Ultrasonic flow sensor and ultrasonic electronic water meter

With the implementation of the new national standards, electronic water meters have gradually come into people's sight. Ultrasonic water meters and their ultrasonic flow sensing technology, which represent the main technology of electronic water meters, have a series of characteristics and advantages in performance, and their influence is expanding day by day, and their usage is also increasing day by day.

Ultrasonic flow sensing technology has many characteristics that other flow sensing technologies do not have, mainly: it can better solve the flow measurement problems of large pipe diameters, large flows, and various open channels and dark channels; there are almost no requirements for the measured fluid medium, and it can measure not only liquids but also gases; because it adopts a non-contact method, it does not destroy the flow field of the measured fluid, and there is no pressure loss; the accuracy of flow measurement is almost not affected by the physical parameters such as temperature, pressure, density, viscosity, etc. of the measured fluid; the price of the instrument does not increase significantly with the increase of the measuring caliber; the flow rate of the fluid in the pipe can be measured on the outside of the measuring pipe.

However, in order to ensure the accuracy of flow measurement, the single-channel ultrasonic flow sensor should have a sufficiently long straight pipe section on its upstream side. The multi-channel ultrasonic flow sensor has a higher measurement accuracy, and the requirements for the straight pipe section can be greatly reduced; ultrasonic flow measurement has certain requirements for the water quality to be measured. When measuring with the time difference method, if there are many bubbles, suspended matter in the water or dirt on the transducer surface, it will hinder the normal propagation of ultrasonic waves, making it impossible to measure; as the diameter of the measuring tube decreases, the ultrasonic flow sensor using the time difference measurement principle will encounter the confusion of increasing measurement errors. At this time, we should try to increase the time difference between forward and reverse measurements, improve the timing resolution, and ensure the accuracy and repeatability of small flow measurements.

Ultrasonic flow sensors and ultrasonic electronic water meters can be installed in an external transducer manner (see Figure 1) or an intrusive transducer manner (see Figure 2).

Principles and characteristics of time-difference ultrasonic flow sensors

Ultrasonic flow sensors are key core components of ultrasonic electronic water meters. They are mainly composed of measuring tubes, ultrasonic transducers, transceiver circuits, timing pulse generators, and precision timing controllers. Currently, ultrasonic flow measurement methods mostly use the time-difference method or the velocity-difference method to measure the velocity, flow rate, and other parameters of the water fluid in the pipeline, and then accumulate them into the actual volume value of the water consumption. The working principle of the ultrasonic time-difference measurement method is shown in Figure 3. The calculation formulas for the forward and reverse propagation time, time difference, and linear average velocity can be found in equations (1) to (3), respectively.

because

so

or

In the formula, t1-2 is the forward propagation time of ultrasonic wave; t2-1 is the reverse propagation time of ultrasonic wave; Δt is the time difference between the forward and reverse propagation of ultrasonic wave; c is the propagation velocity of ultrasonic wave; v is the average linear velocity of fluid in the axial direction; D is the diameter of the pipe; φ is the angle between the propagation direction of ultrasonic wave and the axis of fluid.

Figure 3. Working principle diagram of ultrasonic time difference measurement method

Since the speed of sound c is a function of the temperature and composition of the measured medium, the time difference measurement method developed later uses the difference in the speed of ultrasonic waves in the forward and reverse directions to reflect the flow rate of the fluid, thus avoiding the influence of changes in medium temperature or composition on the accuracy of ultrasonic flow measurement. Therefore, it is also called the speed difference method. Now change the form of formula (1) as follows:

Subtracting equation (4) from each other, we get:

Substituting into formula (5), we get:

Formula (6) has eliminated the ultrasonic propagation sound velocity term. As long as the forward and reverse time (t1-2, t2-1) and the time difference Δt are measured, the linear average value v of the flow velocity on the sound channel can be obtained. [page]

Formula (6) shows that the linear average value v of the flow velocity in the pipeline is linearly related to the mathematical expression composed of the time difference Δt, the forward propagation time t1-2, and the reverse propagation time t2-1. After conversion, formula (7) is obtained:

Where M is a constant, which is only related to the processing and assembly accuracy of the ultrasonic water meter measuring tube (that is, it is related to the inner diameter D of the pipe and the transducer installation angle φ). The change of M value will affect the slope of the theoretical characteristic curve of ultrasonic water meter flow measurement, as shown in Figure 4.

