In the context of approval measurements, indirect measurement procedures (grid measurements) are used to determine air flows. In the EN 12599, the following methods are suggested: Trivial method, centroidal axis method and log-linear method. A mean flow velocity is calculated from the individual velocity measurements, and from that, the air volume flow.
Measurements at duct outlets involve a high level of inaccuracy, and are of only limited suitability for approval measurements. For this reason, they should rather be used for estimation measurements.As described in the previous chapter, the best results for large duct outlets are obtained with a 100 mm vane. There are two possibilites for this:
Multi-point measurement: The measurement points are addressed one by one in a regular pattern over the entire duct cross-section, and a mean value calculated.
Loop method: The probe is moved incrementally over the entire area of the grid.. A timed arithmetic mean value is calculated.
Observe distance from outlet: In the measurement, it is important to maintain a distance of 3 to 5 cm to the outlet.
Avoid uneven movements of the vane: Ideally, several measurements should be carried out, e.g. once moving the probe in vertical loops, and once moving it in horizontal loops.
Influences on the air flow caused by the vane and the person measuring: Flow resistances influence the measurement result. For this reason, a large van with a telescope should be used, so that only the vane probe is situated in the flow profile.
The measurement of air flow velocity and the correct calculation of volume flow at ventilation outlets can present a challenge.
The turbulence created there plus differing flow directions make correct measurement more difficult.
Measurement with a funnel in combination with a vane anemometer or a vane probe allows easy and accurate flow velocity measurement at ventilator grilles and plate valves.
