Measurement techniques and selection guide for anemometers
Probe selection for anemometers
The flow velocity measurement range from 0 to 100 m/s can be divided into three zones: low speed: 0 to 5 m/s; medium speed: 5 to 40 m/s; and high speed: 40 to 100 m/s. Thermal probes for anemometers are used for accurate measurements from 0 to 5 m/s; rotary probes for anemometers are best for measuring flow velocities from 5 to 40 m/s; and Pitot tubes can be used to get the best results in the high speed range. An additional criterion for the correct selection of an anemometer's flow velocity probe is the temperature, typically the thermal sensor of an anemometer is used at temperatures up to about +-70 C. Special anemometers have rotary probes up to 350 C. Pitot tubes are used at +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 should be at least 10 x D (D = pipe diameter in CM) away in front of the measurement point; the end point should be at least 4 x D after the measurement point. The fluid section shall not be obstructed in any way. (corners, heavy overhangs, objects, etc.)
Rotating Wheel Probes for Anemometers
The working principle of the anemometer's rotating wheel probe is based on the conversion of the rotation into an electrical signal, which is first passed through a proximity inductor, "counting" the rotation of the wheel and generating a series of pulses, which are then converted and processed by the detector to obtain the value of the rotational speed. The large diameter probes (60mm, 100mm) of the anemometers are suitable for measuring turbulent flows with small or medium velocity (e.g. at the outlet of pipelines). The small diameter probe of the anemometer is more suitable for measuring the air flow where the cross-section of the pipe is more than 100 times of the cross-section of the probe.
Positioning of the anemometer in the air stream
The correct adjustment of the anemometer's rotating probe is to position the airflow parallel to the rotor axis. When the probe is gently rotated in the air stream, the indicated value changes. When the reading reaches its maximum value, the probe is in the correct measuring position. When measuring in the pipe, the distance from the starting point of the straight part of the pipe to the measurement point should be greater than 0XD, turbulence on the anemometer's thermal probe and Pitot tube has relatively little effect.
Measurement of airflow velocity of anemometer in a pipe
Practice has proved that the 16mm probe of the anemometer is the most widely used. Its size ensures good permeability and can withstand flow velocities up to 60 m/s. The measurement of air velocity in ducts is one of the feasible measurement methods, and the indirect measurement protocol (grid measurement) is applicable to air measurements.
