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What are the factors considered in the selection of ultrasonic flowmeters?

What are the factors considered in the selection of ultrasonic flowmeters?

2022-07-18

The ultrasonic flow meter is used in conjunction with the weir tank to measure the flow of water in the open channel. It is mainly used to measure the sewage discharge outlets of sewage plants, enterprises and institutions, the flow of urban sewers and the channels in farmland water conservancy. Since our ultrasonic flow meter company's instrument uses ultrasonic waves to pass through the air, it measures in a non-contact method. Therefore, it is suitable for the measurement of flow and liquid level under the conditions of sticky and corrosive liquids.


What are the reference factors for selecting an ultrasonic flow meter?


1. Selection of range and accuracy The water level range of open channel flowmeters used in agricultural irrigation channels and sewage discharge channels is within the range of 2-4m, and the blind area of the probe is within the range of 0.3-0.5m. The accuracy is selected according to the actual application needs to achieve economic benefits.


2. The choice of power supply type includes 24VDC.12VDC.5VDC.220VAC, etc. If there is a municipal power supply on site, select the 220V/AC power supply type. If there is no municipal power supply on site, 12V/DC is preferred, and the supporting DTU/RTU is 12V/DC power supply to ensure the consistency of the on-site power supply.


3.Selection and installation of the weir tank. There are three common water weirs: triangular weirs, rectangular weirs and Parshall troughs. Parshall troughs are recommended for agricultural irrigation and sewage discharge. The level-flow relationship of the Parshall trough is calibrated experimentally, and the data are relatively accurate. As long as it is ensured that the water level in the downstream of the Parshall trough is poor, the discharge is smooth, and there is no stagnant water. The water level-flow relationship between triangular weirs and rectangular weirs comes from theoretical calculations, and it is easy to ignore some operating conditions, resulting in additional errors and requirements for the upstream and downstream drop.


4. Select the signal transmission type.


Digital signal: The digital signal shall determine the data interface and communication protocol. Data interface RS485.RS232, etc. Protocols include HART protocol, Modbus protocol, Profibus protocol, etc.


Analog signal: Analog signal includes current (DC4-20ma), voltage, resistance (resistance and voltage are usually rarely used), etc.


Ultrasonic flow meters are mainly used for flow measurement in reservoirs, rivers, water conservancy projects, farmland irrigation and culverts. Since it mainly uses the weir-trough method and the velocity area method to calculate the flow of the water delivery section, its installation method will directly affect the measurement results and accuracy.


The low cost ultrasonic flow meter consists of an instrument for measuring the liquid level and a water tank, which is manufactured according to documents published by the Metrology Institute. Liquid level gauges directly measure the liquid level in a channel or tank. When used for open channel flow measurement, install a weir groove on the open channel. Weir grooves convert water flow in open channels into liquid levels. The instrument measures the water level in the weir tank, and then calculates the flow according to the relationship between the water level and the flow rate in the corresponding weir tank.


There must be a certain gap between the actual measurement accuracy of the high temperature ultrasonic flow meter and the theoretical accuracy (that is, the calibration accuracy of the instrument when it leaves the factory), which is mainly due to factors such as installation location, installation accuracy, data processing method and calibration method.

Therefore, the improvement of measurement accuracy mainly starts from the following points:


1. Adjust the installation position.


Try to choose a straight channel with a standard section to meet the requirements (the upstream straight section of the instrument is 10 times the channel width, and the downstream straight section is 5 times the channel width). If this requirement is not met, the flow state of the water will not be very stable, and the measurement result will be too large or too small. At this time, it needs to be corrected, usually by multiplying a correction factor (generated by on-site calibration) or adjusting the installation position.


2. Improve installation accuracy


It is mainly aimed at the accuracy of the installation angle and position of the speed probe. If the installation angle is deviated, the result will have a fixed error factor. At this time, in order to improve the measurement accuracy, it is necessary to adjust the installation angle or multiply by a correction factor.


3. Data processing method


Mainly refers to the fact that in the actual measurement process, there will be various disturbances on site (such as fish swimming near the flow rate probe during the measurement process), making individual data inaccurate or completely distorted. If the measurement time interval is large, these distorted data will have a greater impact on the measurement results, so it is necessary to increase the density of the measurement time interval and remove or smooth the distorted data.


4. Calibration method


When calibrating the ultrasonic density meter, it is necessary to ensure that the flow rate of the section at the measurement site remains unchanged for a certain period of time, otherwise the same water level will correspond to different flow rates during the process of rising or falling water, resulting in the calibration coefficient being too large (falling water) or too small (rising water). water). Another point to note is that one or several suitable water levels should be selected for calibration, and special water levels cannot be selected. When the exact value of the actual section flow cannot be obtained, the system error can be offset by installing the instrument by the symmetrical method or the reverse method.


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