The principle of an anemometer and how to use it
The probe selection of the anemometer: The measurement range of flow velocity from 0 to 100m/s can be divided into three sections: low speed: 0 to 5m/s; Medium speed: 5 to 40m/s; High speed: 40 to 100m/s. The thermal sensitive probe of the anemometer is used for precise measurement from 0 to 5m/s; The rotary probe of the anemometer has the most ideal effect in measuring flow velocities ranging from 5 to 40m/s; And using a pitot tube can achieve the best results in the high-speed range. An additional standard for selecting the flow rate probe of an anemometer correctly is temperature, which is typically used by the thermal sensor of the anemometer at temperatures of approximately+-70C. The rotor probe of the specially designed anemometer can reach 350C. Pitot tubes are used for temperatures above+350C.
The working principle of the thermal probe of the thermal probe anemometer is based on the cold shock airflow taking away the heat on the thermal element. With the help of a regulating switch, the temperature is kept constant, and the regulating current is proportional to the flow rate. When using a thermosensitive probe in turbulence, airflow from all directions simultaneously impacts the thermal element, which can affect the accuracy of the measurement results. When measuring in turbulence, the reading of the flow velocity sensor of the thermal anemometer is often higher than that of the rotary probe. The above phenomena can be observed during pipeline measurement. According to different designs for managing pipeline turbulence, it can even occur at low speeds.
Therefore, the anemometer measurement process should be carried out on the straight section of the pipeline. The starting point of the straight section should be at least 10 times before the measurement point × D (D=pipeline diameter, in CM); The endpoint should be at least 4 after the measurement point × Location D.
The fluid cross-section must not have any obstruction. The working principle of the rotary probe of the anemometer is based on converting the rotation into an electrical signal. After passing through a proximity sensing start, the rotation of the rotary wheel is "counted" and a pulse series is generated, which is then converted and processed by the detector to obtain the rotational speed value. The large-diameter probe of the anemometer (60mm, 100mm) is suitable for measuring turbulence at medium and small flow rates (such as at pipeline outlets). The small caliber probe of the anemometer is more suitable for measuring airflow with a cross-sectional area greater than 100 times that of the exploration head.
Anemometer Selection Guide: Positioning of Anemometers in Air Flow: The correct adjustment position of the anemometer's rotary probe is that the airflow direction is parallel to the rotary axis. When the probe is gently rotated in the airflow, the reading will change accordingly. When the reading reaches its maximum value, it indicates that the probe is in the correct measurement position. When measuring in a pipeline, the distance from the starting point of the straight part of the pipeline to the measurement point should be greater than 0XD, and the influence of turbulence on the thermal sensitive probe and pitot tube of the anemometer is relatively small. Anemometer for measuring airflow velocity in pipelines: Practice has proven that the 16mm probe of the anemometer is the most widely used. Its size ensures good permeability and can withstand flow rates up to 60m/s.
The measurement of airflow velocity in pipelines is one of the feasible measurement methods, and the indirect measurement regulation (grid measurement method) is applicable to air measurement. Measurement of anemometer in exhaust extraction: The ventilation port will greatly change the relatively balanced distribution of airflow in the pipeline: a high-speed zone will be generated on the surface of the free ventilation port, while the rest will be a low-speed zone, and vortices will be generated on the grid. According to different design methods of the grid, the airflow cross-section is relatively stable at a certain distance (about 20cm) in front of the grid.
In this case, measurements are usually made using the caliber wheel of a high wind speed instrument. Because larger apertures can average uneven flow rates and calculate their average values over a larger range. The anemometer uses a volumetric flow funnel for measurement at the exhaust port: even if there is no interference from the grid at the exhaust point, the air flow path also has no direction, and its airflow cross-section is extremely uneven. The reason for this is the local vacuum inside the pipeline, which funnels the air out into the air chamber. Even in the area close to the extraction, there is no location that meets the measurement conditions for measurement operations.
If the grid measurement method with average value calculation function is used for measurement, and the volumetric flow method is used to determine the volumetric flow rate, only the pipeline or funnel measurement method can provide repeatable measurement results. In this case, measuring funnels of different sizes can meet the usage requirements.
By using a measuring funnel, a fixed cross-section that meets the flow velocity measurement conditions can be generated at a certain distance in front of the disc valve. The center of the cross-section can be measured and fixed, and the center of the cross-section can be measured and fixed here. The measured value obtained by the flow rate probe is multiplied by the funnel coefficient to calculate the extracted volumetric flow rate.
