Anemometer probe selection and use
Anemometer probe selection
There are usually three methods for measuring wind speed: thermal probe, impeller probe, and Pitot tube. So how can we choose the most suitable instrument for us to use when measuring wind speed? In what situations are each of these three measurement methods suitable for use?
In the flow velocity measurement range of 0 to 100m/s, we can divide it into three sections: low speed: 0 to 5m/s; medium speed: 5 to 40m/s; high speed: 40 to 100m/s. The thermal probe of the anemometer is used for measurements from 0 to 5m/s; the impeller probe of the anemometer is ideal for measuring flow rates from 5 to 40m/s; and the pitot tube is used to obtain the best results in the high-speed range.
1. Thermal probe has accurate measurement effect, and the wind speed range is generally 0-30m/s.
2. The impeller probe can select the diameter of the impeller, and impellers of different sizes have different applications. If you choose a large impeller with a diameter of 100mm, you can measure the average wind speed in a circular area with a diameter of 100mm. In addition, the impeller probe can also be attached with a cover to achieve the effect of measuring the air volume of small air outlets.
3. Pitot tubes are generally used to measure wind speed in pipelines and are suitable for large wind speeds. Generally, Pitot tubes are not recommended for wind speeds less than 5m/s.
An additional criterion for the correct selection of anemometer probes is temperature: usually the operating temperature of the thermal sensor of an anemometer is about -20~70˚C. Ordinary impeller probes are also around -20~70˚C, but impeller probes can be specially made to withstand high temperatures of 350˚C. Pitot tubes have the widest range of temperature applications, and even the most ordinary probes can withstand high temperatures of 600˚C.
How different anemometers work
1. Thermal probe of anemometer
Thermal probe is based on the cold impact airflow taking away the heat from 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 a thermal probe in turbulent flow, airflow from all directions hits the thermal element simultaneously, affecting the accuracy of the measurement results.
When measuring in turbulent flow, the indication value of the thermal anemometer flow sensor is often higher than that of the impeller probe. The above phenomena can be observed during pipeline measurement. Depending on the design of how pipe turbulence is managed, it can occur even at low speeds. Therefore, the anemometer measurement process should be carried out on the straight part of the pipe. The starting point of the straight line part should be at least 10×D (D=pipe diameter, unit: CM) in front of the measuring point; the end point should be at least 4×D after the measuring point. There must be no obstruction in the fluid section. (edges, overhangs, objects, etc.)
2. Impeller probe of anemometer
The working principle of the anemometer's impeller probe is based on converting the rotation into an electrical signal. First, through a proximity induction start, the rotation of the impeller is "counted" and a pulse series is generated. After conversion and processing by the detector, the rotation speed can be obtained. value. The large-diameter probe (60mm, 100mm) of the anemometer is suitable for measuring turbulent flows with medium and small flow rates (such as at the pipe outlet). The small-diameter probe of the anemometer is more suitable for measuring airflow where the cross-section of the pipe is more than 100 times larger than the cross-sectional area of the exploration head.
3. Pitot tube probe of anemometer
The Pitot tube can be used to measure the dynamic pressure characteristics of the fluid, and according to the following formula, the velocity of the fluid can be calculated. 1) In the formula: Pd——the dynamic pressure of the fluid, Pa;
W——fluid velocity, m/s;
r——fluid weight, N/m3;
g——gravitational acceleration, m/s2.
This is how a Pitot tube measures wind speed.
