Pressure is a notion used very often during scuba diving. A good understanding of pressure is obtained when you assimilate its effects on scuba divers and on sucba diving equipment. Pressure comprehension permits to assimilate other notions related to scuba diving such as air consumption, buyoancy... It also permits to explain the accidents caused by pressure : the barotraumas.

Pressure is an effect wich occurs when a force is applied on a surface :

 

 

Equation : P = F / A

P is the pressure

F is the force

S is the surface area that is in contact

 

 

The pressure has SI units of pascals, 1 pascal (Pa) = 1 Newton(N) / m².

Bars (b) are pressure units used during scuba diving, 1 bar is equal to a 1kg force applied on a 1 cm² area.

 

So the pressure is higher when :

- the force is higher.

- the surface area is smaller.

 

The pressure exerted by a fluid (air, water...) is exerted uniformly and in all directions. As an example when we inflate a balloon, the air that is compressed takes all the available space and the balloon becomes round.

We notice in this example that until the balloon doesn't reach its elasticity limit, its volume increases and so the air pressure doesn't change. If the volume can't change (as it is the case for a tank) then it is the fluid pressure that increases (Boyle's law).

 

The air weights 1.293 gr/l so it exerts a pressure on all objects that are on contact with it. At ocean level and when the temperature is at 15° Celsius, the atmospheric pressure is approximatively equal to 1 bar.

The atmospheric pressure decreases when the altitude increases. As an example when the altitude is equal to 1000 m the atmospheric pressure is equal to 0.88 bar.

All object, when it is submerged into water, is under the pressure of the water column that is above it. This pressure is called the hydrostatic pressure or relative pressure.

So at 10 m of depth each cm² of a submerged object is under a pressure that is equal to the force exerted by a 10 meter high water column over a surface of 1 cm2 (that is to say the weight of 1000 cm3 of water, approximatively 1 kg). So at 10 m of depth the hydrostatic pressure is equal to a 1kg force applied on a 1 cm² area, that is to say 1 bar.

Likewise at 20 m of depth the hydrostatic pressure is equal to 2 bar, at 30 meters of depth it is equal to 3 bar...

 

The relative pressure increases by 1 bar for every 10 m of depth.

 

 

The absolute pressure is the sum of the atmospheric pressure and the hydrostatic pressure: Pabsolute = Prelative + Patmospheric.

 

 

 

You can find in the following table the comparison of Absolute Pressure with Relative Pressure at different depths.

 

We notice that the greatest variations in absolute pressure occur near the surface. Therefore the absolute pressure doubles between the surface and a depth of 10 meters.