Our normal idea of diving is that a diver descends from sea level, 1 ATA, and returns when the dive has finished. There is a series of variations from this situation. A diver may have to dive in a mountain lake where the pressure on the surface is less than 1 ATA. Another variation occurs when a diver starts from an environment where the pressure is greater than 1 ATA. This happens when divers operate from a pressurized compartment or underwater habitat. These conditions introduce complexities that require understanding of the physics involved.
A diver operating in a high mountain lake is returning to a lower surface pressure than a diver at sea level. This decreases the pressure at which the diver is while releasing inert gas after a dive and so increases the tendency to form bubbles. Therefore, the diver may need to modify the decompression plan. Another minor correction will be required if it is a fresh water lake. Fresh water is less dense than salt water, so the diver is exposed to a slightly lower pressure change per unit depth.
In addition, this diver will have to exhale faster during ascent. A diver who ascends from 10 metres (2 ATA) to the surface (1 ATA) without exhaling would find that the volume of gas in the lungs has doubled. Most divers realize this and exhale at an adequate rate during ascent. However, they may not realize that a similar doubling in gas volume occurs during the last 5 metres of ascent to the surface, if the pressure at the surface was 0.5 ATA.
High-altitude diving may require that the depth or duration of dive and the rate of ascent be reduced to allow for the lower than normal surface pressure at the end of the dive. Tables are available for diving at higher altitudes, and many dive computers are programmed to compensate for this.
A diver living in a human-made environment where the pressure is high can operate to deeper than normal depths. This system is used in saturation diving, where the diver operates from a base at increased pressure and becomes equilibrated with it. The eventual return to the surface can take many days. The use of such environments has proved to be invaluable where deep or long dives are required (see Chapter 67).
Another pressure-related problem can occur when a diver dives and then flies or ascends into mountains. Some dives and ascents will require the diver to ensure that adequate time is spent at the surface before ascending to high altitude, to avoid DCS. This problem is encountered by a diver tourist who wants to fly home after diving or one who needs to pass over hills or mountains when returning from a dive. It is also encountered when it is necessary to transport a diver with DCS. There may be an increase in manifestations of DCS when the pressure is decreased, even by a relatively small amount.