Mailing List Archive Mailing List Archive
In his message, Michael, parker@ca*.wa*.ed*, raised some
interesting questions about standards pertaining to the use of NITROX
and where they should be moving over the next few years. I'd like to
follow up on three issues:
1. Should ALL NITROX tanks be oxygen clean?
2. Should ALL NITROX tanks be color-coded?
3. Is NITROX really suitable for the non-technical
recreational diver, anyway?
1. Should ALL NITROX tanks be oxygen clean?
Firstly, let's ask what oxygen cleaning is, and why it is done at all.
When ANY two materials are moved relative to one another, friction
occurs, producing heat. If the circumstances are right, this heat can
be made to ignite one of the materials, producing fire. This is the
principle the cave dwellers used, millenia ago, when they banged two
sticks together! Combustion depends on three factors: the amount of
heat produced, the presence of a flammable substance, and the
presence of an oxidation agent to cause the ignition. In a SCUBA tank,
these three factors are all sufficiently present to be of concern.
The flammable substance could be any hydrocarbon-based product or
contaminant that is present, such as various lubricants in the tank
valve, or O-rings, or even small particles of dirt that have become
lodged in the airway or tank.
The heat comes from the friction produced by trying to squeeze a great
deal of high-pressure air through the tiny hole in the tank valve.
Tanks are supposed to be filled SLOWLY, but at some stations, this rule
is ignored, and I've once picked up a freshly filled tank to find the
valve was downright HOT.
The third component necessary for combustion is present in the air
itself - it is the oxygen. Now in AIR, it is generally accepted that
there is insufficient oxygen to make ignition and combustion (also known
as an explosion) a serious enough risk. However, it is also assumed
that tanks will be filled SLOWLY and that there will not be a great deal
of heat present there, either. If an air fill is pumped too fast, the
amount of heat increases, and so does the risk of explosion.
When gasses other than air are pumped, the risk changes. If the amount
of oxygen in the gas is increased, the risk of explosion increases.
When pumping NITROX, this is the case. NITROX is (usually) air with a
higher oxygen content, with 32%, 36%, 40% or 50% oxygen being common
mixes. The hotter the mix, the greater the risk of explosion. When
pumping pure oxygen the risk is greatest.
To minimize this risk, tanks and valves can be oxygen cleaned. This
process involves a thorough cleansing of the tank and valve components
and replacement of all lubricants and O-rings with oxygen compatible
versions. If 100% oxygen is to be pumped, this procedure is essential.
The question is, to what extent must the procedure be performed when
pumping the less hot NITROX mixes?
Current thinking, and teaching, tells us that pumping up to about 40%
oxygen is OK in an un-cleaned SCUBA tank. It also teaches us to pump
real slow, to minimize the heat.
Obviously, having an oxygen clean tank will do no harm. It will
certainly increase safety. But, are there any reasons why one might not
want to do this? Cost is the first negative that springs to mind. It
is fairly expensive to have your tank oxygen cleaned. Not only does the
valve have to be cleansed, but any oxidation (rust) on the inside tank
walls must be removed by tumbling. These procedures are time and labor
intensive. Also, oxygen compatible O-rings and lubricants are more
expensive than regular ones, too. When pumping air, or similar
low-oxygen mixes, the combustion risk is not sufficiently high to
justify the expense.
Before deciding whether or not to bother oxygen cleaning a tank and
valve, it's also worth thinking about how you obtain your NITROX. If
you receive your NITROX at a filling station where ready mixed gas is
injected into your tank from a single pipe, from a continuous mixer or a
partial pressure mixer, all your tank will ever see is the mixed gas.
If it is receiving a 32% oxygen fill, it will probably be OK, even if it
isn't cleaned.
But, if you are mixing the NITROX in the tank itself, you are initially
pumping 100% oxygen into that tank, and then adding air. In such a
case, I'd say that it's pretty essential to have the tank oxygen
cleaned.
As for where the industry should go? I'm not an expert on combustion,
so I can't say at what point the risk of ignition becomes high enough.
I am safety conscious, though, so I would approve of an industry that
insisted on oxygen clean tanks for all NITROX fills (I'd also approve of
an industry that insisted on clean tanks for air fills, too). But the
sheer cost of requiring this for NITROX, will act as a disincentive for
most people to use NITROX, and will thus impede the spread of NITROX
into the recreational sector.
