In
scuba diving
Scuba diving is a mode of underwater diving whereby divers use breathing equipment that is completely independent of a surface air supply. The name "scuba", an acronym for "Self-Contained Underwater Breathing Apparatus", was coined by Chris ...
, the rule of thirds is a
rule of thumb
In English, the phrase ''rule of thumb'' refers to an approximate method for doing something, based on practical experience rather than theory. This usage of the phrase can be traced back to the 17th century and has been associated with various t ...
used by divers to plan dives so they have enough
breathing gas
A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common and only natural breathing gas, but other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed h ...
remaining in their
diving cylinder
A diving cylinder or diving gas cylinder is a gas cylinder used to store and transport high pressure gas used in diving operations. This may be breathing gas used with a scuba set, in which case the cylinder may also be referred to as a scu ...
at the end of the dive to be able to complete the dive safely.
This rule generally only applies to diving in overhead environments, such as caves and wrecks, where a direct ascent to the surface is impossible and the divers must return the way they came.
For divers following the rule, one third of the gas supply is planned for the outward journey, one third is for the return journey and one third is a safety reserve.
However, when diving with a buddy with a higher breathing rate or a different volume of gas, it may be necessary to set one third of the buddy's gas supply as the remaining 'third'. This means that the turn point to exit is earlier, or that the diver with the lower breathing rate carries a larger volume of gas than he alone requires.
Reserves are needed at the end of dives in case the diver has gone deeper or longer than planned and must remain underwater to do
decompression stop
The practice of decompression by divers comprises the planning and monitoring of the profile indicated by the algorithms or tables of the chosen decompression model, to allow asymptomatic and harmless release of excess inert gases dissolved in ...
s before being able to ascend safely to the surface. A diver without gas cannot do the stops and risks
decompression sickness
Decompression sickness (abbreviated DCS; also called divers' disease, the bends, aerobullosis, and caisson disease) is a medical condition caused by dissolved gases emerging from solution as bubbles inside the body tissues during decompressio ...
. In an
overhead environment
Underwater diving, as a human activity, is the practice of descending below the water's surface to interact with the environment. It is also often referred to as diving, an ambiguous term with several possible meanings, depending on contex ...
, where it is not possible to ascend directly to the surface, the reserve allows the diver to donate gas to an out-of-gas buddy, providing enough gas to let both divers exit the enclosure and ascend to the surface.
By the rule of thirds system the gas in stage cylinders is managed in the same way as the primary supply, whether the primary is carried as back gas or sidemounted. A third of the gas in the stage cylinder is used before the drop, leaving two thirds in the cylinder, the minimum amount for two divers to exit on one cylinder. The cylinder may be carried a few minutes beyond the point at which the first third was used, but is not breathed for this extra distance, to conserve the gas for the return, as this allows it to be reached a bit earlier if one diver loses all gas at the end of the next stage when gas supply is at critical pressure. If all goes to plan, the divers will surface with stages and primary cylinders each containing about one third of the original content.
With the rule of thirds, the duration of the dive is limited by the point at which the gas reaches 1/3 the starting quantity, by not exceeding the planned decompression obligation, and by returning along the same route in similar conditions. Where a more specific or varied dive profile is planned, the "rock bottom"
gas planning procedure is more versatile but more complex to calculate. Other rules of thumb for scuba gas planning exist and may be used where appropriate.
See also
*
*
*
*
*
*
*
References
External links
National Speleological Society Cave Diving Section
{{Underwater diving, divsaf
Underwater diving safety
Thirds, rule of
Dive planning