Let us begin fixing concepts: The air that we breathed is made up of a 78.08% of nitrogen (N2), a 20.95% of oxygen (O2), a 0.035% of carbon dioxide (CO2) and a 0.94% of inert gases like argon and neon.

In order to dive we take this same air and we compressed it within a bottle that is the one that we used to dive, the pressure to that normally the bottles are loaded is to 200 atmospheres, that is to say, 200 times the normal pressure that it exists at the level of the sea, reducing the volume of a liter 200 times (law of Boyle). This means that if the bottle has a volume of 12 liters, we can introduce in its interior:

 200 Atm x 12 ls = 2400 liters of compressed air.

If we suppose that the pulmonary volume of a person is of 5 liters and that it breathes 5 times per minute makes a total of 25 liters to the minute, therefore with this bottle it will be able to breathe during:

 2400 liters/25 liters per minutes = 96 minutes.

As we see has sufficient air for but of average hour and in surface, but that happens when we submerged?

The increase of pressure is linear with the depth at the rate of an atmosphere each 10 meters. If now we lowered to a depth of 10 meters the pressure that supports our body is more of 1 atmosphere than in the surface, that is to say, of 2 Atm.

This implies that now to fill our lungs that continue having a volume of 5 liters we are going to need 5 2 liters but to Atm, the diving regulator is in charge to balance the pressure of exit of the air of the bottle to the pressure exerted by the depth to which we are of automatic way, in this case it provides the 5 liters to us to two atmospheres that serious the double (law of Boyle) of volume which if we were in the surface with which the bottle would last half 96/2 = 48 minutes, if now we lowered to 30 meters the serious pressure of 3 Atm and the time of diving would be used excessive respect one third part that is to say, 96/3 = 32 minutes.

The time and the depth of diving not only also come determined by the amount from air which we have, but from the physiological behavior of the different gases that form the air. When it increases to the pressure with the depth the impact pressure of the air is the sum of the partial pressures (pp) of each one of the components (law of Dalton), the margin considered surely as far as the partial oxygen pressures in a gaseous mixture is between 0,16 Atm and 1,6 Atm. There is risk of undergoing hypoxia and sudden loss of the smaller knowledge with one ppO2 of 0,16 Atm. and when ppO2 surpasses the 1,6 Atm, from the 70 meters it can produce neurotoxic symptoms that become espasmos and convulsions reason why he is better not to exceed this depth.

In the level of 40 meters they appear symptoms of narcosis, associate to the increase of the partial pressure of nitrogen, the famous borrachera of the depths that disappears when we raised. In the recreational diving we must be taken care of of not exceeding this level then in addition, where we will be able to see more life and to enjoy more the immersions they are in the first meters of depth.

Nitrogen also has another peculiarity and is that it is dissolved in the blood and different weaves from the body (law of Henry) something that at first is innocuous, but that when we ascend this dissolved gas is freed and is dragged by the sanguineous current until the lungs and of there to the outside of the body without problems.

This nitrogen liberation must be gradual and of controlled form so that it gives time all to weaves to be expelling it, without microbubbles take place, for that reason run-down times to different depths have settled down, that increase with the depth and the time of bottom.

Actually we waste time of the immersion doing stopped of decompression to eliminate nitrogen, mainly when successive immersions become that time between immersions does not give to eliminate residual nitrogen and every time is growing the run-down time and being reduced therefore the time of diving itself. How can be diminished this? We are going to see it:

If we varied the proportion of the air mixture diminishing the nitrogen concentration and increasing the one of oxygen it is evident that if we breathed less amount of nitrogen it will be necessary to eliminate less and that will result in a smaller run-down time of decompression, we finished inventing the NITROX.

Another advantage is that the narcotic effect of nitrogen is reduced and the fatigue sensation diminishes after the immersion, nevertheless we must have a series of precautions and is that now our maximum bottom to which I oxygenate was neurotoxic has diminished then its partial pressure has increased.

At the moment the use of NITROX with the appearance of membranes has become popular that allow to load of form needs the bottle diving with the concentration wished of I oxygenate.

LAW DE BOYLE

Relation between the pressure and the volume of a gas when the temperature is constant

Boyle in 1662 was discovered by Robert. Edme Mariotte also reached the same conclusion that Boyle, but did not publish his works until 1676. This is the reason for which in many books we found this law with the name of Law of Boyle and Mariotte.

The law of Boyle establishes that the pressure of a gas in a closed container is inversely proportional to the volume of the container, when the temperature is constant.

So that it happens this?

When increasing the volume, the particles (atoms or molecules) of the gas take more in arriving at the walls of the container and therefore they hit less times by unit of time against them. This means that the pressure will be smaller since this one represents the frequency of shocks of the gas against the walls.

When the distance diminishes the volume that they have to cross particles is smaller and therefore more shocks in each unit of time take place: it increases the pressure.

What Boyle discovered is that if the amount of gas and the temperature remain constant, the product of the pressure by the volume always has he himself value.

Since we have seen, the mathematical expression of this law is:

PV = k

(the product of the pressure by the volume is constant)

Let us suppose that we have a certain volume of gas V1 that is in the beginning to a P1 pressure of the experiment. If we varied the volume of gas until a new V2 value, then the pressure will change to P2, and it will be fulfilled:

P1 V1 = P2 V2

that it is another way to express the law of Boyle.

Gas mixture: Law of DALTON

In a gas mixture that does not react to each other, each molecule moves independently, of an analogous form to as if she was totally isolated.

In that mixture, each gas is distributed uniformly by all the space available, as if no other gas was present. The molecules exert the same pressure on the walls of the container that contains it that the one that would exert if there were not any other present gas.

In 1803, Dalton (1766-1844) enunciated the law of the partial pressures: “in a gas mixture, the impact pressure exerted by such is the sum of the pressures that each gas would exert if he were single in the same conditions”.

If several gases, To, B and C, are placed in a same container, end up forming a homogenous mixture. The pressure that each gas exerts individually in a mixture denominates partial pressure.

The law of Dalton of the partial pressures is expressed:

Ptotal = PA + PB + PC +…

As much the mixture of gases as each individual component fulfills the equation of ideal or perfect gases:

Ptotal × V = (nA + nB + nC+…) RT

Law of HENRY

In general, the amount of a gas that dissolves in a liquid depends on the external pressure and the temperature of the liquid. According to the law of Henry, the solubility of a gas in a liquid is proportional to the pressure of the gas on the liquid.  If C to the concentration is denominated to molar of the gas dissolved in the liquid and P their pressure, then: 

constant k = to a temperature

 The law of Henri explains, for example, the nitrogen narcosis, or poisoning that is pronounced in the divers who breathe air in bottles when the pressure by the depth dissolves great amounts of nitrogen in the blood. High concentrations of this gas produce a narcotizante effect. In addition, the law of Henri also explains because when returning to the surface the divers must raise stepped to allow that the nitrogen dissolved in the blood frees when diminishing the pressure. Of thus not doing it, the diver runs the risk of experiencing the symptoms of the decompression, resultants of the gas bubbles that are come off the blood when returning to the atmospheric pressure.