Summary Definitions of Pressure
AIR MEDIUM
Experimentally it is known that one liter of atmospheric air weighs 1.293g or approximately 1.3g.
Thus, all bodies on the surface of the earth are subject, per surface unit, to the weight of the air column whose height corresponds to the height of the air layer that surrounds the globe, which constitutes the troposphere (17,000m at the equator and 6,000 m at the poles), and can be taken as an average value of 12,000 meters.
PRESSURE IS THE WEIGHT (OR FORCE) EXERTED PER SURFACE UNIT
We can say that the pressure exerted by atmospheric air on the bodies, situated at sea level is:
P = 1033 gram per square centimeter (g/cm2) or approximately 1kg/cm2
Pressure can also be measured in other units, such as atmospheres, millimeters of mercury column, bar, Pascal, etc.
- P = 1 atmosphere
- P = 1.013bar
- P = 760mm mercury
- P =1.01325×105
This pressure decreases with altitude, but this variation is not proportional to the two parameters, as we can see in the following graph. It should be noted that for the pressure value to be halved it is necessary that the height reaches approximately five thousand meters.
AQUATIC MEDIUM
Similarly to the air, it is known experimentally that a liter of seawater (1000cm3) weighs approximately 1000g.
This means that a column of seawater only ten meters high exerts on the surface of a body approximately the pressure of 1kg/cm2 . That is, for every ten meters of water column there is a pressure variation of 1kg/cm2 (or 1 atmosphere or 1bar), a pressure that is equal to the pressure exerted by atmospheric air at sea level.
We can therefore say that the pressure exerted on a dipped body, absolute pressure , is the sum of atmospheric pressure plus the pressure exerted by the water column, whose height is equal to the depth to which the body is immersed and which is designated by hydrostatic pressure .
| DEPTH | HYDROSTATIC PRESSURE (Ph) | ABSOLUTE PRESSURE (Pa) |
|---|---|---|
| 0 m | — | 1 bar |
| -10 m | 1 bar | 2 bar |
| -20 m | 2 bar | 3 bar |
| -30 m | 3 bar | 4 bar |
| -40 m | 4 bar | 5 bar |
| -50 m | 5 bar | 6 bar |
| -60 m | 6 bar | 7 bar |
| -70 m | 7 bar | 8 bar |
| -80 m | 8 bar | 9 bar |
| -90 m | 9 bar | 10 bar |
The pressure at a given depth is equal to the number of tens of meters, corresponding to that depth, plus one:
P = n/10 + 1
To recap, Absolute Pressure is the sum of Hydrostatic Pressure with Atmospheric Pressure:
Pa = Ph + Patm
Out of curiosity, if we dive at six feet and try to breathe the air from the surface through a tube we can’t! The pressure that is exerted on our rib cage does not allow us to inspire, since the inspiratory muscles cannot overcome the pressure increase of only 0.18kg/cm2, exerted by the column of water we have on us.
We can now understand the importance of the action that increased pressure exerts on the human body when it is immersed.