Tank oxidation contributes to death

I was taking a break from exams and reading the DAN Incident Report (For those of you who are members, read it. For those who aren't, buy it or join DAN and read it) when I came upon an incident:

01-63 Instructor without designated buddy
found on wreck; tanks shown to contain
improper gas mix
Cause of Death: Asphyxia
This 42-year-old male was an experienced dive
instructor who worked in a dive shop and
filled his own tanks. He made a wreck dive
with eight other divers but without a designated
buddy to 112 fsw (33.6 msw).
Approximately 35 minutes into the dive,
another diver found the decedent on the wreck
and unconscious. The stricken diver was
brought to the surface and, after some delay,
they managed to get him into the boat.
Resuscitation efforts were unsuccessful. An
evaluation of the dive equipment revealed that
one of the two tanks the decedent had been
using contained less than 1 percent oxygen.
That particular tank contained several ounces
of rust-colored water, and the tank itself had
extensive rust and pitting throughout its inner
surface. The logical conclusion is that this tank
had not been used recently. The oxygen that
was originally present in the tank was likely
consumed in the oxidation (rust) process. The
tank did not have current visual inspection
and hydrostatic testing. Contributing
factor/significant diagnosis included obesity.

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They are claiming that oxidation in the tanks reduced the amount of oxygen available in the tank to less than one percent! That sounds entirely illogical. If the tanks are aluminum, the chance of this is virtually impossible, so I'll assume that they are steel. Even if they are steel and not galvanized (which virturally eliminates rust, especially inside the tank where there is nothing to break the impermeable zinc oxide coating), I still find it almost impossible to believe that this kind of reaction has occurred in a period of say 10-15yrs (even with salt water in the tank) and did not absolutely destroy the tank (ie. BOOM) especially without noticably reducing the pressure. Additionally, it must have been intended as only air (or else any diver would check the O2 in an old tank). Even so, what would someone with the experience that is claimed not check the oxygen content of their tank before using? I have much more chemistry knowledge than the average layman, but there has to be something wrong here. Does anyone have any ideas or more information on this specific case? It also goes to show you how preventable diving accidents are.

brandon

FIrst of all, a steel tank that is hot dip galvanized is onyl galvanized on the OUTSIDE of the tank, not the INSIDE!!!

SO, from reading the description of RUST inside the cylinder, it had to be steel. Secondly, the guy let his tank go out of hydro, and out of vis. This guy shoudl know better as an Instructor.
A steel tank can get significant rusting from a drop of water inside the cyl;inder over the course of a year. it sounds like this cylinder had a substantial amount of water inside it. It could have done serious damage, and the rusting process would have dropped the O2 content.

No surprises here...

from the description of the accident it sounds like it wans' thte 10-15hrs that made the oxygen content drop but that the tank itself was unused for a very long time, probably left filled up to operational pressure... and over the time of weeks, months, years ??? the rusting took place and took away the oxygen from the air.

that would also explain why he woudlnt' check the mix percentage before the dive - it was an air fill so it "should" have had 21 % oxygen.

well.. that's my take on it

My BS meter is pegged on this one...
Math is always interesting to play with when we hear some incredible story that has a "hint of truth" in it but is just a little too far over the line...
If the tank in question were an old steel 72, for example, filled with air, it would have about one and a third pounds of oxygen in the tank before this great oxidation took place. (71.2*.08*.21*32/28) = 1.37
Now, since a rust molecule is two atoms of iron combined with three atoms of oxygen, it has an atomic weight of about 160, of which 112 is iron, or 70% iron by weight. That means that for every pound of oxygen in rust, there are two and a third pounds of iron... so, in our hypothetical rusty tank that's consumed over 20 of the 21% oxygen through internal rusting, that oxygen had to eat up over three pounds of steel, and there would be about four and a half pounds of rust inside the tank! Highly unlikely that you could chew up three pounds of steel without a tank failure.
Of course using up 20% of the gas to eat up the tank would also drop the internal pressure by 20% too, from 2475 to 1980 psi. Our rocket scientist in this story would have to not top up this tank in order to keep the oxygen level below 1% also........
Is the story getting a bit too thin??? It is for me.
Rick

So steel tanks aren't galvanized on the inside? I am going to insist on having mine done inside and out . I guess that's what happens when you assume.

I took the time to try to look up the rate law (an equation that predicts the concentration of the components of the reaction at a certain time) for the formation of iron oxide. It turns out that there is no simple solution to this problem since oxidation of iron is an electrochemical process (besides there are too many variables for any reliable numbers). So I cannot back up the reaction time I suggested with any concrete numbers. Still, under such a low concentration of oxygen the reaction should be extremely slow.

brandon

yeah, something doesnt sound right about this story...

I think that someone may have embellished the story a little...

voidware once bubbled...
So steel tanks aren't galvanized on the inside? I am going to insist on having mine done inside and out . I guess that's what happens when you assume.

I took the time to try to look up the rate law (an equation that predicts the concentration of the components of the reaction at a certain time) for the formation of iron oxide. It turns out that there is no simple solution to this problem since oxidation of iron is an electrochemical process (besides there are too many variables for any reliable numbers). So I cannot back up the reaction time I suggested with any concrete numbers. Still, under such a low concentration of oxygen the reaction should be extremely slow.

brandon

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Where are you going to take your cylinder to have it galvanized on the inside?

Secondly, why would you want to? I mean, PST uses a zinc coating. I dont know what that would do when mixed with o2 at high pressures. But, I dont think i would want to breath it...


Padi Instructor huh??? Where did you get your instructor's certification???

Interiors of tanks are not galvanised because Zinc is HIGHLY toxic to humans.

Rick, I think there is more than just Fe2O3 going on with rust. Rust self propogates by a means of attaching more oxygen than it needs and donating that down the line. The carbon is also leached from the steel and creates a weak acid which accelerates the whole thing. Add pressure to the mix and who knows what is going to happen. I have heard of steel tanks with less than %10 O2 over many years... but I have to agree with you, that %1 sounds way light.

1% would only equate to less than %5 PO2 at that depth. It is surprising that it took that long for him to pass out... not to mention the taste that air must have had.

My BS meter is waivering near it's top peg on this one as well.

No point in repeating what Rick had to say, he made a very convincing case.

I'll simply add that the "rust" color they are refering to, would in fact mean Red and not Black. That would lend even more credibility to Ricks argument seeing that Red rust is Fe2O3 Iron Oxide / Diron Trioxide with an formula weight of 159.688 (almost 160), whereas Black rust would be Fe3O4 Iron Oxide / Triiron Tetraoxide with a formula weight of 231.533. If it was Black "rust", then enriched air may have been present, however highly unlikely. As the remaining elements in order to create Fe3O4 aka Magnetite could not have possibly existed in any environment habitable to humans prior to this dive. Which is where this tank was presumably stored. (yeh yeh, I know it's not scientifically sound to presume)

So Red it is, and Rick is right. This is probably just a typo if the report is real. Or it's an April fools joke, a little late.


Class dismissed. :doctor:

This 42-year-old male was an experienced dive instructor

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I don't see any mention of Padi.

Rachel