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The methods described in the section on ‘Measurement of airﬂow rate in a

duct’ (above) may of course be used to measure the ai rﬂow rates through grilles,

but speciﬁc instruments may be easier to use or bring a more accurate result.

Inﬂatable bag

This method consists in measuring (using a chronograph) the time required to

ﬁll a plastic bag of a known volume when its opening is placed against the grid.

It is not very accur ate and creates a counter-pressure that could perturb the

airﬂow, but the equipment is very cheap: a simple plastic bag, such as a rubbish

bag.

Flowmeter

Any type of ﬂowmeter ﬁxed to a box or cone adapted to the shape of the grille

may be used to measure the airﬂow rate through the grille, provided this

ﬂowmeter does not require too large a pressure drop across it.

Compensated ﬂowmeter

The compensated ﬂowmeter (see Figure 2.4) is equipped with a fan, the speed

of which is adjusted so that the pressure diﬀerential through the instrument is

negligible during the measur ement. This allows an accurate measure of the ﬂow

rate going throu gh the instrument without perturbing the measured grid or

duct. The fan itself could be the ﬂowmeter, since the fan speed at zero pressure

diﬀerential is directly related to the airﬂow rate.

Airﬂow rate measurements in air handling units

Tracer gas injection ports

Injecting several tracer gases at various locations and analysing the air at other

appropriate locations allows the assessment of se veral airﬂow rates simulta-

neously.

Fan

Flowmeter

Differential

manometer

Airﬂow

Figure 2.4 Schematics of a compensated ﬂowmeter

Note: The diﬀerential manometer adjusts the fan so that there is no pressure drop

through the ﬂowmeter.

20 Ventilation and Airﬂow in Buildings

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Sharp JX-9400 Technical Information Seite 41