Airflow Instruments AV 30 Manuale di istruzioni per l'uso - Pagina 6

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6. Use in Airways

In large airways the presence of the instrument will have a negligible

effect, but in small airways the blockage caused by the instrument, hand

and arm will cause the airstream to accelerate slightly as it passes the

rotating vane. This effect is somewhat variable depending on the size of

the airway and the distance from the duct walls. The error can be virtually

eliminated by mathematical correction to allow for the reduction of free

area caused by the obstruction. For this purpose the effective front area of

the instrument (not including hand or arm) can be taken as 0.019m

(0.204ft

). The effect can be ignored completely if the duct exceeds about

500mm diameter (1' 9").

7. Possible sources of error

The above method ignores the effects of the reduced velocity at the duct

walls. A more precise method is shown in BS1042 Part 2.1 Log

Tchebycheff method.

This procedure is satisfactory for use in ducts, and at unobstructed

apertures.

Significant errors may occur if the aperture is covered by a grille,

particularly if this is of the type having adjustable direction vanes and/or

dampers. The airstream issuing from such a grille is invariably very

disturbed, consisting of many small areas of high velocity interspersed with

areas of low velocity.

The transitions between these areas are highly turbulent, and there may

even be some reversed flow. If maximum accuracy is required, it is

advisable to make up a short length of test ducting which is just larger than

the overall dimensions of the grille. This duct can be of any convenient

rigid material (eg stiff cardboard) and should have a length about twice the

diagonal measurement of the grille. The duct should be placed over the

grille, and sealed to the wall with adhesive tape. Measurements of flow can

now be conducted, as already described, at the unobstructed end of the test

duct. Use the cross sectional area of the duct (not the grille) for the

calculations.

It should be noted that using an AV30 instrument to take spot readings

across a duct as described earlier can result in an exaggerated velocity

indication in applications where there is a significant variation in velocities

across the test area. This is caused by the inability of the rotating vane to