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The height which the column of water must achieve to equal the atmospheric pressure is 10,3
m
.
In the International System the unit of measurement of pressure is the
Pascal
(Pa)
and is equal to the pressure of
a Force of 1
N
over 1
m
2
.
1
Pa
=1
N ⁄m
2
Amuch usedmultiple of
Pa
is the bar (even though it doesn’t belong to the International System):
1
bar
≈
100.000
Pa
≈
0,1
MPa
The table below illustrates themost common units of measure:
bar
Pascal
Kg/cm
2
cmH
2
0
mmH
2
O mmHg
P.S.I.
1
101.757
1,03
1033
10.330
760
14,69
76 cm
vacuum
sea level
1
76cm
S=1cm
2
columnof water
mercury column
S= 1cm
2
1030 cm
sameweight
10,33
Newton
2
3
Fig. 2
Absolute and relative pressure
In this chapterwe observe the phenomenawhereby all gaseous bodies have a tendency to
expand
;we also observe
the influence the atmospheric pressure has on a container full of water.
Figure 3
An empty balloon closed by a tap is positioned inside a glass container. At atmospheric pressure, the balloon is
floppy, although it contains a certain amount of air. Removing the air from the container we observe that the
balloon gradually inflates inside it, because as the air in the containerwas being gradually extracted, it exerted less
and less Force on the balloon and resistance to the expansion of the air in the balloonwas reduced. The air inside
the balloon had a pressure value greater than that inside the bell.
This property is evident with balloons that float upwards: whereby increasing the altitude, the external pressure
decreases and the gas inside the balloon expands until the balloon bursts.
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