How you can avoid measurement errors
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.
- When measuring in ducts with a small cross-section the guideline value applies: if the flow-impacted area of the probe is larger than 1:100 in relation to the free duct cross-section, then the inserted probe itself disturbs the flow behaviour in the duct. The display is then increased by the percentage proportion of the cross-section narrowing.
- Important in the selection of the measurement location: if possible, maintain a length of 10 duct diameters upstream of the probe and 4 to 6 duct diameters downstream of the duct free of disturbances, and seal the measurement location well.
- The probe must be rotated in the duct so that the maximum velocity is measured. Only then can the measurement value be recorded.Rotating into the main flow direction is especially important when measuring with a Pitot tube.
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.
- The funnel should fit completely over the plae valve or the ventilator
- For measurements at so-called swirl outlets, which have a high level of drift, a straightener should be used, in order to be able to obtain considerbaly better results thanks to a straightened flow.