Basic principles and types of ultrasonic flowmeters

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When ultrasound propagates through a flowing fluid, it carries information about the flow velocity of the fluid. Therefore, the flow velocity of the fluid can be detected using the received ultrasonic waves, which can then be converted into flow rate. Based on the detection method, ultrasonic flowmeters can be classified into different types such as the velocity difference method, Doppler method, beam deflection method, noise method, and related methods. An acoustic flow meter is a type of non-contact instrument that has come into use over the past decade or so, thanks to the rapid development of integrated circuit technology. It is suitable for measuring fluids that are difficult to access and observe, as well as flow rates in large-diameter pipes. It can be used in conjunction with a water level gauge to measure the flow rate of open-flow water. By using ultrasonic flow measurement, there is no need to install measuring elements in the fluid, which means that the flow pattern of the fluid remains unchanged and no additional resistance is generated. The installation and maintenance of such instruments do not affect the operation of the production pipelines, making it an ideal energy-saving flow meter. As is well known, current industrial flow measurement faces the challenge of measuring large-diameter flows with high volumes. This is because conventional flow meters encounter difficulties in manufacturing and transportation as the diameter of the measurement tube increases, resulting in higher costs, greater energy losses, and other installation-related issues – all of which can be avoided by ultrasonic flow meters. Since various ultrasonic flowmeters can be installed outside the pipeline and perform non-contact flow measurement, their cost is basically independent of the diameter of the pipeline being measured. In contrast, the cost of other types of flowmeters increases significantly as the diameter grows; therefore, for larger diameters, ultrasonic flowmeters offer a better cost-performance ratio compared to other types of flowmeters with similar functions. Considered to be a good flow measurement instrument for large diameters, the Doppler ultrasonic flow meter is capable of measuring the flow rate of two-phase media, making it suitable for measuring dirty flows such as those in sewers and wastewater systems. In power plants, using portable ultrasonic flowmeters to measure large-diameter flow rates such as the water inflow to turbines and the circulating water volume in steam turbines is much more convenient than using Pitot tubes in the past. Ultrasound flow meters can also be used for gas measurement. The applicable range of pipe diameter is from 2 cm to 5 m; it can be used in open channels and buried channels several meters wide, as well as in rivers up to 500 m wide. Furthermore, the flow measurement accuracy of ultrasonic measuring instruments is hardly affected by parameters such as the temperature, pressure, viscosity, and density of the fluid being measured. Moreover, they can be designed as non-contact and portable instruments, which enables them to address the flow measurement challenges associated with highly corrosive, non-conductive, radioactive, and flammable/explosive media, issues that are difficult to handle with other types of instruments. Furthermore, given the characteristics of non-contact measurement, and with appropriate electronic circuits, one instrument can be used for measuring various pipe diameters as well as across a range of flow rates. The adaptability of ultrasonic flowmeters is also incomparable to that of other instruments. Ultrasonic flowmeters possess some of the advantages mentioned above; as a result, they are receiving increasing attention and are evolving toward standardized and generalized product lines. Standard, high-temperature, explosion-proof, and wet-type models with different sound frequencies have now been developed to meet the needs of flow measurement in various media, applications, and pipeline conditions. The main drawbacks of ultrasonic flowmeters at present are that the temperature range of the fluid that can be measured is limited by the temperature tolerance of the ultrasonic transducer and the coupling material between the transducer and the pipe, as well as the lack of accurate data regarding the sound propagation speed of the fluid under high temperatures. At present, in our country it can only be used to measure fluids below 200°C. Furthermore, the measurement circuit of an ultrasonic flowmeter is more complex than that of a conventional flowmeter. This is because, in general industrial measurement applications, the flow velocity of liquids is often several meters per second, while the speed of sound in liquids is around 1500 m/s. The maximum change in the speed of sound caused by variations in the flow velocity (or flow rate) of the fluid is on the order of 10^-3. If a precision of 1% is required for measuring the flow velocity, then the precision needed for measuring the speed of sound must be on the order of 10^-5 to 10^-6. Therefore, a sophisticated measurement system is necessary to achieve this, and this is precisely why ultrasonic flowmeters can only be put into practical use with the rapid development of integrated circuit technology. An ultrasonic flowmeter consists of an ultrasonic transducer, electronic circuitry, and a flow display and accumulation system. The ultrasonic transmitting transducer converts electrical energy into ultrasonic energy, which is then emitted into the fluid to be measured. The ultrasonic signals received by the receiver are amplified by electronic circuits and converted into electrical signals representing flow rate, which are fed to display and accumulation instruments for display and calculation. This enables the detection and display of traffic. Ultrasonic flowmeters commonly use piezoelectric transducers. It utilizes the piezoelectric effect of piezoelectric materials; an appropriate transmitting circuit is used to apply electrical energy to the piezoelectric elements of the transmitting transducer, causing it to generate ultrasonic vibrations. Ultrasonic