The measuring section should be selected on a straight pipe section with uniform airflow, and it should be 4-5 times the diameter of the upstream local resistance pipe fitting and 2 times the diameter of the downstream local resistance pipe fitting, as shown in Figure 1. Generally, pitot tube and piezometer are used to measure the velocity of each point on the selected section. The straight pipe of the pitot tube must be perpendicular to the pipe wall, and the probe of the pitot tube should face the air flow direction and be parallel to the axis of the air duct. The detection position is shown in figure 1-2.

2. Layout of measuring points

1) Rectangular air duct: The section of rectangular air duct is divided into several equal segments with an area close to a square, the area is generally not more than 0.05m2, and the side length is

2) Circular air duct: the section of circular air duct is divided into several concentric rings with equal area, and the measuring points are arranged on the area bisector of each ring, and two or four measuring holes should be arranged on two diameters perpendicular to each other. See Figure 4 for the determination and arrangement of measuring points of circular section.

3. Measuring instrument

1) Air volume and wind pressure measuring instruments: pitot tube and micro-pressure gauge. When the dynamic pressure is less than 10Pa, it is recommended to use thermoelectric anemometer or digital anemometer for air volume measurement.

2) Atmospheric pressure measuring instrument: atmospheric pressure gauge.

4. Measuring steps

1) Check whether the system and unit are operating normally and adjust to the detection state;

2) Determine the specific location of air volume measurement and the number and arrangement of measuring points;

3) according to the operating rules of the instrument, adjust the testing instrument to the measuring state;

4) Measure point by point, and each point shall be measured for more than 2 times.

5) When measuring with pitot tube, the straight tube of pitot tube must be perpendicular to the tube wall, and the probe of pitot tube should face the air flow direction and be parallel to the air duct axis. When measuring, the connecting hose between the pitot tube and the micro-pressure gauge should be smooth and airtight;

6) Record the temperature and pressure measured at that time.

5. Data processing

1) When pitot tube is used for measurement, the air volume of computer group or system shall be calculated as follows.

Average dynamic pressure, in general, the arithmetic average of each measuring point can be taken as the average dynamic pressure. When the data of each measuring point changes greatly, the average dynamic pressure should be calculated according to the root mean square value as follows:

The average cross-sectional wind speed is calculated as follows:

The measured air volume of the device or system is calculated by the following formula:

For the calculation of standard air volume, when there is a clear requirement, you can press the following formula to replace the measured air volume with the air volume under standard air conditions.

2) When measuring air volume with thermoelectric anemometer or digital anemometer, the average cross-sectional wind speed is the average of the wind speeds measured at each measuring point, and the calculation methods of the measured air volume and standard air volume are the same as those of pitot tube measurement.