In recent years, with the development of sensor technology and instrumentation technology, the use of ultrasonic pulses to measure fluid flow has developed rapidly. Based on different principles, various types of ultrasonic flowmeters suitable for different applications have been widely used. Also for the majority of users how to understand the ultrasonic flow meter, how to choose the right type, so that the application should pay attention to what a series of problems, this article on the current situation of domestic ultrasonic flowmeter and the application of many years of practice, to discuss the above issues.
2 principle
At present, two types of ultrasonic flowmeters are generally used, one is a Doppler ultrasonic flowmeter, and the other is a time-lapse ultrasonic flowmeter. The Doppler type uses the phase difference method to measure the flow rate, that is, a sound wave with a known frequency moves in the fluid. Since the liquid itself has a moving speed, the frequency or phase of the ultrasonic wave occurs between the two receivers (or transmitters). Relative change, by measuring the relative change can be obtained liquid velocity; time difference type is the use of time difference method to measure the flow rate, that is, a certain speed of sound waves due to fluid flow caused by the time it travels between the two receivers (or transmitter) occurs Changes can be obtained by measuring this relative change. Guangdong Shaoguan Iron and Steel Group Co., Ltd. currently uses a time-lapse ultrasonic flowmeter.
Ultrasonic flowmeter consists of three parts: ultrasonic transducer, electronic circuit and flow display and accumulation system. Ultrasonic emission transducer converts electrical energy into ultrasonic energy and transmits it to the measured fluid. Receiver receives ultrasonic signals. The electronic signal amplified by the electronic circuit and converted into an electrical signal representing the flow is supplied to the display and integrator for display and integration, thus realizing the detection and display of the flow.
3 Ultrasonic Flowmeter Selection
In the face of many types of ultrasonic flowmeters, users must rationally select models based on actual conditions and measurement needs.
3.1 Doppler type ultrasonic flowmeter
It can only be used to measure fluids containing particles or air bubbles that reflect an appropriate amount of ultrasonic signals, such as factory effluent, untreated effluent, and factory process fluids with stable levels of impurities. It should be noted that it requires more stringent requirements on the measured medium, that is, it cannot be clean water, and the impurity content must be relatively stable before it can be measured. Moreover, the instrument performance of different manufacturers and the requirements on the tested manufacturers are not the same. The selection of such ultrasonic flowmeters requires a thorough understanding of the medium to be measured, as well as the performance, accuracy, and requirements for the measured medium of the selected ultrasonic flowmeter.
3.2 Portable Ultrasonic Flowmeter
Suitable for temporary measurements, mainly used to calibrate the running status of other flow meters installed on the pipeline, to conduct a fluid balance test within a zone, check the current flow conditions of the pipeline, etc., if not fixedly installed for these uses, The use of a portable ultrasonic flow meter is both convenient and economical.
3.3 Time Difference Ultrasonic Flowmeter
At present, the most widely used and most widely used is the time-difference ultrasonic flowmeter. It is mainly used to measure the flow rate of clean fluid. It is widely used in the water company and industrial water fields. In addition, it can measure even fluid with low impurity content (contaminant content less than 10g/L, particle size less than 1mm), such as sewage The flow of medium, and the accuracy can reach ± ​​1.5%. The practical application shows that the use of time-lapse ultrasonic flowmeters can achieve satisfactory results for the corresponding fluid measurements.
3.4 Pipeline Ultrasonic Flowmeter
With the highest accuracy of ±0.5%, it is not limited by the material of the pipe and the lining. It is suitable for occasions where the flow measurement accuracy is high. However, with the increase of the pipe diameter, the cost will increase. Under normal circumstances, the Small and medium-diameter pipe-type ultrasonic flowmeters are economical.
3.5 Fixed ultrasonic flowmeter
If there is enough space for installation, the use of a plug-in transducer instead of an external mount transducer completely eliminates the influence of the tube lining, fouling, and the tube wall on the attenuation of the ultrasonic signal, resulting in higher measurement stability and greatly reduced Maintain workload. Moreover, because the plug-in transducer can also be installed continuously, its application is being continuously promoted.
Some manufacturers have introduced an internal ultrasonic flowmeter for digital circuits, which is characterized by the use of digital circuits to process the signal, the error correction capability is enhanced, the sampling is timely, the precision is improved (the accuracy of the analog circuit is ± 1.5%, the digital circuit can reach ± ​​1.0% ), And the integration is improved, the volume of the instrument is greatly reduced, there are a variety of signal output modes for selection, in the actual application has also achieved good results. The user can compare with the ultrasonic flowmeter of the analog circuit in use.
The function of ultrasonic flowmeter can be selected by the user according to the actual situation. If you are measuring two-way fluids, you must select an ultrasonic flowmeter with positive and negative metering functions; if you need to regularly understand the flow of fluid at a certain period of time, you can choose an ultrasonic flowmeter with a printer. In short, the function of the selected ultrasonic flowmeter must be satisfied