CHAPTER 10 — Nitrogen-Oxygen Diving Operations
Change A 10-11
Compressed air for NITROX mixing shall meet the purity stan-
dards 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.
3.
Mixing Using a Membrane System. Membrane systems selectively separate
gas molecules of different sizes such as nitrogen or oxygen from the air. By
removing the nitrogen from the air in a NITROX membrane system the
oxygen percent is increased. The resulting mixture is NITROX. Air is fed into
an in-line filter canister system that removes hydrocarbons and other
contaminants. It is then passed into the membrane canister containing
thousands of hollow membrane fibers. Oxygen permeates across the
membrane at a controlled rate. The amount of nitrogen removed is determined
by a needle valve. Once the desired nitrogen-oxygen ratio is achieved, the gas
is diverted through a NITROX approved compressor and sent to the storage
banks (see Figure 10-4 and Figure 10-5). Membrane systems can also
concentrate CO2 and argon.
4.
Mixing Using Molecular Sieves. 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.
5.
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 for oxygen (Table 4-3) and nitrogen (Table 4-5).
Table 10-2. Oil Free Air.
Constituent
Specification
Oxygen (percent by volume)
20-22%
Carbon dioxide (by volume)
500 ppm (max)
Carbon monoxide (by volume)
2 ppm (max)
Total hydrocarbons [as Methane (CH4) by volume]
25 ppm (max)
Odor
Not objectionable
Oil, mist, particulates
0.1 mg/m3 (max)
Separated Water
None
Total Water
0.02 mg/1 (max)
Halogenated Compounds (by volume):
Solvents
0.2 ppm (max)
