Analysis of factors affecting the measurement accuracy of portable ultrasonic flowmeter

The factors affecting the measurement accuracy of portable ultrasonic flowmeters are analyzed, and the influence of several factors on the measurement is analyzed, and solutions are proposed.

Portable ultrasonic flowmeters are widely used to measure the volume flow of fluids in round pipes. They have the characteristics of high measurement accuracy, small size and light weight, easy operation and portability. Due to this feature, the use of portable ultrasonic flowmeters is usually carried out under relatively harsh field conditions. Therefore, in actual use, due to field conditions or operators, the measurement accuracy of portable ultrasonic flowmeters will be affected. This article briefly analyzes the factors that affect the measurement accuracy.

1 Measurement principle and method 1.1 Overview of measurement principle Since the difference between the speed of sound in a moving fluid and the speed of sound in a stationary fluid is proportional to the instantaneous velocity of the fluid under any existing conditions, the average flow velocity along the path can be measured by emitting a sound pulse along a path and measuring its propagation time accordingly. Ultrasonic flowmeters use the time difference of ultrasonic waves propagating in the fluid in the forward and reverse directions to measure the flow velocity and flow rate of the fluid.

1.2, Measurement method: By inputting the measured pipeline parameters into the portable ultrasonic flowmeter, two ultrasonic probes are installed on the outside of the pipe wall. They are connected to the host through a signal transmission line. The flow data can be obtained.

2 Analysis of factors affecting measurement accuracy 2.1 Number of pipelines Generally speaking, whether the input pipeline parameters are correct or not directly affects the installation of upstream and downstream probes of portable ultrasonic flowmeters, which is the primary condition for obtaining accurate measurements. The installation spacing of the probes is directly related to the number of pipelines.

Its accuracy affects whether the host can obtain the signal. Take the installation of method 2 as an example. For a specific analysis, see the installation spacing of the probe; i bamboo suitable wall. The ultrasonic incident angle has been set at the factory; according to the following formula r, bamboo wall speed c fluid sound speed, under the condition that the pipe material and fluid are known, they are all constants. From formula 2, two different fixed values ​​can be obtained, thus. The installation spacing is As can be seen from formula 3, when the pipe and fluid are known, the installation spacing will be determined by the wall thickness and the inner diameter of the pipe. In actual measurements, the wall thickness of the pipe is usually difficult to know accurately. Therefore. The wall thickness and inner diameter of the measured pipe must be actually measured by certain technical measures to ensure that the input parameters are correct. In order to minimize the measurement error. Therefore, pipeline parameters are the primary factors affecting measurement accuracy.

2.2 The straight pipe section required for the installation of upstream and downstream probes is generally equipped with various resistance parts on the pipeline for conveying fluids, such as elbow valves, gradually expanding and contracting pipes, pumps, etc. Near them, the flow pattern of the fluid in the pipeline will change, and vortices, etc., will be easily generated, causing the flow velocity distribution to be disturbed. Since ultrasonic measurement is calculated under the theory of approximate constant flow, serious changes in the flow pattern will cause serious distortion of the measured value. Moreover, the bubbles generated near the resistance parts will seriously hinder the propagation of ultrasonic waves, making it difficult for the host to receive signals and affecting the measurement. Therefore, when installing upstream and downstream probes in actual measurements, it is necessary to choose a distance of more than 50 times the inner diameter of the pipeline away from these resistance parts to reduce the impact of these resistance parts. Ma Wen'an, male, 1 Shu, 1 Fan, July, 2009, North China University of Technology, Ke Yi 2.3 The contact surface between the probe and the pipe wall The outer wall of the pipeline for conveying fluids is generally coated with a protective layer, such as paint, asphalt, etc. The presence of these substances will be transferred to page 43 4.1. Blank absorbance values ​​are 0.07.20.1 5.3 1 time after adding potassium ferricyanide solution. The standing time has an impact on the determination results. The standing time after adding potassium ferricyanide and before chloroform extraction must be strictly controlled to make the blank and standard standing time consistent, so as to obtain good results. In order to ensure the consistency of the standing time, an electric shaker can be used. If 6 samples can be shaken each time, only 6 samples of reagents are added, and the timing is started after adding potassium ferricyanide, and chloroform is added immediately without shaking. Only when it is close to 1; shake slightly to release air, place it on the shaker, and shake it again when it is accurate. 13 5.4 Distillation recovery and collection liquid accumulation Since phenol is not easy to evaporate and is always evaporated with water vapor, the distillation recovery is related to the volume of the collected distillate. Experiments have confirmed that the volume of the collected effluent must be equal to the original water sample volume. If not, it will affect the recovery rate. Add phenol-free water of 10% of the water sample volume before starting distillation. Distill until the volume of the distillate is equal to the volume of the water sample, which can ensure a recovery rate of 95.100. This operation is simple and safe and can replace the double distillation method.

