106 NAUI Nitrox Diver
Glossary
Partial Pressure Mixing: The mixing method by which
gases are serially combined using proportions of each
gas to determine the composition of the resulting
mixture. Each gas is added to the containing cylinder
so that its partial pressure gives it the desired fraction
of the resulting mix.
PO2: Partial pressure of oxygen.
Pressure: Force acting on a unit of area. See also Absolute
pressure and Gauge pressure.
Pressure, Atmospheric: The measure of the weight of
the surrounding “column” of air. One atmosphere
is equal to 1.01325 bar, 14.7 pounds per square inch
(psi), 33 fsw, or 760 millimeters of mercury (mmHg).
Pressure Gradient: The difference in pressure between
two separate locations. Used in diving to describe the
driving force on a gas that causes it to move in or out
of a body tissue.
Pressure Swing Absorption: A method of separating
gases in a mixture using the adsorption properties of
a porous material, usually a “molecular sieve” over
which the gas mixture is passed. In nitrox blending
by pressure swing absorption, the adsorbing material
removes nitrogen from the air flowing over it.
R
Reduced Gradient Bubble Model (RGBM): A set of
decompression algorithms and diving tables based
on a dual-phase model of tissue ingassing and
offgassing–that is, based not only on gases dissolved
in one’s tissues but also on the ever-present gaseous
micronuclei or “bubble seeds” in our bodies.
Repetitive Dive: Any dive following a previous dive
before complete offgassing of nitrogen. According to
the NAUI Dive Tables, any dive within 24 hours of a
previous dive is a repetitive dive.
Residual Nitrogen: The nitrogen remaining in the body
because of a previous dive or dives.
Residual Nitrogen Time (RNT): The amount of time a
diver must consider as already having been spent at
depth for a planned repetitive dive. The time is based
on the residual nitrogen remaining in the diver’s body
from the previous dive.
Rule of Halves: A modification to the safety stop
procedure to further reduce bubble formation. By the
Rule of Halves, the ascending diver stops for two to
three minutes (with two and one-half minutes being
optimum) at a depth that is one half the deepest depth
of the dive, followed by a one-minute stop at a depth
of 3-6 msw (10-20 fsw) before surfacing.
S
Safety Stop: A precautionary delay in ascent from a
no-stop dive. The safety stop is taken at approximately
5 msw (15 fsw) for three to five minutes at the end of
the dive.
Saturation: In diving, the presence of the most dissolved
gas possible in the body tissues of the diver for the
existent depth/pressure.
SIT: Surface Interval Time: the time spent between dives.
Solubility: The amount of one substance that will
dissolve in another substance under specified
conditions. See Henry’s Law.
Solute: A dissolved substance.
Solvent: The liquid substance that dissolves another
substance.
Square Profile Dive: A dive that involves descending to
one particular depth, staying there for the entire dive
time and then ascending directly to the surface, with
the exception of safety and decompression stops. All
dives are treated as square profile dives when using
dive tables.
Surface Interval: The time spent on the surface between
dives. A surface interval must be at least 10 minutes
to qualify the next descent as a repetitive dive. NAUI
recommends that the surface interval be a minimum
of one hour.