Anemometer measurement techniques and selection guide
Probe selection for anemometers: The 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; and high speed: 40 to 100m/s. The thermal probe of anemometers is used for accurate measurements from 0 to 5m/s; the rotary probe of anemometers is best for measuring flow velocities from 5 to 40m/s; and the use of Pitot tubes gives the best results in the high speed range. The use of a Pitot tube gives 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, which is typically about ±70°C for an anemometer's thermal sensors. Specialised anemometers have rotor probes for temperatures up to 350°C. Pitot tubes are used above 350°C. Pitot tubes are used above 350°C.
Thermal probes for anemometers: The principle of operation of thermal probes for anemometers is based on the fact that a cold impulse airflow carries away the heat from the thermal element with the help of a regulating switch, which keeps the temperature constant, and regulates the current in direct proportion 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 sensor tends to show a higher value than the rotor probe. The above phenomena can be observed during duct measurements. Depending on the design of the duct turbulence, they can occur even at low speeds. Therefore, the anemometer measurement process should take place in a straight part of the duct. The starting point of the straight section should be at least 10 x D (D = diameter of the pipe in CM) away in front of the measurement point; the end point should be at least 4 x D behind the measurement point. The fluid section must not be obstructed in any way. (corners, heavy overhangs, etc.).
Anemometer's rotating wheel probe: The working principle of anemometer's rotating wheel probe is based on converting the rotation into an electrical signal, which first passes through a proximity inductor, "counts" the rotation of the rotating wheel and generates a series of pulses, which are then converted and processed by the detector to obtain the rotational speed value. The large diameter probes (60mm, 100mm) of the anemometers are suitable for measuring turbulent flows (e.g. at the outlet of a pipeline) at medium to small velocities. The small diameter probes of the anemometers are more suitable for the measurement of airflow in ducts with a cross-section more than 100 times larger than the cross-section of the probe.
Anemometers measure the relatively balanced distribution of airflow in ducts with large ventilation openings in the exhaust air: a high velocity zone is generated on the surface of the free ventilation opening, the rest of the duct is characterised by a low velocity zone and vortices are generated on the grill. Depending on the design of the grille, the airflow cross-section is more stable at a certain distance in front of the grille (approx. 500 px). In this case, a large aperture rotor anemometer is usually used for the measurement. This is because the larger aperture allows the averaging of uneven flow velocities and the calculation of the average value over a wide range.
Anemometers use a volumetric flow funnel for measurements at the suction hole: even if there is no grid interference at the suction, the air flow path is not directional, and the cross-sectional area of the air flow is not uniform. The reason for this is the local vacuum in the pipe, which funnels the air out of the chamber, and there is no position in which the measurement conditions can be fulfilled, even in the area close to the extraction. If measurements are carried out using the grid method with a mean value calculation and by means of the deterministic volumetric flow method, only the pipe or funnel method can provide reproducible measurement results. In this case, measuring funnels of different sizes are available. With the measuring funnel it is possible to generate a fixed cross-section that meets the conditions for flow rate measurement at a certain distance in front of the lamella valve, to locate the centre of the cross-section and to fix the cross-section there. The measured value obtained by the flow rate probe is multiplied by the funnel coefficient to calculate the volume flow rate being pumped.
