NITROX mixing can be accomplished by a variety of techniques to produce a
final predetermined nitrogen-oxygen mixture. The techniques for mixing NITROX are listed as follows:
1. Continuous Flow Mixing. There are
two techniques for continuous flow mixing:
a. Mix-maker. A mix-maker uses a precalibrated mixing system that proportions
the amount of each gas in the mixture as it is delivered to a
common mixing chamber. A mix-maker performs a series of functions
that ensures accurate mixtures. The gases are regulated to the same
temperature and pressure before they are sent through precision metering
valves. The valves are precalibrated to provide the desired mixing
pressure. The final mixture can be provided directly to the divers or be
compressed using an oil-free compressor into storage banks.
b. Mix-maker. Oxygen induction uses a system where low pressure
oxygen is delivered to the intake header of an oil-free compressor,
where it is mixed with the air being drawn into the compressor. Oxygen
flow is adjusted and the compressor output is monitored for oxygen
content. When the desired NITROX mixture is attained the gas is
diverted to the storage banks for diver use while being continually
monitored for oxygen content (Figure 10-3).
2. Mixing by Partial Pressure. Partial pressure mixing techniques are similar to
those used in helium-oxygen mixed gas diving and are discussed in Chapter
16.
a. Partial Pressure Mixing with Air. Oil-free air can be used as a Nitrogen
source for the partial pressure mixing of NITROX using the following
procedures:
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Prior to charging air into a NITROX bottle, the NITROX mixing
technician shall smell, taste, and feel the oil-free air coming from
the compressor for signs of oil, mist, or particulates, or for any
unusual smell. If any signs of compressor malfunction are found,
the system must not be used until a satisfactory air sample has been
completed.
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Prior to charging with oxygen, to produce a NITROX mix, the
NITROX-charging technician shall charge the bottle to at least 100
psi with oil-free air. This will reduce the risk of adiabatic compression
temperature increase. Once 100 psi of oil-free air has been
added to the charging vessel, the required amount of oxygen should
then be added. The remaining necessary amount of oil-free air can
then be safely charged into the bottle. The charging rate for
NITROX mixing shall not exceed 200 psi per minute.
WARNING Mixing contaminated or non-oil free air with
100% oxygen can result in a
catastrophic fire and explosion.
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Compressed air for NITROX mixing shall meet the purity standards
for “Oil-Free Air,” (Table 10-2). All compressors producing
air for NITROX mixing shall have a filtration system designed to
produce oil-free air that has been approved by NAVSEA 00C3. In
addition, all compressors producing oil-free air for NITROX
charging shall have an air sample taken within 90 days prior to use.
Table 10-2. Oil-Free Air.
3. Mixing Using a Membrane System. Molecular sieves are columns of solid, highly
selective chemical absorbent which perform a similar function to membrane
systems, and are used in a similar fashion. Molecular sieves have the added
advantage of absorbing CO2 and moisture from the feed gas.
4. Purchasing Premixed NITROX. Purchasing premixed NITROX is an
acceptable way of obtaining a NITROX mixture. When purchasing premixed
NITROX it is requisite that the gases used in the mixture meet the minimum
purity standards listed in volume 3.
