the nitrox blend will be analyzed, and the proportions
adjusted as necessary.
Partial-pressure blending is popular in dive centers
both because it is easy and relatively inexpensive to set
up and because it lends itself to preparing small quantities
of a variety of different blends. Each cylinder’s fill is
unique. With partial-pressure blending, the diver can
request any nitrox blend for a single cylinder or several
different mixes in several cylinders, and the dive center
can easily supply the need.
CONTINUOUS-FLOW MIXING
This is the method that was first used by NOAA.
The continuous-flow method injects a measured flow
of pure oxygen into the air before it reaches the intake
of the compressor. The oxygen-air mixture is then compressed.
As the high-pressure nitrox exits the compressor,
the oxygen fraction is analyzed on-line, and the oxygen
flow at the low-pressure intake is fine-tuned, either
manually or by an automatic feedback control, until the
desired blend is attained. The high-pressure nitrox is
routed to a bank of storage cylinders from which it will
be drawn to fill individual scuba cylinders. In commercial
diving, the mix may be delivered directly to the diver
through a hose attached to the helmet or band-mask.
The oxygen is well homogenized with the air
before being drawn into the compressor and some oxygen
injection systems advertise that the blend is so thoroughly
mixed before entering the compressor that the
compressor does not have to be hydrocarbon-free.
Whether this is true or not, the nitrox will be used to fill
a nitrox-dedicated cylinder, so the compressor should
not introduce any hydrocarbons that might contaminate
the cylinder, which at a later time may be filled by
partial-pressure blending. Continuous-flow mixing is
used when larger quantities of a given nitrox blend are
needed. Although the method could be used to fill individual
scuba cylinders, it would be inefficient and
wasteful of oxygen.
PRE-MIX
Ordering one’s gas from a commercial gas supplier
is the simplest and most straightforward way to
obtain quality nitrox. The gas supplier will usually
mix oxygen and nitrogen rather than oxygen and air.
They will also blend the gases by weight rather than by
partial pressure in order to prevent any errors that
ideal gas calculations might introduce. A commercial
supplier will deliver very accurately mixed nitrox with
the oxygen and nitrogen fractions exactly as ordered.
It is also the most expensive way to obtain nitrox, and
the mix accuracy available is not of real necessity in
nitrox diving.
PRESSURE SWING ABSORPTION
This method removes nitrogen from air rather than
adding oxygen to it. Pressure swing absorption (PSA)
uses a material called a molecular sieve that selectively
adsorbs a thin layer of nitrogen onto its surface from the
air that passes over or through it. (Technically, the
NAUI Nitrox Diver
72 Oxygen Precautions and Preparing Nitrox
FIGURE 6-4: AN EXAMPLE OF A CONTINUOUS FLOW BLENDING UNIT