How to use super flow meter

I. Overview

In recent years, based on different principles, various types of ultrasonic flowmeters suitable for different occasions have been widely used. At the same time, they also put forward how to understand ultrasonic flowmeters, how to choose the appropriate type, and what to pay attention to when using them. A series of questions. In this paper, the current development of domestic ultrasonic flowmeters and the practice of many years of application are discussed.

The classification of two ultrasonic flowmeters

1 Classified according to use principle

(1) Doppler ultrasonic flowmeter

Doppler ultrasonic flowmeters use Doppler effect principles to measure fluid flow.

As shown in Fig. 1, the transducer 1 emits an ultrasonic signal of frequency f1. After passing through the suspended particles or bubbles in the liquid in the pipeline, the frequency shifts to reflect the frequency of f2 to the transducer 2. This is multi-spectrum. Lehrer effect, the difference between f2 and f1 is the Doppler frequency difference fd.

Let the fluid velocity be v, the ultrasonic sound velocity be c, and the Doppler frequency shift fd be proportional to the fluid velocity v, that is:

So the fluid velocity

After the pipeline condition, transducer installation position, launch frequency, and sound velocity are determined, c, f1, and θ are constants, and the fluid flow rate is proportional to the Doppler frequency shift. By measuring the frequency shift, the fluid flow rate can be obtained, and then obtained. Fluid flow.

(2) Time difference ultrasonic flowmeter

The time difference type ultrasonic flowmeter uses the principle that the time difference between the forward and backward propagation of sound waves in the fluid is proportional to the flow velocity of the fluid to measure the fluid flow.

As shown in FIG. 2 , the transducer 1 transmits an ultrasonic signal to the transducer 2, which is in the forward flow direction, and its propagation time is:

On the contrary, the propagation time in the upstream direction is:

The time difference is:

Since c>>v, so

So, the fluid velocity

Similarly, c, L, and q are constants, and the fluid velocity v can be obtained by measuring the time difference Δt , and the fluid flow rate can be obtained.

2 Classified by use situation

According to different occasions of ultrasonic flowmeters, it can be divided into fixed ultrasonic flowmeter and portable ultrasonic flowmeter.
The main difference between these two types of ultrasonic flowmeters is:

(1) Different occasions apply

The stationary ultrasonic flowmeter is installed in a certain fixed position to measure the flow of fluid in a certain pipeline for a long time without interruption. The portable ultrasonic flowmeter has great mobility and is mainly used for temporary flow of fluid in different pipelines. measuring.

(2) Different power supply methods

The fixed ultrasonic flowmeter runs continuously for a long period of time, so to use 220V AC power, the portable ultrasonic flowmeter can use the on-site AC power supply, and also has a built-in rechargeable battery, which can work continuously for 5~10h, which greatly facilitates the temporary flow of different occasions. The need for measurement.

(3) Different functions

Fixed ultrasonic flowmeter usually has 4~20mA signal output and other functions, which can be used for remote transmission display. However, only one pipeline parameter can be stored in it. The portable ultrasonic flowmeter is only for on-site viewing of the current flow rate and cumulative time. Flow, so generally no output signal function, but in order to facilitate measuring the flow of different pipelines, it has a wealth of storage capabilities, you can store dozens of different parameters of the pipeline, ready for use at any time.

3 Classified by transducer power supply

According to the different power supply methods of the transducer, it can be divided into three kinds of ultrasonic flowmeters: external paste type, plug-in type and pipe segment type.

(1) External Mounted Ultrasonic Flowmeter

It is the earliest production, the user is most familiar with the most widely used ultrasonic flowmeter, the installation of transducer without pipe cut off, that is, paste it, it fully reflects the ultrasonic flow meter installation is simple, easy to use features.

(2) Ultrasonic flowmeter

Some pipes are caused by loose materials, poor sound conduction, or serious rust, and gaps in the lining and pipes, which cause the attenuation of ultrasonic signals to be severe. Ultrasonic flowmeters cannot be measured normally. Therefore, a segment-type ultrasonic flowmeter has been produced.