Closed pipes usually use the velocity surface average value as the basis for the calibration of water meter flow measurement characteristics and the evaluation of measurement errors. Since the relationship between the line average value v obtained by ultrasonic measurement and the surface average value v of the velocity distribution is very complicated at different Reynolds numbers and corresponding velocity distributions, the time difference expression and the velocity surface average value v show obvious nonlinearity in different flow sections, as shown in Figure 5. This requires different characteristic correction methods for ultrasonic flow sensors in different velocity distribution areas.

Figure 5. Characteristics of ultrasonic flow sensor between line and surface average velocity in different flow velocity ranges

Ultrasonic electronic water meter composition and main performance indicators

The ultrasonic electronic water meter is composed of an ultrasonic flow sensor with signal processing circuit and data communication function added. Its working principle is shown in Figure 6.

Figure 6. Block diagram of the working principle of ultrasonic electronic water meter

The better performance indicators that ultrasonic electronic water meters can currently achieve are (taking DN100 as an example):

● Flow measurement range: Q3=100m3/h; Q1=0.2m3/h; Q3/Q1=250～250;
● Maximum allowable error: low area ≤±3.0%; high area ≤±1.0%;
● Maximum working pressure: 1.6 MPa;
● Protection level: IP68;
● Temperature range of measured medium: 0.1℃～50℃;
● Battery life: ≥10 years;
● Data communication function: wireless short-distance communication (point-to-point)/GPRS wireless public network communication/M-BUS, etc.

Key technologies and development trends of ultrasonic electronic water meters

● Since the monophonic ultrasonic water meter has only one channel, it is very sensitive to the flow velocity distribution in the pipeline. It is necessary to have a long front and rear straight pipe section to ensure that the flow velocity distribution in the pipe is in a fully developed stable symmetrical flow state, so that the correction point between the linear flow velocity and the surface flow velocity does not change due to the distortion of the flow velocity distribution. Because of the multiple sound channels, dual-channel or even multi-channel ultrasonic water meters can be calibrated at different positions of the flow velocity distribution, which basically solves the correction error caused by the distortion of the flow velocity distribution in the pipe due to the installation of flow-blocking devices (such as elbows, tees, valves, etc.) before and after the water meter. Therefore, shorter straight pipe sections can be used before and after, and it also provides a good technical means for the realization of high-accuracy electronic water meters.

● High-performance ultrasonic transducers are an important guarantee for ensuring the measurement accuracy and long-term working stability of ultrasonic water meters. The development of new electromechanical transducer materials, the stability treatment of transducer material characteristics, the design and assembly technology of transducers, and the measurement and screening technology of transducer devices are all key technologies to ensure high-performance ultrasonic transducers.

● The timing pulse technology and timing control strategy with higher time resolution are used to improve the timing accuracy of the time difference ultrasonic water meter at low flow rates, so that the measurement characteristics of the ultrasonic water meter can be expanded to a wider flow measurement range to meet the special requirements of water metering applications.

● Ultrasonic water meters are usually powered by batteries. In order to ensure that the battery does not need to be replaced during the calibration period, the micro-power consumption design of the water meter is particularly important. In addition to the use of extremely low-power electronic circuits and embedded microsystems, the improvement of software algorithms is also very important for reducing the power consumption of the whole machine. The research and development of new high-energy batteries has played a key role

in the large-scale promotion and application of various types of electronic water meters such as ultrasonic. ● As a measurement node in the urban pipe network, ultrasonic and other electronic water meters, as a new type of flow sensor, will play a very important role in the pipe network measurement and control application and even the Internet of Things application. Therefore, the communication interface technology of ultrasonic water meters is an important network access technology. At present, the communication methods of electronic water meters mainly include short-distance wired communication, wireless communication and long-distance wireless communication. The network technology mainly includes self-organized local area network and the use of public network platform.

● The optimization design of the fluid dynamics performance of the ultrasonic flow sensor measuring pipe section, the reasonable setting of the transducer position and the sound channel, the selection of the metal material of the measuring pipe section, and the application of sealing and protection technology will also determine the measurement performance, service life and measurement reliability of ultrasonic electronic water meters.

The emergence of ultrasonic flow sensors and ultrasonic electronic water meters will definitely play a very important role in the management of water resources and trade settlement of water consumption in China. With the gradual advancement and application of Internet of Things technology, the new electronic water meter that integrates functions such as data collection and measurement, data transmission and communication, and network valve control will definitely have a very broad application prospect.