and long-term reliable operation of the flow meter.