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Chapter 7 Knowing What You Breathe 77
LEARNING GOALS
Comprehension Objectives
Learn about oxygen analyzers and why analysis
is necessary.
Understand how to analyze your nitrox mixture.
Learn how your cylinder should be labeled after it
is filled.
Understand why a fill station logbook is kept and
how to enter your fill information in the log.
As an enriched air nitrox diver, you will be responsible
for your cylinder's contents (Figure 7-1) and therefore
must know how to analyze the oxygen content of the
gas that you will be breathing. The two most important
things to remember when verifying the gas that you are
breathing are: 1) verify the calibration of the oxygen
analyzer and 2) verify the analysis of YOUR gas mixture.
The blending technician will fill your cylinder,
and his or her analysis of the blend is part of the filling
procedure. But, performing or verifying a final gas
analysis and recording the pertinent information in a fill
Figure 7-2 Oxygen analyzer.
log and on the cylinder is the responsibility of you, the
end-user. You will use an oxygen analyzer to perform this
procedure, so you should know how to use one as well as
have some acquaintance with how they work. OXYGEN ANALYZERS
There are many styles of oxygen analyzers available,
ranging from large units to small analyzers designed
for use in the field (Figure 7-2). Most of the analyzers
used for nitrox diving have a digital readout, but analog
readouts are also available. Ideally, an analyzer should
display oxygen content to an accuracy of one-tenth of
one percent. That is, the analyzer would display 31.7%
rather than just 32%.
The sensor commonly used in nitrox analysis is
electrochemical. In this type of sensor, the oxygen in the
gas diffuses through a membrane where it is ionized as
it comes in contact with an electrode. A small electric
current is generated by the oxygen ions, which is
proportional to the amount of oxygen in the sample. The
analyzer measures the resulting current and displays a
read-out in percent oxygen. Electrochemical analyzers
have the advantage of being rugged and portable. They
are also relatively less expensive than other types.
An essential operation in using any analyzer is
calibrating it. Analyzers may exhibit some instability or
drift as they age, and so they should be calibrated before
each use. In scientific use, calibration often includes
zeroing the analyzer to a known inert gas that has no
oxygen content. For nitrox use, an analyzer is normally
Figure 7-1 As a nitrox diver, you are responsible for
your cylinder's contents.