A diver’s life used to be simple. After filling your tank, you
inhaled and if it was clean air, everything was OK. If it had
an oily taste, you felt sick during the dive. If it was contaminated
with carbon monoxide, you died. The deeper you went,
the less time you had, and you had to consider how much
nitrogen you might have absorbed. That was what decompression
tables were for. That much hasn’t changed.
Then along came Nitrox. By increasing the amount of oxygen
in “air,” the effects of nitrogen dissolving in the body at
depth are reduced, which means, compared to air (Nitrox 21),
longer dive times for any depth. If the no-stop times are kept
the same as for air, an increased level of safety is introduced.
Increasing Nitrox’s Oxygen Level Comes With a Price
If you breathe oxygen at a partial-pressure greater than,
say, 1.6 bar (that’s equivalent to pure oxygen at more than 1.6
atmospheres), there is an increased risk of oxygen poisoning.
But arguments rage as to what the safe level is. After the statistical
evidence for 1.6 was put forward at Planet Tec in the
mid ’90s, the training agencies, ever mindful of litigation, set
a maximum of 1.4 bar. With ordinary air, that would give it a
maximum depth of 175 feet. However, Bret Gilliam once held
the record for the deepest air dive to 475 feet. It’s been broken since. I watched Rob Palmer do a sequence of air dives to 390
feet, and he got away with it for five of six dives -- he died on
that sixth dive. So we can only really say that oxygen poisoning
is accumulative and affects each individual differently.
Because air is often supplied alongside
Nitrox, even air divers need to analyze
their tanks to be sure they’re filled with
air and not mistakenly filled with Nitrox. |
Since Nitrox contains increased oxygen, the amount being
up to whoever mixes it, we need to analyze it ourselves to be
satisfied we have the right mix for our dive plan. Because air
is often supplied alongside Nitrox, even air divers need to analyze
the gas in their tanks to be sure that its air and that they
haven’t erroneously been filled with Nitrox.
Analyzer Calibration
A Nitrox analyser has an oxygen sensor that generates
a voltage that varies according to the percentage of oxygen present, and gives a readout on a voltmeter calibrated in
percentages. An analyzer reading is affected by air pressure
(weather) and the aging of the sensor cell, so before each
dive, it needs to be calibrated at 20.9 percent O2 with clean
air passing around it. If you calibrate using pure oxygen
instead of air, the sensor readings will be three times more
accurate.
Recently, I was on a liveaboard with a diver who was a
gas analyst from Siemens. He merely checked his tank each
time to see that he didn’t have air. He told me that the analyzer
on board was so inaccurate, it was only good for telling
whether you got air or something else, but nothing more. He
just checked to see if it went over 21 percent but didn’t bother
with a final reading. Furthermore, he said, the little oxygen
analyzer that I had held so much faith in was almost totally
inaccurate. I was shocked. So what was wrong with the simple
equipment that was available to us?
The problem stems from gas flowing over the sensor. He
pointed out that the gas had to be stationary or controlled
to a known rate. By simply increasing the flow, the apparent
oxygen content increased. Cracking open a tank valve “just
a little’” and holding the cell over it is evidently not accurate
enough for him. Temperature also makes a difference, and gas
depressurizing from a tank can be very cold.
Some analyzers use the direct-feed hose to the BC, using
the regulator first-stage as a flow control. It takes time to get a
reading and this seems unpopular with divers impatient to get
in the water. Divers will always look for the most convenient
method. I have witnessed Nitrox being analyzed by holding
the sensor in the mouthpiece of a regulator while the purge
button was pressed.
But manufacturers have come up with designs to get a
stationary sample of gas at the same ambient temperature and
pressure with which the analyzer was calibrated. Some restrict
the flow of gas over the cell while others attempt to capture a
stationary sample. It seems that the bit of plastic tube that interfaces
between the tank valve and the sensor is very important.
If your Nitrox is made by the older partial pressure blending
system, you need to know your analyzer is exact, because
there you may inadvertently get very high percentages of oxygen,
if by mistake.
Oxygen Sensors
Depending on how much work an oxygen sensor does, its
performance declines. Even when you are not using one, it
will still be working if the sensor is surrounded by air. So sensors
tend to have a shelf life of two years (some manufacturers
now claim up to five years), provided their gas-barrier packaging
has not been opened. They should provide a millivolt
reading between 10 mv for air and 60 mv for pure oxygen.
Used sensors become unpredictable when they reach the end
of their useful life. While it does no harm to keep an analyzer
in an airtight container, some experts tell me this adds little to
the lifespan of a sensor because there are so many other factors
to consider.
