4.4.2. Influence of Air Pressure
36
The heat transfer from the probe will also depend on the
air
density and consequently
the barometric pressure. The
daily variations in barometric pressure at sea level are
of
no importance, but
if
the instrument is used at high elevations e.g., in the mountains
or
in aircraft, the
drop
in air pressure should be taken into account. The corrections that
need
to
be added
to
the displayed air velocity to give true air velocity values
for
mea-
surements made
at
altitudes greater than approx. 1 000 m above sea level are shown in
Fig. 4.6.
Important: If the purpose
of
the measurement is to determine the cooling effect
of
a
draught then the
displayed value is the correct one and should not be corrected
for
air
pressure variations.
[cm/s)
36
34
32
t1
v
Correction
to
be
added
to the
displayed
air
velocity
ft/min
Height
and
air
pressure
30~6=0~------~--------~----------~-------.---------.
28
26 50
24
22
20 40
18
16
30
14
12
10
8
6
4
2
80 100 120 140
160 180 200
ft/min
0~--~----~--~~--~----~---+~--~--~~--~~--~~
0 1 0
20
30 40 50 60 70 80
90
100
cm/s
850451
Fig. 4.6.
Air
pressure
correction
for
the
Air
Velocity Transducer Type
MM0038
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