Measurement technology and selection guide for anemometers
Probe selection for anemometer: The velocity measurement range from 0 to 100m/s can be divided into three sections: low velocity: 0 to 5m/s; medium velocity: 5 to 40m/s; high velocity: 40 to 100m/s. The thermal probe of the anemometer is used for accurate measurement from 0 to 5m/s; the rotating wheel probe of the anemometer is ideal for measuring the flow velocity from 5 to 40m/s; result. An additional criterion for correctly selecting the velocity probe of the anemometer is the temperature. Usually, the temperature of the thermal sensor of the anemometer is about ±70°C. The rotor probe of the special anemometer can reach 350°C. Pitot tubes are used above 350°C.
Thermal probe of the anemometer: The working principle of the thermal probe of the anemometer is based on the cold impact airflow taking away the heat on the heating element, with the help of an adjustment switch to keep the temperature constant, the adjustment current is proportional to the flow rate. When using thermal probes in turbulent flow, airflow from all directions impinges on the thermal element simultaneously, which can affect the accuracy of the measurement results. When measuring in turbulent flow, thermal anemometer flow sensors tend to give higher indications than rotary wheel probes. The above phenomenon can be observed in the pipeline measurement process. Depending on the design of the pipe turbulence can occur even at low speeds. Therefore, the anemometer measurement process should be carried out on the straight part of the pipeline. The starting point of the straight line should be at least 10×D (D=pipe diameter, in CM) before the measurement point; the end point should be at least 4×D behind the measurement point. The flow section must not be obstructed in any way. (edges, heavy suspensions, etc.).
Rotary wheel probe of anemometer: The working principle of the rotary probe of anemometer is based on converting the rotation into an electrical signal, first passing through a proximity sensor, "counting" the rotation of the wheel and generating a pulse series, and then The rotational speed value can be obtained after conversion by the detector. The large-diameter probe (60mm, 100mm) of the anemometer is suitable for measuring turbulent flow with medium and small flow rates (such as at the pipe outlet). The small-bore probe of the anemometer is more suitable for measuring the air flow whose cross-section of the pipe is more than 100 times larger than that of the probe.
The anemometer measures the relatively balanced distribution of the airflow in the pipe with a large vent in the extraction and exhaust: a high-speed area is generated on the surface of the free vent, and the rest is a low-speed area, and a vortex is generated on the grid. According to the different design methods of the grid, at a certain distance (about 500px) in front of the grid, the airflow section is relatively stable. In this case, a large-diameter runner anemometer is usually used for measurement. This is because the larger bore is able to average the unbalanced flow rate and calculate its average value over a larger range.
The anemometer uses a volumetric flow funnel for measurement at the suction hole: Even if there is no grid interference at the suction point, the air flow route has no direction, and its airflow cross-sectional area is not uniform. The reason is that the partial vacuum in the pipeline draws out the air in the air chamber in a funnel shape. Even in the area very close to the air extraction, there is no position that meets the measurement conditions for measurement operations. If the measurement is carried out with the grid measurement method with averaging function and by means of which the volumetric flow method is determined, only the pipe or funnel measurement method can provide reproducible measurement results. In this case, measuring funnels of different sizes can meet the requirements of use. The measuring funnel can be used to generate a fixed cross-section meeting the flow velocity measurement conditions at a certain distance in front of the sheet valve, measure and locate the center of the cross-section and fix the cross-section here. The measured value obtained by the flow rate probe is multiplied by the funnel coefficient to calculate the drawn volume flow.
