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78 NAUI Nitrox Diver
Figure 7-3 Gas sampling flow-rate restrictor. Figure 7-4 Electrochemical oxygen sensor.
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 as the
planned depth of the dive does not exceed the maximum
operating depth of the mix.
ANALYZING YOUR GAS
When you pick up your cylinder of nitrox, the facility
will ask you to analyze your mix and record your analysis
in the fill station log. The facility that filled your cylinder
will have an oxygen analyzer available for this purpose.
You will also record the mix information on the cylinder
itself or verify what has already been recorded on the
cylinder by the blending technician.
Accurate analysis depends not only on the reliability
of the oxygen analyzer but also on the flow rate of the gas
through the oxygen sensor. If the flow rate is too high,
the analyzer will detect an excess amount of oxygen, and
the displayed oxygen percentage will be too high. If the
flow rate is too low, the displayed oxygen percentage may
also be too low. The acceptable flow rate is about one
liter per minute and should be between one-half and two
liters per minute. Many analyzer manufacturers supply
a specially designed flow restrictor or flow-rate regulator
that attaches directly to a low-pressure inflator hose
fitting on a regulator first stage (Figure 7-3). The flow
restrictor is either self-setting or will have a meter that
allows its valve opening to be set to an appropriate flow
rate. The downstream side of the flow restrictor attaches
to the oxygen sensor (Figure 7-4) of the analyzer.
Accurate oxygen analysis also depends on accurate
calibration of the analyzer. Before analyzing the gas in
your oxygen-enriched air cylinder, you must calibrate it
using a known source. This is usually a scuba cylinder
that is filled with air. Attach the flow restrictor/regulator
to the air supply and the oxygen analyzer to the downstream
side of the flow restrictor. Turn on the analyzer
and the air supply. If the flow restrictor is not preset,
adjust the flow rate to about one to two liters per minute.
Allow the air to flow to the analyzer for a minute until
you are sure that the analyzer has stabilized. Then, set the
analyzer’s readout to 20.9% or 21.0%.
Once the analyzer is calibrated, you can analyze the
mix in your enriched air nitrox cylinder (Figure 7-5).
Without disturbing the analyzer’s setting, shut down
the air-supply cylinder, allow the analyzer system to