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.