waves are transmitted into the fluid at a certain angle, then captured by a receiving transducer, and converted into electrical energy via piezoelectric elements for detection. The transmitting transducer utilizes the inverse piezoelectric effect of piezoelectric elements, while the receiving transducer makes use of the piezoelectric effect. The piezoelectric elements of ultrasonic flowmeter transducers are typically made into circular thin sheets that vibrate along their thickness. The diameter of the thin sheet should be more than 10 times its thickness to ensure the directivity of vibration. Lead zirconate titanate is commonly used as the material for piezoelectric elements. To fix the piezoelectric element and direct ultrasonic waves into the fluid at an appropriate angle, the element must be placed within an acoustic wedge to form the entire transducer (also known as the probe). The material of the acoustic wedge is required not only to have high strength and resistance to aging, but also to result in minimal energy loss of ultrasonic waves passing through it, meaning that the transmission coefficient should be close to 1. The commonly used material for acoustic wedges is plexiglass, as it is transparent, allowing one to observe the assembly of the piezoelectric elements within the acoustic wedge. Additionally, certain rubbers, plastics, and bakelite can also be used as sound wedge materials. The electronic circuit of an ultrasonic flowmeter includes transmission, reception, signal processing, and display circuits. The measured instantaneous and cumulative flow rates are displayed as digital or analog values. Based on the principles of signal detection, ultrasonic flowmeters can be roughly classified into types such as the propagation speed difference method (including direct time difference method, time difference method, phase difference method, and frequency difference method), beam deflection method, Doppler method, correlation method, spatial filtering method, and noise method, as shown in the figure. Among them, the method based on noise principles has the simplest structure, is easy to measure and carry, is inexpensive, but has lower accuracy; it is suitable for use in situations where high accuracy in flow measurement is not required. Since the basic principle of each of the direct time-difference method, time-difference method, frequency-difference method, and phase-difference method is to determine the flow velocity of the fluid by measuring the difference in the propagation speeds of ultrasonic pulses in the forward and reverse directions, they are collectively referred to as the propagation speed difference methods. Among them, the frequency difference method and the time difference method overcome the errors caused by changes in sound speed with fluid temperature, offering high accuracy; therefore, they are widely used. Depending on the configuration method of the transducer, the propagation speed differences are further classified into: the Z method (transmission method), the V method (reflection method), the X method (cross method), etc. The beam deflection method relies on the fact that the direction of propagation of the ultrasonic beam in a fluid changes as the flow velocity of the fluid changes, and this change is used to determine the flow velocity. Its sensitivity is very low at low flow velocities, resulting in limited practicality. The Doppler method utilizes the principle of acoustic Doppler effect; by measuring the Doppler shift of the ultrasonic waves scattered by particles in the fluid, it is possible to determine the flow rate of the fluid. This method is suitable for measuring the flow rate of fluids containing suspended particles or bubbles. The correlation method utilizes correlation techniques to measure flow rate. In principle, the accuracy of measurement using this method is independent of the sound speed in the fluid; as a result, it is unaffected by factors such as fluid temperature or concentration, which ensures high measurement accuracy and a wide range of applicability. But correlators are expensive and have relatively complex circuits. This drawback can be overcome after microprocessors became widely used. The noise method (sound detection method) utilizes the principle that the noise generated by fluid flow within a pipe is related to the flow velocity of the fluid, and it determines the flow velocity or flow rate by detecting this noise. Its method is simple and the equipment is inexpensive, but the accuracy is low. The above methods each have their own characteristics, and the choice should be made based on factors such as the properties of the fluid to be measured, the flow velocity distribution, the location where the pipeline is installed, and the requirements for measurement accuracy. Generally, since the temperature of the working fluid in industrial production often cannot be kept constant, the frequency difference method and the time difference method are commonly used. The direct time difference method is used only when the pipe diameter is very large. The general principle for choosing the transducer installation method is: when the fluid flows parallel to the tube axis, the Z method should be used ； The V-method or X-method is used when the flow direction is not parallel to the pipe axis, or when the location of the piping limits the spacing between transducers. When the flow field distribution is uneven and the straight pipe section ahead of the sensor is short, multiple channels (such as dual or quad channels) can also be used to overcome the flow measurement errors caused by flow velocity disturbances. The Doppler method is suitable for measuring two-phase flows, and it has developed rapidly because it avoids the problems associated with conventional instruments, such as blockage, wear, and deposition caused by suspended particles or bubbles, which can lead to their malfunction. With the development of industry and energy-saving efforts, as well as the use of kerosene-blended (COM) and coal-water blended (CWM) fuels, along with energy-saving methods such as adding water to fuel oils to enhance combustion, these developments have opened up broad prospects for the application of Doppler ultrasonic flowmeters.