Measurement location selection
The determination of the wind speed and air volume in the ventilation duct is obtained by converting the measured pressure. To measure the real pressure value of the gas in the pipeline, in addition to the correct use of the anemometer, the reasonable selection of the measurement section and the reduction of airflow disturbance have a great influence on the measurement results. The measurement section should be selected as far as possible on the straight pipe section with stable airflow. When the measuring section is set in front of special-shaped parts such as elbows and tee (relative to the direction of airflow), the distance from these parts should be greater than 2 times the diameter of the pipe. When the measuring section is set behind the above components, the distance from these components should be greater than 4 to 5 times the diameter of the pipe. When the test site is difficult to meet the requirements, the measurement points can be appropriately increased to reduce the error. However, the minimum distance from the position of the measuring section to the special-shaped part is at least 1.5 times the diameter of the pipe.
When measuring the dynamic pressure, if any one of the measuring points is found to have a zero value or a negative value, it indicates that the airflow is unstable, and the section should not be used as the measuring section. If the airflow direction deviates from the centerline of the air duct by more than 15°, the section should not be used as a measurement section (check method: the end of the tube is facing the direction of the airflow, slowly swing the tube to maximize the dynamic pressure value. The angle between the vertical lines of the outer wall of the air duct is the deviation angle between the airflow direction and the center line of the air duct). When choosing a measurement section, the convenience and safety of the measurement operation should also be considered.
Test holes and test points
Due to the inhomogeneity of the velocity distribution, the pressure distribution is also inhomogeneous. Therefore, it is necessary to measure multiple points on the same section, and then obtain the average value of the section.
- For a circular air duct, set two measuring holes perpendicular to each other on the same section, and divide the pipe section into a certain number of equal-area concentric rings. For a circular air duct, the more measuring points, the higher the measurement accuracy.
- Rectangular air duct, the air duct section can be divided into several small rectangles of equal area, the measuring point is arranged in the center of each small rectangle, the length of each side of the small rectangle is about 200 mm, and the distance between the measuring point of the circular air duct and the pipe wall Coefficient (based on pipe diameter).