5.5 Use of the instrument The separatory funnel and colorimeter must be kept dry before use. Otherwise, the colorimetric solution will become turbid. If it is contaminated by phenol, it needs to be washed with potassium dichromate solution. Then rinse thoroughly with tap water. The lower piston of the separatory funnel should be removed to avoid residual 6+ and interference with the determination.

The following contents are: 4. The glass measuring instrument for the determination of volatile phenols in water by spectrophotometry must be accurately calibrated and kept for special use. It must not be mixed with other analytical items to avoid mutual interference.

1 National Environmental Protection Agency Water and Wastewater Monitoring and Analysis Methods Editorial Committee. Water and Wastewater Monitoring and Analysis Methods. Beijing China Environmental Science Press. 1989.409.

2 Zhang Hongtao, ed. Notes on the Standard Test Method for Daily Drinks. Chongqing, Chongqing University Press. 1993.8789.

3 Water quality analysis office editor. Private quality analysis encyclopedia. Science and Technology Editor Xu Weiwei Continued from Page 4 It will affect the propagation path and intensity of ultrasonic waves. Therefore, when installing the probe, the outside of the pipe wall at the installation location must be processed. Polish the metal surface to a bright finish, and there must be no pits. Apply coupling agent to the emitting surface of the probe to make the emitting surface in close contact with the pipe wall. This can avoid the attenuation of ultrasonic waves caused by the presence of air between the emitting surface of the probe and the outside of the pipe wall.

In actual measurement, sometimes it is found that the portable ultrasonic flowmeter data display is unstable or cannot display data continuously. Usually this is because the outer wall of the pipe is not properly treated. This phenomenon can be eliminated by re-treating and applying coupling agent. Therefore, how the contact surface between the probe and the outer wall of the pipe is treated is also a factor that affects the measurement accuracy of the portable ultrasonic flowmeter.

2.4 Zero point electronic components have been working for a long time, or their working environment has changed significantly. This will cause the working point of the electronic components to change, thereby causing the circuit working state to change. Portable ultrasonic flowmeters are often used in the field. Due to the influence of the environment. After a period of use, the displayed value will not be zero when tested on a zero flow pipeline. This kind of portable ultrasonic flowmeter with a changed zero point is bound to cause inaccurate measurement values ​​during on-site measurement. Therefore, the portable ultrasonic flowmeter should be calibrated frequently during actual use. 2.5 Interference from the outside world Portable ultrasonic flowmeter is a high-precision electronic measuring instrument. If there are strong electromagnetic waves in the surrounding environment. It will interfere with the normal operation of the instrument and affect the measurement accuracy. Therefore, when using a portable ultrasonic flowmeter in practice, stay away from high-speed motors and high-frequency transformers and other facilities that are prone to interference.

3 Conclusion Through the above analysis, when using a portable ultrasonic flowmeter, try to eliminate factors and interference that affect the measurement accuracy, and formulate a calibration cycle based on actual usage. Perform traceability calibration according to the cycle. Ensure that the portable ultrasonic flowmeter is in good working condition. Only in this way can accurate and reliable flow data be obtained.