At present, two types of ultrasonic flowmeters are commonly used, one is a Doppler ultrasonic flowmeter, and the other is a time difference ultrasonic flowmeter. The Doppler type uses the phase difference method to measure the flow rate, that is, a sound wave of a known frequency moves in the fluid. Because the liquid itself has a speed of movement, the frequency or phase of the ultrasonic wave between the two receivers (or transmitters) occurs. Relative change, the liquid velocity can be obtained by measuring this relative change; the time difference type is to use the time difference method to measure the flow rate, that is, the sound wave of a certain speed due to the fluid flow causes its propagation time between the two receivers (or transmitters) Changes, the fluid flow rate can be obtained by measuring this relative change.
Ultrasonic flowmeter installation method introduction: At present, three installation methods are generally used: W type, V type, and Z type. Choose the installation method according to different pipe diameters and fluid characteristics. Generally, W type is suitable for small pipe diameters (25 ～ 75mm), V type is suitable for medium pipe diameters (25 ～ 250mm), and Z type is suitable for large pipe diameters (above 250mm) ) In short, in order to improve the accuracy and sensitivity of the measurement, select the appropriate installation method, so that the measurement signal (ie, the difference) and the secondary instrument match.
In order to ensure the measurement accuracy of the instrument, the location of the site that meets certain conditions should be selected: usually the straight pipe section of 10D upstream and 5D downstream is selected; disturbance equipment such as pumps and valves cannot be installed in the upstream 30D.
Ultrasonic flowmeter probe installation method: the specific use of "coordinate method installation", that is, first clean the outer surface of the pipeline, apply a special coupling agent, first fix the position of one of the probes, use a paper tape around the pipeline, measure the circumference Make a half-fold mark, and determine the position of the other probe track at the circumference of 1/2. Similarly, the track should be parallel to the axis of the pipeline, and then determine the relative distance between the two probes on the track according to the installation distance displayed by the instrument to ensure ultrasound With sufficient signal strength, the signal strength displayed on the panel is usually greater than 2%. When the reading display is stable, it indicates that the installation and debugging are completed and the meter can work normally.
Use of ultrasonic flowmeter:
(1) Zero flow inspection
When the liquid in the pipeline is still, and there is no strong magnetic field interference or strong vibration around it, the meter display shows zero. At this time, the zero point is automatically set to eliminate the zero point drift. A small signal must be cut off during operation. The flow rate is usually less than the full range 5%, automatically cut off. At the same time, the zero point can also be adjusted through the menu.
(2) Keyboard operation on the instrument panel
Before starting the operation of the instrument, you must first set the parameters effectively, for example, use the unit system, installation method, pipe diameter, pipe wall thickness, pipe material, pipe roughness, fluid type, distance between two probes, flow rate unit, minimum speed, maximum Speed ​​etc. Only if all parameters are entered correctly, the meter can display the actual flow value correctly
(3) Periodic calibration of the flowmeter
In order to ensure the accuracy of the flowmeter, periodic verification is required. Usually, a higher-precision portable ultrasonic flowmeter is used for direct comparison, and the measured data is used for calculation: error = (measured value-standard value) / standard value, use The calculated relative error and the correction factor make the measurement error meet the error of ± 2%, which can meet the measurement requirements. The operation is simple and convenient, which can effectively improve the accuracy of measurement. (Organized by China Educational Equipment Purchasing Network)
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