Cited Passage
3-26 NOAA Diving Manual
When a compartment fills to capacity, it is called saturat-
ed. Given enough time, the pressure of nitrogen in all the dif-
ferent compartments will eventually equal ambient pressure,
and thus, the entire body is saturated. On most dives, there is
not enough time for total saturation. Faster compartments
may be saturated, while slow compartments may be practical-
ly empty, while other compartments attain middling pressure. Differences in solubility and rates of gas diffusion
give different gases different half-times. Helium is much
less soluble in tissues than nitrogen, but it diffuses faster,
so helium equilibration occurs faster than for nitrogen. Still, the basic principles of absorption and elimination
apply for any inert gas breathed. On ascent, for example,
the diver’s tissues, especially the slow compartments,
may continue to absorb nitrogen. During most dives,
there isn’t time for slower compartments to equilibrate
with ambient pressure; these compartments have a lower
pressure than the surrounding water. During ascent,
ambient pressure can drive nitrogen into slow tissues,
even as higher pressure, fast compartments off-gas. Not
all nitrogen passes directly back into the blood stream for
direct off-gassing by exhalation. Nitrogen may pass from
the higher pressure in one part of the body to the lower
pressure in an adjacent one (Hamilton, pers. com. 2000). Fast tissues not only on-gas quickly, they also off-gas
quickly. Decompression or safety stops taken near the sur-
face on a recreational-type dive are favorable. They allow
some extra gas to be taken up by the slow tissues, but allow
more gas to be given off by the faster tissues, while holding
at a pressure slightly greater than the surface (Hamilton,
pers. com. 2000). After ascending to the surface (or to a shallower level),
equilibration at the new level may require 24 hours or so,
even though the dive was far shorter in duration. Half-time
gas elimination is the reason. It takes six half-times before a
compartment can fill or empty. No matter how much gas a
compartment starts with, it takes six half-times to empty. A
60-minute compartment will half fill (or empty) with nitro-
gen in 60 minutes. After another 60 minutes, or two hours
total, the compartment will be 3/4 or 75 percent full (or
depleted). It will take another 60 minutes for the remaining
1/4 to move, making the compartment 7/8 or 87.5 percent
full (or empty) in three hours (1/2 + 1/4 + 1/8 = 7/8). In
four hours, the compartment will be 93.8 percent exchanged
and in five hours it will be 97.0 percent. It takes six half-
times for any compartment to become about 99 percent full
or empty. For practical purposes 99 percent is completely
saturated or de-saturated. This means a 60-minute compart-
ment is full or empty in six hours, since six half-times x 60
minutes = 360 minutes or six hours. A fast compartment
like a five-minute compartment fills or empties in only 30
minutes (6 half-times x 5 minutes = 30 minutes). The slow
120-minute compartment fills and empties in 12 hours (6
half-times x 120 minutes = 720 minutes or 12 hours). Several complicated factors can slow the release of
nitrogen from the body. However, for practical applica-
tions like calculating decompression tables, off-gassing is
considered to proceed at the same half-time rate as on-
gassing.
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