The anemometer's flow rate measurement range spans from 0 to 100 m/s, typically divided into three segments: low speed (0–5 m/s), medium speed (5–40 m/s), and high speed (40–100 m/s). Each range is best suited for a specific type of probe. Thermal probes are ideal for low-speed measurements due to their high accuracy in the 0–5 m/s range. For medium speeds, the rotary probe offers reliable performance, while the pitot tube is most effective at higher velocities above 40 m/s. Additionally, temperature plays a crucial role in selecting the right probe. Thermal sensors generally operate within a range of around ±70°C, whereas specialized rotary probes can handle up to 350°C. Pitot tubes are typically used for temperatures above 350°C.
Thermal probes function by measuring the cooling effect of airflow on a heated element. A constant temperature is maintained, and the current required to sustain that temperature is proportional to the airspeed. However, in turbulent conditions, the airflow comes from multiple directions, which can reduce measurement accuracy. In such cases, thermal anemometers may show higher readings compared to rotary probes. This issue is commonly observed during pipeline measurements and can occur even at low speeds depending on the pipe’s design. To ensure accurate results, measurements should be taken in a straight section of the pipe. The straight section should extend at least 10 times the pipe diameter (D) before the measurement point and 4 times D after it. No obstructions, such as bends or valves, should be present in this area.
Rotary probes work by converting the rotation of a rotor into an electrical signal. A proximity sensor detects the rotor’s movement, counts the rotations, and generates a pulse series. This data is then processed to determine the airspeed. Larger bore probes (e.g., 60mm or 100mm) are suitable for measuring turbulence at medium and small flow rates, such as at pipe outlets. Smaller probes are better suited for applications where the cross-sectional area of the airflow is significantly larger than the probe’s size, ensuring more accurate readings.
Proper positioning of the rotary probe is essential. The probe should be aligned so that the airflow is parallel to the rotor’s axis. When gently rotating the probe in the airflow, the reading will fluctuate. The maximum reading indicates the correct measurement position. When measuring inside a pipeline, the distance from the start of the straight section to the measurement point should be greater than 0xD, minimizing the impact of turbulence on both thermal and pitot tube measurements.
For pipeline airflow measurement, the 16mm probe is often considered the most versatile. Its compact size ensures good penetration and can measure airspeeds up to 60 m/s. Pipeline airflow measurement is a practical method, and the gate measurement technique can also be used for indirect air velocity assessment. Choosing the right probe based on flow range, temperature, and turbulence levels is key to obtaining accurate and reliable results.
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