5
The instrument will function satisfactorily in an angular position but
should not be used in airstreams which are smaller than the entire face
area of the measuring head (113mm diameter). The AV30 is calibrated
for use in free air conditions. For smaller airstreams TSI
Instruments
Ltd. thermal type anemometers are recommended.
4.2
Checking air velocity or volume flow rate over larger areas.
When checking air velocity or volume flow rate over larger areas, a
number of ‘spot’ readings should be taken and recorded to give
coverage over the whole area.
When taking ‘spot’ readings, it should be noted that quite large
variations might be observed between individual readings. In general,
the more readings taken, the more accurate the result will be. It does
not matter if the positions of the readings overlap somewhat, so long as
they are equally spread to cover the entire area.
5.
Use on Grilles
Note:
See Comments under section 7 ‘Possible sources of error’. Avoid
intrusion of the hand, arm or handle of the instrument into the face area of
the grille. The blockage effect created by this would cause artificially high
velocity over the remainder of the grille, leading to additional errors.
Better measuring conditions can be obtained on grilles with adjustable
direction vanes if the vanes on the grilles are temporarily straightened
before making measurements. This should not significantly affect the flow
rate so long as any built in dampers are not accidentally disturbed. It is
advisable to use the aperture, not the surface area of the grille in any flow
calculations.
The instrument is suitable for both supply and extract grilles, and the
procedure for both is the same except that the measuring head must be
rotated through 180
o
to align the direction arrow correctly.
Whilst it is acceptable to hold the anemometer head against the grille in
extract it is usually recommended to hold it slightly away from the grille
face on supply to avoid excessive turbulence and any vena-contracta
effects.
