The flow rate measurement range of 0 to 100 m/s can be divided into three distinct sections: low speed (0–5 m/s), medium speed (5–40 m/s), and high speed (40–100 m/s). Each section requires a different type of probe for optimal accuracy. Thermal probes are ideal for low-speed measurements, as they offer high precision in this range. For medium speeds, the rotary probe is more suitable, while the pitot tube is best suited for high-speed applications. Additionally, temperature plays a crucial role in selecting the right probe. Thermal sensors typically operate within a range of around ±70°C, whereas specialized rotary probes can handle up to 350°C. Pitot tubes are recommended for temperatures above 350°C.
Thermal probes work by measuring the cooling effect of airflow on a heated element. A built-in adjustment mechanism maintains a constant temperature, making the current proportional to the flow rate. However, in turbulent flow conditions, air from multiple directions can impact the sensor, reducing 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 due to turbulence caused by pipe fittings or bends. To ensure accurate results, measurements should be taken in a straight section of the pipe. The straight section should extend at least 10×D before the measurement point and 4×D after it, with no obstructions present.
Rotary probes function by converting the rotation of a rotor into an electrical signal. A proximity sensor counts the rotations, generating a pulse series that is then processed to determine the flow speed. Large-bore probes (e.g., 60mm or 100mm) are well-suited for measuring turbulence at medium to low flow rates, such as at pipe outlets. Smaller probes, on the other hand, are more appropriate for larger cross-sectional areas where the airflow is more spread out.
When positioning the rotary probe, it's essential that the airflow is parallel to the rotor axis. Gently rotating the probe in the airflow will cause the reading to fluctuate. The maximum reading indicates the correct position for accurate measurement. For pipeline measurements, the distance between the start of the straight section and the measurement point should be greater than 0×D, minimizing the impact of turbulence on both thermal and pitot tube probes.
The 16mm probe is considered the most versatile option for pipeline airflow measurement. Its size ensures good penetration and can handle flow rates up to 60 m/s. Pipeline airflow measurement is a reliable method, and the gate measurement technique is often used as an indirect approach for air flow analysis.
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