Heat shortens a sensor’s life. Keep your analyzer out of the
sun and away from any hot place. Other factors include the
cleanliness of the lead anode and the exact composition of the
electrolyte. Vacuum packing and freezing have inherent risks
of damage. Some say that you should store it in an inert gas
but this raises questions of wake-up time for the cell after it
is brought out of storage. If you get a reading more than one
percent different from your expectation, or are using a sensor
that has been mishandled, do not use it to analyze the oxygen
content of an unknown gas.
So should you trust your dive operator to analyze your
Nitrox accurately, or should you carry your own analyzer?
While you can trust most operators, today more divers have
decided they want to know the precise amount of oxygen in
their Nitrox, and tote their own - - and a good one at that.
I looked at several analyzers to see how convenient they
were to operate. I also checked each against the same tank of
Nitrox to see if there were differences in measurement. All
the analyzers I tested were calibrated using the same tank of
air, and all gave a reading of my sample gas within 0.5 of a percentage point of each other. Knowing the inaccuracy of
decompression theory, that’s probably accurate enough.
VN202 This provides a choice of methods for sampling
gas. The sensor cell is at the end of a flexible cable, and can
be plugged into a conventional Quick-Ox sampling tube.
Otherwise, it can be connected to a DIN tank connection (not
a pressure-reducing valve), complete with a not similar chamber
to the Quick-Ox and a long exit tube. Sample by turning
the gas on for five seconds and off for five more. It proved
quick and easy to calibrate. (Approximately $245 with Quick-
Ox and $255 with DIN tank connection; purchase on the
manufacturer’s Web site www.vandagraph.co.uk)
Tek-Ox An analyzer designed to make use as convenient
as possible. Simply hold the attached Quick-Ox sampling tube
over the cracked tank valve, and let the gas migrate up inside
the unit to its sensor cell. Then turn off the tank so that the
trapped sample is static. I found it probably simplest to pull off the sampling tube and wave it around in fresh air to get
an initial setting, being careful to avoid backdrafts from boat
exhausts, which could give errors while under way. The reaction
time was quick and the display is big. It was my favorite
among those intended for individual divers. (Approx. $260; purchase
at the manufacturer’s Web site www.vandagraph.co.uk)
Dynatron OxySpy This Swiss-made analyzer is similar
in concept to the simpler-looking but more expensive Nuvair.
Knowing that divers can be lazy, OxySpy is designed to be as
convenient to use as possible. I thought the calibration procedure
was a little confusing and also quite time-consuming, but
it was very convenient to use once this was done. (Approx.
$290; www.dynatron.biz)
Analox O2EII A popular compact analyzer often supplied
on dive boats. Simply wake it up by pressing a button.
Calibrate it in air by adjusting a knob to get the right percentage
according to a compensation chart provided on the LCD.Then offer up the sampling hemisphere of the analyzer to the
slightly opened valve of the Nitrox tank to be checked until
the reading stabilizes. It may not be the most accurate way to
do it according to the experts, but it is good for confirming an
anticipated Nitrox mix. ($325; www.analox.net)
Nuvair O2 Quickstick This is merely a plastic cylinder
containing the cell, a voltmeter, and the LCD readout with a
simple hemispherical head containing a tiny orifice that you
place firmly against the tank valve after cracking it open a
little. It is calibrated for air (O2 20.9 percent) and supplied
with a fine long-handled screwdriver. Alas, switching it off
means taking the battery out, a cumbersome operation. ($250;
www.nuvair.com)
Alpha-1 This tough little unit in an anodized metal box
comes with a long hose complete with constriction in-line
offering a connection that conveniently clips over your tank
valve. It proved simplest to use on a known tank of air as a
supply for calibration. It was a workmanlike answer to the
problem of knowing what gas mix I had, and its metal construction
will give it certain longevity. ($500; for sale at Andi
International’s Web site www.andihq.com)
OMS. Favored by many technical training agencies, this
one comes with a DIN-connection regulator first-stage to
operate as a pressure-reducing valve at the end of a hose.
It also has an anatomical shape to help with holding it, an on/off switch and a rotating
knob for calibration.
I suspect most divers would just
whip off the hose connection
where it presses in at the sensor
cell end, and wave the unit
around in fresh air to calibrate
it. Attaching it to a tank, I was
able to get an apparently accurate
reading that was quick, easy
and free from user error. ($482;
www.omsdive.com)
So which one to buy? All of the
analyzers included here were adequate for the job. However,
what separates them is not the core hardware but the method
of gas-sampling. Only the Vandagraph analyzers with the
Quick-Ox sampling tube attachment meet the requirement
to analyze a stationary sample of gas. And of all these,
Vandagraph’s VN202 and Tek-Ox analyzers seemed the most
convenient to use.
John Bantin is the technical editor of DIVER magazine in the United
Kingdom. For 20 years, he has used and received virtually every piece of
equipment available in the U.K. and the U.S., and makes around 300 dives
per year for that purpose. He is also a professional underwater photographer.