The pipe segment type ultrasonic flowmeter integrates the transducer and the measuring tube to solve the problem that the externally attached type flow meter has a problem in measurement, and the measurement accuracy is also higher than other ultrasonic flowmeters, but at the same time, the external ultrasonic flow is also sacrificed. The continuous flow installation of this advantage requires that the pipeline be installed to cut the transducer.

(3) Insertion type ultrasonic flowmeter

Between the above two. In the installation can be continuous flow, the use of special tools in the water pipe drilling, the transducer is inserted into the pipeline to complete the installation. Because the transducer is in the pipeline, its signal is transmitted and received only through the measured medium without passing through the pipe wall and the lining, so its measurement is not limited by the pipe quality and the pipe lining material.

Three ultrasonic flowmeters

In addition to the above types of ultrasonic flowmeters, digital ultrasonic flowmeters using digital circuits and integrated ultrasonic flowmeters that combine transducers and transducers have emerged in recent years.

(1) Doppler-type ultrasonic flowmeters can only be used to measure fluids containing particles or bubbles that reflect a suitable amount of ultrasonic signals, such as factory discharge fluids, untreated sewage, and factory process fluids with stable levels of impurities. It should be noted that it requires more stringent requirements for the measured medium. It cannot be clean water, and the impurity content requirements are relatively stable before normal measurements can be made. The instrument performance of different manufacturers and the requirements for the measured medium are not the same. The selection of such ultrasonic flowmeters requires an in-depth knowledge of the measured media, as well as the performance, accuracy, and requirements for the medium being measured.

(2) The portable ultrasonic flowmeter is suitable for temporary measurement, and is mainly used for the operation status of other flow meters installed on the nuclear school pipeline, to conduct fluid balance testing within a zone, and to check the current flow conditions of the pipeline. If not for a fixed installation, these portable ultrasonic flowmeters are convenient and economical when used for these purposes.

(3) The time difference ultrasonic flowmeter is currently the most widely used ultrasonic flowmeter and has the most extensive application range. It is mainly used to measure the flow of clean liquids and is widely used in water companies and industrial water applications. In addition, it can also measure the flow of homogeneous fluids with a low impurity content (contaminant content less than 10g/L, particle size less than 1mm), such as sewage, and the accuracy can reach ± ​​1.5%. The practical application shows that the time-difference type ultrasonic flowmeter has a satisfactory measurement effect on the corresponding fluid.

(4) The ultrasonic ultrasonic flowmeter has the highest accuracy, which can reach ±0.5%, and is not limited by the material of the pipeline and the lining. It is suitable for applications where the flow measurement accuracy is high. However, with the increase of pipe diameter, the cost will increase. Under normal circumstances, the use of small and medium-diameter pipe-type ultrasonic flowmeter is more economical.

(5) For fixed ultrasonic flowmeters, if there is enough space for installation, use a plug-in transducer instead of an external mount transducer to completely eliminate the effect of tube lining, fouling, and wall attenuation on ultrasonic signal attenuation. The stability is higher and the maintenance workload is also greatly reduced. Moreover, because the plug-in transducer can also be installed continuously, its application is being continuously promoted.

(6) At present, some manufacturers have introduced an ultrasonic flowmeter with a digital circuit inside. The feature is the use of digital circuits for signal processing, enhanced error correction capability, timely sampling, and improved accuracy (accuracy of analog circuits is ±1.5%, digital The circuit can reach ± ​​1.0%, and the integration is improved, the instrument volume is greatly reduced, there are a variety of signal output modes for selection, in the practical application has also achieved good results. The user can compare with the ultrasonic flowmeter of analog circuit in use.

(7) For the function selection of the ultrasonic flowmeter, the user can determine according to the actual situation. If you are measuring two-way fluids, you must select an ultrasonic flowmeter with positive and negative metering capabilities; if you need to regularly understand the flow of fluid at a certain time, you can choose an ultrasonic flowmeter with a printer. In short, the function of the selected ultrasonic flowmeter must meet the needs of the user, and it is not necessary to seek for more than one, resulting in many functions being idle and increasing the purchase cost.

Application of four ultrasonic flowmeters

1 Correct selection

This is the basis for the normal operation of ultrasonic flowmeters. If the type is not properly selected, it will result in unmeasurable traffic or user inconvenience. The specific selection principle has already been described in detail earlier.

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