Lesson 1, Topic 1
In Progress

Pressure and Volume

Let’s experimentally look at what’s going on when we dip a container full of air in the water, mouth down. Suppose that the container has the capacity of one liter (1L) and that we take it progressively to an increasing depth.

So at sea level, at pressure P0 = 1 bar, the air volume, V0, is one liter.

At 10 meters deep, at pressure P1 = 2 bar, the volume was reduced to half the initial volume V1 = 1/2l.

At 20 meters deep, at pressure P2 = 3 bar, the volume was reduced to one third of the initial volume V2 = 1/3l.

At 30 meters deep, at pressure P3 = 4 bar, the volume was reduced to one quarter of the initial volume V3 = 1/4l.

If we relate the volume to the pressure, we can write:

  • V0 x P0 = 1 x 1 = 1
  • V1 x P1 = 1/2 x 2 = 1
  • V2 x P2 = 1/3 x 3 = 1
  • V3 x P3 = 1/4 x 4 = 1
  • V4 x P4 = 1/5 x 5 = 1
  • V x P = constant

It is verified that the product is constant and that, in addition, for the same difference in depth, the variation of the volume is all the more important the closer to the surface this variation occurs.

In fact, between 0 and -10 meters, for a pressure variation of 1kg/cm2, the volume is halved (0.5l), while between -30 and -40 meters, for the same pressure range of 1kg/cm2, the volume is reduced from 1/4 liter to 1/5 liter (i.e. only 0.05l), so 10 times less than in the first case.


What was said for the descent is also absolutely valid for the climb. If at 90m depth we introduce one litre of air into a container with a capacity of 10 litres:

  • At -60m the air volume in the container is 1.42l.
  • At -30m the air volume in the container is 2.5l.
  • At -10m the air volume in the container is 5L.
  • Finally at sea level the air volume is 10 liters, completely filling the container.


The volume variations caused by the variation of the pressure on the diver’s body are manifested in the cavities with air, especially the ears and perinasal sinuses (maxillary, frontal sinuses, etc.), teeth and lungs. This forces the diver to perform compensation maneuvers (using the air he breathes and is at outside pressure) to maintain a constant balance between the outside and inside pressures. If these compensation maneuvers are not performed, the difference in pressures may lead to barotraumatic accidents of greater or lesser severity.

Also the air contained in some pieces of diving equipment (mask, vest and isothermal suit) is subject to volume variations caused by variations in the pressure of the environment, and it should be taken greater care to maintain the perfect control of these volumetric variations, to avoid barotraumatic or other injuries, which will be treated in detail in the respective modules.