104 NAUI Nitrox Diver
Glossary
fsw: Depth (or pressure) measured in feet of seawater.
The pressure exerted by one fsw is approximately
equal to 1/33 of a standard atmosphere.
G
Gas Analysis: The measurement of separate component
gases in a gas mixture. In recreational nitrox diving,
analysis is for oxygen content only.
Gas Fraction: The quantity of a single gas in a mixture
of gases expressed as a decimal fraction of the whole.
Gas fractions are used in mathematical calculations.
Gas Laws: The physical statements that predict how
an ideal gas will behave with changes in pressure,
temperature, and volume.
Gas Percentage: The quantity of a single gas in a mixture
of gases expressed in relation to 100 representing the
whole mixture. Gas percentages must be converted
to gas fractions in mathematical calculations. For
example, a mix that is 32% oxygen has an oxygen
fraction of 0.32.
Gas Tension: The equivalent of partial pressure for a
dissolved gas.
Gauge Pressure: The pressure indicated by a gauge
that ignores atmospheric pressure. Depth gauges,
which show hydrostatic pressure in msw or fsw, are
examples of gauge pressure read-out.
H
Henry’s Law: The statement of the relationship of the
pressure of a gas on a liquid to the solubility of the gas
in the liquid. Henry’s law states: “The amount of gas
that will dissolve in a liquid at a given temperature is
directly proportional to the partial pressure of that gas
on the liquid.”
Hypoxia: A deficiency in the amount of oxygen reaching
the tissues of the body and the condition brought on
by that deficiency.
I
Ideal Gas Laws: The description of the relationship of
temperature, pressure, and volume of a theoretical
“ideal” gas, that is, one with molecules of negligible
size moving randomly and colliding with completely
elastic collisions. The behavior of a real gas more
closely approximates that of an ideal gas when it is at
relatively low pressure and high temperature.
Inert Gas Narcosis: A semi-stuporous condition
caused by the elevated partial pressure of inert gases
(commonly nitrogen) in the breathing gases at
depth. The condition is alleviated by ascending to a
shallower depth or by changing the components of
the breathing mixture, as in trimix.
Ingassing: The diffusion of additional dissolved gasses
into the tissues as ambient pressure increases.
M
Maximum Dive Time (MDT): Also called the
no-decompression time or no-decompression limit
(NDL). The length of time that may be spent at
a given depth without being required to make a
mandatory decompression stop during ascent to the
surface.
Maximum Operating Depth (MOD): The maxi-mum
depth to which a particular gas mixture can be used
without exceeding the oxygen partial pressure limit.
In normal diving the maximum PO2 is 1.4 ata.
MDT: See Maximum Dive Time
Membrane Separation: The process of separating gases
by forcing a gas mixture through a differentially
permeable membrane that allows some gases to pass
more readily than others.
MOD: See Maximum Operating Depth.
msw: Meters of seawater. The pressure exerted by one
msw equals 1/10 bar.
N
Narcosis: See Nitrogen narcosis and Inert gas narcosis.