I will leave it to those more expert in this matter to provide the
technical justifications showing the risk of combustion at various
oxygen mixtures. I currently feel comfortable pumping up to 40% oxygen
into my non-cleaned tanks. If technical justification is provided that
this is very dangerous, I will change my mind, and get my tanks cleaned.
2. Should ALL NITROX tanks be color-coded?
OK, let's ask why tanks should be color-coded at all? The color coding
is simply one way of labeling the content of a tank. It originates from
the coding system used for pure gas (a green tank contains oxygen, a
yellow one contains air, black is nitrogen, and brown is helium). You
can tell at a glance what gas a tank has in it. There are also codings
for mixed gasses: yellow and green for NITROX, and brown and green for
HELIOX. Having a color coded tank would avoid mistakes such as the one
Bill, mayne@pi*.cs*.fs*.ed*, experienced with the enthusiastic employee
topping off a mixed gas tank with air.
But there is another, more important, reason SCUBA tanks must be
labeled. Not only should the type of gas or mixture be recorded, but
also the mix - the percentage of oxygen in the tank. This is highly
important because oxygen actually becomes TOXIC as its partial pressure
increases. Pure oxygen is used to resuscitate a patient on the surface.
But, only 6 meters down, it is a poison. NITROX 32% becomes a poison at
40 meters down. NITROX 36% can only be used to 30 meters.
Given this, each mixed gas SCUBA tank is individually analyzed (using
something like the MiniOx analyzer mentioned in earlier messages), and
the exact percentage of oxygen in it is written on a tag and affixed to
the tank before use. The oxygen content will determine the maximum
depth to which that tank may be safely used. Tank color coding is, in
itself, not a sufficient indicator of the content of the tank, the color
coding must be supplemented with a mix analysis tag.
Again, I find myself saying: sure, it's fine by me if the industry
insists that I color code all my tanks. It's safer that way. I'd
probably paint all my tanks with the TRIMIX coding (is it
green/brown/yellow?) and then use the analysis tag to indicate what
I currently have in the tank.
But, if we're trying to push the adoption of NITROX into the
recreational arena, where it is more important that the color of one's
tank matches that of one's BC and fins, and must be something like neon,
or pink, or orange, or bright blue, we're going to be creating yet
another obstacle against NITROX.
3. Is NITROX really suitable for the non-technical
recreational diver, anyway?
I have a big concern about suggesting that NITROX should be used by all
recreational divers for dives down to 40 m.
When diving on gas, you are acutely aware of the maximum depth limit of
the gas mix you have. You do not go deeper than that. Let's say, for
the sake of argument, that the recreational industry switched to using
NITROX 32% as its standard mix. This is gernerally considered suitable
for dives to 40 m, based on a PO2 of 1.6. Now, 1.6 is already pretty
high on the oxygen toxicity scale, anyway. Personally I would keep
below 1.5, so I'd need NITROX 30% mix to get me to 40 m.
How often does a recreational diver at the maximum depth of their dive
plan see something of interest a bit below, and just nip down to take a
look? I know I do. On air, that's OK. But, on NITROX, this places me
in significant danger from increased oxygen toxicity. Now, having been
trained in NITROX, and being a technically-aware person, anyway, I would
think twice about `just nipping down to take a look'. My concern is,
even if NITROX were taught as a standard part of a PADI (etc) course,
how many divers would really understand the importance of the maximum
depth limits? Would be see a serious increase in the fatality rate,
resulting from people `just nipping down to take a look'?
To be on the safe side, the recreational industry would probably have to
use NITROX 28%, which at 40 meters hs a PO2 of 1.4. This would allow
people to `just nip down' as far as 47 meters, and still be within the
1.6 limit. Much safer. But, given that air is effectively NITROX 21%
and that pumping anything other than air is much more expensive, is the
recreational industry ever going to switch?
-jr
Navigate by Author:
[Previous]
[Next]
[Author Search Index]
Navigate by Subject:
[Previous]
[Next]
[Subject Search Index]
[Send Reply] [Send Message with New Topic]
[Search Selection] [Mailing List Home] [Home]