Ahlborn ALMEMO 2390-5 Manual de Serviço descarregar pdf (Página 22)

Volume Flow Measurement

Determination of the Average Flow Velocity

The most important parameter for the measurement of the volume flow is the

average flow velocity vM. As the velocity has the highest value in the center of

a channel and because it is significantly smaller at the walls, it is necessary to

use one of the following methods for averaging it over the cross section.

Time-based Averaging:

With air volume measurements at grill-screens the mean flow velocity can be

determined by a time-based averaging:

1. Set the averaging mode for the time-based averaging

2. Place the rotating vane properly at one end and start averaging.

3. Proceed uniformly over the whole cross section.

4. When the other end has been reached, stop the averaging.

Area Measurements:

If flow measurements are performed for acceptance tests, according to the

guidelines VDI/VDE 2640, the medium flow velocity is determined in a net of in-

dividual measuring points within the measuring cross section, which is placed

vertically with respect to the flow direction. The averaging over single measure-

ments should be used for these net measurements (see device manual). An

overview of the different methods can be found on the following page. Precise

measuring results can be obtained with a Pitot tube or a micro anemometer,

which has a minimal influence on the flow. Depending on the measuring meth-

od the average value of the single measurements vM must be corrected by the

correction factor k: v = k v

3-5-22 ALMEMO

Sensors

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Ahlborn ALMEMO 2390-5 Manual de Serviço Página 22