Atmospheric Pressure and Gauge Pressure

Let us learn about Atmospheric pressure,
The pressure of the atmosphere at any point is
equal to the weight of a column of air of unit
cross sectional area extending from that point
to the top of the atmosphere. At sea level it is
1.013 × 105 Pa (1 atm). Italian scientist
Evangelista Torricelli (1608-1647) devised for
the first time, a method for measuring
atmospheric pressure. A long glass tube closed
at one end and filled with mercury is inverted
into a trough of mercury as shown in Fig.
This device is known as mercury barometer. The
space above the mercury column in the tube
contains only mercury vapour whose pressure
P is so small that it may be neglected. The
pressure inside the column at point A must
equal the pressure at point B, which is at the
same level. Pressure at B = atmospheric
pressure = Pa
Pa = ρgh
where ρ is the density of mercury and h is the height of the mercury column in the tube.
In the experiment it is found that the mercury column in the barometer has a height of about
76 cm at sea level equivalent to one atmosphere (1 atm). This can also be obtained using the
value of ρ in Eq. A common way of stating pressure is in terms of cm or mm of mercury
(Hg). A pressure equivalent of 1 mm is called a torr (after Torricelli).
1 torr = 133 Pa.
The mm of Hg and torr are used in medicine and physiology. In meteorology, a common unit
is the bar and millibar.
1 bar = 105 Pa
Now, let us learn Measurement of Atmospheric Pressure.