Not flying after having dived is one of the golden rules every diver has to bear in mind. All of us have been taught in courses about this, and we have been also asked the classical question “what time does your flight leave?” when we are finishing our dives. With some simple rules, based in the changes in our body, we can take the plane with no problem.
Remembering Physics
Here we have to remember the known Henry’s law, it is, the dissolution of gases in liquids. Solutions are the mixtures of two different bodies. When gases and liquids contact, das molecules dissolve in the liquid through a diffusion process in a period of time. So, bit by bit, the gas dissolves until reach equilibrium point or saturation.
This happens in the moment when the gas dissolved in the liquid keeps a tension equal to the partial pressure in the gas mixture. This is the phenomenon described by the statement of the famous law: “At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.”
When conditions of one of the two change, the equilibrium changes also, and a saturation or unsaturation phase. If partial pressure of the gas increases, we are in the last stadium, and if is reduces we are in a moment of supersaturation.
Decompression
These theoretical approaches are extremely important in diving, as we all know, specially the supersaturation phase. This phenomena takes place when we finish our dive and go to the surface, since in this moment the pressure reduces and we have to stop to let the dissolved gas in our body eliminate, and not appear like dangerous bubbles, cause of the decompression accident.
At atmospheric pressure 1 l of nitrogen is dissolved in the corporal liquid tissue (blood, adipose tissue, aqueous tissues, etc.) If we double pressure, just in the 10 m., the dissolved gas will double, and so on, given that the contact gas-liquid happens in enough time. It is that excess nitrogen that has to leave our body, bit by bit.
Flying
What happens when we take the plane? It is supposed that our body has desaturated the residual nitrogen in terrestrial pressure at sea level. But when we take a plane we rise, and it will influence the pressure. This pressure will decrease in any case. Depending on the sort of flight and plane it will do it more or less. In pressurized aircraft, environmental pressure is usually placed, gradually in the equivalent of 2,000 m. This height is usually exceeded by unpressurized small planes, not in helicopters, which normally fly lower.
In this situation there is a different between the internal pressure of tissues (that still have nitrogen) and the environmental pressure, and the feared bubbles can appear again. It is the known effect of a fizzy drink. If we open the bottle very fast, bubbles go out fast and part of the liquid pours because of the great difference in pressure between the inside and the outside of the bottle. On the contrary, if we open it carefully the situation will remain stable, the gas going out will go out softly and the liquid will remain inside the bottle.
There are still more reasons to be cautious: the possibility of suffering an accidental depressurization in the aircraft cabin. In this case, the consequences for passengers will be a scare, but for a diver, still saturated with nitrogen, they can be terrible. A decompression accident in flight is very difficult to solve.
The decompression tables
Almost all the decompression models calculate the pressures of the theoretical tissues based on the rise of the dive to the atmospheric pressure at sea level. There can be problems if the diver flies or goes over the sea level before the body is desaturated. But it does not need to be flying. Different kinds of accidents are registered with divers who have gone after diving, for example, to the plateau, with altitudes between 600 and 800 meters above sea level.
The atmospheric pressure, reduced with the altitude, increases the pressure gradient between tissue and environmental pressures, and it is possible that it causes big bubbles in the body. The same problems happen if diving over sea level without taking into account the reduced atmospheric pressure in special dive tables or in a computer designed for diving in altitudes. Once the decompression sickness appears due to altitude, it generally does not alleviate when coming back to sea level.
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