LEARNING GOALS
In this chapter you will learn:
• About oxygen analyzers and why analysis
is necessary.
• How to analyze your nitrox mixture.
• How your cylinder should be labeled after it is
filled.
• Why a fill station logbook is kept and how to
enter your fill information in the log.
As an enriched air nitrox diver, you must know how
to analyze the oxygen content of the gas that you will be
breathing. In a very few situations, such as a dive destination
in which a contract fill station routinely supplies
EAN32 to many dive operators, you may not be tasked
with analyzing your own gas, but in most cases–and
especially if you have requested a custom blend–analyzing
the gas that you will breathe is an essential and critical
part of preparing to dive.
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 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. 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 readout
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
calibrated using standard air. That is, with the analyzer
exposed to air, it is set to read 20.9% or 21.0%.
Although an analyzer should read in tenths of a
percent, it is generally accepted that a mix need be only
±1% of the target value. NAUI EAN32 and EAN36 Dive
Tables have a tolerance of ±1% in their use. If the mix
is off more than 1% and tables are to be used, the blending
technician should adjust the mix in the cylinder. A
nitrox dive computer can be set to the actual percentage
of the mix, so a greater error would be tolerable as long
NAUI Nitrox Diver
80 Knowing What You Breathe
FIGURE 7-1: OXYGEN ANALYZER