Measurement of wind pressure, wind speed, and air volume

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I. Selection of measurement point locations: The measurement of wind speed and volume in ventilation ducts is obtained by measuring pressure and then making conversions. To obtain the true pressure value of the gas in the pipeline, in addition to using pressure measuring instruments correctly, it is also important to select the measurement section appropriately and minimize the impact of airflow disturbances on the measurement results. The measurement section should be selected on a straight pipe section with stable airflow. Due to the unevenness in the velocity distribution, the pressure distribution is also uneven; therefore, measurements must be taken at multiple points on the same cross-section, and then an average value is calculated. The circular duct is equipped with two mutually perpendicular measurement holes in the same cross-section, dividing the duct cross-section into a certain number of equal-area concentric rings. A rectangular duct can divide the duct cross-section into several small rectangles of equal area, with measurement points placed at the center of each small rectangle. II. Measurement of pressure in the air duct. During testing, it is necessary to measure the static pressure, dynamic pressure, and total pressure of the gas. The hole for measuring total pressure should face the direction of the airflow, while the hole for measuring static pressure should be perpendicular to the direction of the airflow. The difference between total pressure and static pressure is the dynamic pressure. The measurement of gas pressure is usually done by using a pressure probe inserted into the air duct to capture the pressure signal; common instruments for this purpose are pitot tubes and pressure gauges. A standard pitot tube is a double-layered concentric tube bent at 90°. Manometers include U-tube manometers and inclined micro-manometers. A pitot tube, in combination with a pressure gauge, is used to measure pressure. III. Measurement of wind speed. Common methods for measuring wind speed in pipes include indirect and direct-reading methods. In the indirect method, the dynamic pressure at a certain point inside the duct is first measured, and then the wind speed at that point is calculated. Although this method is cumbersome, it is widely used in ventilation testing systems due to its high precision. The direct-reading anemometer is a hot-ball thermoelectric anemometer; its sensor is affected by the speed of the surrounding air flow, and the speed of the airflow can be determined based on the degree of temperature rise. IV. Measurement of wind speed at local intake and exhaust openings: 1. Constant-speed movement method: Using an impeller anemometer, move it at a constant speed along the cross-section of the opening to determine the average wind speed there. 2. Fixed-point measurement method: Using a hot-ball anemometer, the opening area is divided into several small squares of equal size; the wind speed is measured at the center of each square, and the average value is taken. V. Measurement of air volume at local intake and exhaust outlets: 1. Use the dynamic pressure method to determine the dynamic pressure at the cross-section, calculate the wind speed, and thereby determine the air volume. 2. When it is difficult to find a stable pressure measurement section using the dynamic pressure method, the static pressure method is employed to determine the air volume.