atm This is a % increase in pressure from surface level. At a depth of km, the increase in pressure is atm! Submarines are designed to withstand such enormous pressures. ⊳ .
. Atmospheric Pressure and Gauge 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 . × Pa ( atm).
Italian scientist Evangelista Torricelli ( – ) 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. . (a).
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. Thus, the pressure at Point A= . The pressure inside the coloumn at Point B must be the same as the pressure at Point C, which is atmospheric pressure, P a .
P a = ρ gh ( . ) 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 cm at sea level equivalent to one atmosphere ( 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 mm is called a torr (after Torricelli).
torr = Pa. The mm of Hg and torr are used in medicine and physiology. In meteorology, a common unit is the bar and millibar. bar = Pa An open tube manometer is a useful instrument for measuring pressure differences.
It consists of a U-tube containing a suitable liquid i.e., a low density liquid (such as oil) for measuring small pressure differences and