Lesson 1, Topic 1
In Progress

Weight and Drive

Let’s see what happens when we plunge a body into the water. Take as an example a body with the volume of 1dm3, that is, 1 liter.

  • The body is suspended by a water meter in the air, and the indicated weight is 10kg.
  • When we immerse the body in the water, the weight indicated by the dnaamrome becomes 9kg.
  • The volume of water displaced by the body is equal to the body volume (1 liter).
  • As a liter of water weighs 1kg, so the force that pushes the body up will have that value.

At this vertical force , which is exerted under the top, it is called an leap.

We will now analyze the situations in which one can find a dipped body, depending on its weight and volume and, therefore, the drive to which it is subjected.


The weight (p) of the body is equal to the drive (I). In this case the body does not rise or fall. It’s in hydrostatic balance. It is the ideal situation for the diver, who is obtained with a correct ballast.


The weight (p1) of the body is greater than the drive (I). In this case the body sinks. That’s what’s going on with an incorrectly-splinted diver (with too much weight).


The weight (p2) of the body is lower than the drive (I). The body rises, pushed to the surface by the thrust. That’s what’s going on with an incorrectly-splinted diver (with less weight).


Upon reaching the surface, the body described in the 3rd case comes out of the water until its weight equals the weight of the displaced water volume. The new value of the drive (I2

It is because of the relationship between weight and the drive that the diver wears a ballast belt. The lead used in the belt has a weight much higher than the drive that suffers and serves to correct the hydrostatic balance of the diver, which usually has a weight lower than the drive, especially if it uses an isothermal suit in neoprene, which gives it a very large fluctuation.

The drive is also felt on the diving bottles. Usually, when they are full, the bottles have a greater weight than the drive and sink. However, after being used, some are almost equal to or even lower than the drive they suffer, in this case they tend to fluctuate.

Drive VS Breath

A properly balanced diver can use the volume variations of his rib cage and vest to vary the drive on it exerted and consequently its hydrostatic balance.

Suppose a diver of p1 weight. Breathing normally, the average volume of your Rib Cage V1 and being subject to i1, is in hydrostatic balance.

It is intuitive that if this diver makes a deep inspiration (higher than normal), which makes him significantly increase the volume of the rib cage (V2), this increase in volume will translate into an increase in the drive (I2) in relation to his weight (p1), which leads to his rise.

On the contrary, if you do a very deep exhalation (higher than normal) by decreasing the volume of the rib cage (V3), this decrease in volume will result in a decrease in the drive (I3) in relation to its weight (p1), which leads to its descent.

In | Go up | Down


We all know that for a body to change its state of equilibrium (at rest or in motion) an external cause (a force) is required. This property of the bodies is called inertia .

This is the situation that happens when we try to change our hydrostatic balance, to go up or down. In this specific case, the resistance offered by the water causes the start of the movement to begin to manifest only a few seconds after the diver has modified its hydrostatic balance, whether using the volume of the rib cage or vest.

It is necessary to always be well aware of the existence of this delay between action and reaction, in order to avoid the tendency (which the diver initially has) to reinforce the injection or air purge of the vest, as you want to go up or down, when verifying that no immediate movement is manifested.

The result of this excessive injection or air purge is that there is too much or less air in the vest when the movement starts, which causes a higher or lower drive to achieve the new desired equilibrium situation.

On the other hand, the air insufflation must be done judiciously, so as not to fill the vest sharply, which will bring the diver wildly to the surface, if in the meantime he cannot control the ascent speed by purging the excess air.

This sudden rise can lead to serious accidents, such as pulmonary overpressure, if during the ascent the air in the lungs is not exhaled quickly enough, or a decompression accident if the rate of ascent imposed by the table is not respected. (Modules 10 and 12 – Diving-related accidents).