Measurement Methods and Applications of Anemometers and Thermal Anemometers
Measurement of air flow speed instrument. It has more types, the most commonly used for meteorological stations for the wind cup anemometer, which consists of three mutual 120 ° fixed in the bracket of the parabolic cone cups composed of induction part, the concave surface of the cups are to a direction. The whole induction part is mounted on a vertical rotating shaft, under the action of the wind, the wind cup rotates around the shaft at a speed proportional to the wind speed. Another type of rotating anemometer is the spinning-propeller anemometer, which consists of a three- or four-bladed propeller forming the sensing part, which is mounted in front of a wind vane so that it is always aligned with the direction of the incoming wind. The propeller rotates around a horizontal axis at a speed directly proportional to the wind speed. Commonly used types of anemometers are also: the use of the heated object's heat dissipation rate and wind speed related to the principle of anemometers; the use of acoustic wave transmission speed affected by the wind speed and thus increase and decrease the principle of ultrasonic anemometer.
Anemometer probe selection
0 to 100m / s flow velocity measurement range can be divided into three zones: low speed: 0 to 5m / s; medium speed: 5 to 40m / s; high speed: 40 to 100m / s. Anemometer's thermal probe for the measurement of 0 to 5m / s; anemometer's rotating wheel probe to measure the flow velocity of 5 to 40m / s the best results; and the use of Pitot tube can be in the high-speed range to get the best results. An additional criterion for the correct choice of anemometer flow velocity probe is the temperature, usually the thermal sensor of the anemometer can be used up to about +-70 C. The rotor probe of the customized anemometer can reach up to 350 C. The Pitot tube is used for +350 C and above.
Thermal Probes for Anemometers
The principle of operation of the thermal probe of anemometers is based on the fact that the cold impulse air flow carries away the heat from the thermal element, and with the help of a regulating switch, which keeps the temperature constant, the regulating current is directly proportional to the flow rate. When using a thermal probe in turbulent flow, air currents from all directions simultaneously impinge on the thermal element, thus affecting the accuracy of the measurement results. When measuring in turbulence, the thermal anemometer flow rate sensor tends to show a higher value than the rotating wheel probe. The above phenomenon can be observed during duct measurements. Depending on the different designs used to manage duct turbulence, they can occur even at low speeds. Therefore, the anemometer measurement process should be carried out in a straight part of the duct. The starting point of the straight section shall be at least 10 x D (D = pipe diameter in CM) away in front of the measurement point; the end point shall be at least 4 x D behind the measurement point. The fluid section shall not be obstructed in any way. (corners, heavy overhangs, objects, etc.)
