Cylinders must be prepared for designated nitrox
service because most of the time pure oxygen will be
used in preparing a nitrox fill. Although there are several
ways to blend nitrox, some of which do not require
using pure oxygen, all will use extra-clean air that
meets a standard of greatly reduced hydrocarbon content.
So, preparing a cylinder and designating it as “for
nitrox service” is valid, even for those blending methods
that remove nitrogen rather than adding oxygen. When
the cylinder is hydrostatically tested, it will have to be recleaned
before being returned to nitrox service.
HOW NITROX IS MADE
Most nitrox blending involves adding pure oxygen
to air. Other methods are available that remove nitrogen
from air. One “de-nitrogenation” technique has become
more widespread in the past several years. As the popularity
of nitrox has grown, many resort locations where
oxygen is not readily available or is very expensive are
using nitrogen removal techniques to make nitrox.
There are three nitrox preparation methods that
add oxygen to air (partial-pressure mixing, continuousflow
mixing, and mixing by weight) and two methods
that remove nitrogen (pressure swing absorption and
membrane separation).
PARTIAL-PRESSURE MIXING
Partial-pressure mixing systems are the easiest to
set up, requiring only a source of high-pressure oxygen
and a source of clean, high-pressure air. The air used
for nitrox blending must meet the higher standard of
purity with a lower hydrocarbon level than ordinary
scuba air. In partial-pressure mixing, the blending
technician first puts a measured amount (pressure) of
oxygen into the cylinder and then fills the cylinder to its
service pressure with air. For simplicity, blending a
nitrox fill normally begins with the cylinder emptied of
any previous mix, but it could start with adding oxygen
and then air to whatever remained in the cylinder from
the previous use.
Chapter 6 - Oxygen Precautions and Preparing Nitrox
Calculations for partial-pressure blending are simple.
Using Dalton’s Law of partial pressures, a measured
amount of pure oxygen (oxygen fraction equals 1.00)
plus a measured amount of air (oxygen fraction equals
0.21) produces a full cylinder of nitrox at whatever oxygen
fraction is desired. If the cylinder does not begin at
empty, it is just a matter of blending three gas mixtures
instead of two. Small adjustments may have to be made
because Dalton’s Law calculations are “ideal gas” calculations,
and the blending technician is mixing “real”
gases. In practice, the blending technician will consult a
table (rather than performing the calculation for each
fill) to determine what pressure of oxygen should be
added to the empty cylinder that will then be topped off
with air.
Because high-pressure oxygen is being introduced
into the nitrox cylinder, the cylinder and its valve must
be oxygen clean and oxygen compatible. The cylinder
will be filled slowly to minimize temperature increases
as the oxygen and then the air are compressed into it.
After filling the cylinder, the mix will be allowed to cool,
Oxygen Precautions and Preparing Nitrox
71
FIGURE 6-3: FILLING A NITROX CYLINDER