6
72 NAUI Nitrox Diver
Figure 6-3 Filling a nitrox cylinder. Figure 6-4 An example of a continuous flow
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 (Figure 6-3).
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 (Figure 6-4). The oxygen-air mixture is
then compressed. As the high-pressure nitrox exits the
compressor, the oxygen fraction is analyzed online, 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 bandmask.
blending unit.
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 partialpressure
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