I was watching a program on PBS last night about “Hitler’s lost sub”. To make a long story short these SCUBA divers were diving down to 230 feet to retrieve wreckage from this U-Boat. Anyway, they said that they couldn’t use conventional SCUBA air mixtures because at that depth Oxygen is poisonous. They were using a mixture called Tri-Mix that had Nitrogen, Helium and very little oxygen. My question is how does the oxygen become posionous? I assume that it has something to do with the pressure, but why poisonous exactly I don’t know.
Oops. pardon my incorrect spelling of poisonous.
Above a certain level oxygen can cause convulsions which tend to be somewhat fatal for scuba divers.
The level that the convulsions occur at is not a specific level but varies from person to person and day to day. The old Navy limit was a pressure of 1.6 atmospheres, but that was dropped to 1.4 atmospheres and I believe has been decreased even further now.
1.4 atmospheres is about 21 PSI which will be encountered by a diver at about 200 feet down.
(The 21 PSI is the partial pressure of the oxygen at 200 feet, the diver will be breathing air at about 103 PSI)
I’m not even going to pretend that I comprehend this page, but it might get you started. Some of the symptoms look to me to be similar to hyperventilation.
ok, O2 isn’t really ‘poisonous’ in the normal sense of the word, but its high reactivity causes it to have serious adverse affects at higher pressures. essentially, the higher pressure allows oxygen to participate in more chemical reactions, and the cumulative effect of all these reactions is generally detrimental to overall well-being.
-b
Actually, I believe that oxygen is poisonous. We don’t think of it as such, because the % of oxygen in the air we breathe is low.
And wait a minute Diver- when I used to dive back in the day, an atmosphere was 33 feet. Is that not still so?
My brother dives on Nitrox or Tri-Mix, and teaches as well. I have been down to 135 feet on regular air, and would not feel at all comfortable going any deeper without a different mix.
EJsGirl said "And wait a minute Diver- when I used to dive back in the day, an atmosphere was 33 feet. Is that not still so? "
33 feet underwater = 2 atmospheres since 0 feet = 1 atmosphere.
I wouldn’t dive below about 130 feet without a special mix either, but getting into that didn’t seem to be directly related to the question, so I didn’t address that issue.
If the truth be known, as a rule, I have little desire in diving below 90 feet or so. There isn’t much to see down that deep that I have a lot of interest in - unless there is a wreck or something like that involved.
Most of my dives range from 30 to 70 feet or so max depth.
ejsgirl,
i guess it depends on one’s definition of ‘poisonous’. anyhoo, yes, each 33 feet is one additional atm of pressure, however when diver stated that 200 feet = 1.4 atm, he/she was referring to the partial pressure of only the oxygen. iirc, partial pressure = % of gas in question times total pressure. (200 ft * 1atm/33ft) + 1 atm for the actual atmosphere = 7 atm total pressure at 200 ft. 7 atm * .2 (assuming 20% O2) = 1.4 atm partial pressure of the oxygen. hope that helps.
-b
Helps? I may have permanently hurt myself just thinking about it… 
Concentrated oxygen is poisonous. Apart from what has already been mentioned above theres this: we have 21% oxygen in our atmosphere. If it were above 25% then most organic substances would become highly flammable. If you breathed in pure oxygen it could oxidise your lungs.
Oxygen can also cause “Retinopathy of Prematurity” (or retrolental fibroplasia) which is described as follows on the National Library of Medicine website: “A bilateral retinopathy occurring in premature infants treated with excessively high concentrations of oxygen, characterized by vascular dilatation, proliferation, and tortuosity, edema, and retinal detachment, with ultimate conversion of the retina into a fibrous mass that can be seen as a dense retrolental membrane. Usually growth of the eye is arrested and may result in microophthalmia, and blindness may occur.” (Dorland, 27th ed)
I am assuming that by % of gas you are talking volume wise. Wouldn’t you have to multiply that by the mass ratio (gas X’s mass/ total mass) of the gas whose partial pressure you are calculating?
Yoda or Yogurt? A dilemma for the ages
<slight hijack>
If oxygen is poisonous in high concentrations, then what’s the deal with oxygen bars? Wouldn’t they cause serious health problems?
</slight hijack>
Oxygen is only poisonous in high concentrations over long time periods (such as those periods during which a premie might be stuck in a pure-O[sub]2[/sub] incubator). Breathing pure oxygen for a few minutes at normal (atmospheric) pressure will not harm you in the least. I have done it as recently as when I had my wisdom teeth out (they tried to give me nitrous; I freaked; they put me on pure oxy until I was normal again).
Breathing pure oxygen at lower pressures (high altitudes) becomes very important.
AFAIK, though, even “pure” breathing oxugen is tainted with a small bit of carbon dioxide, mainly because if your blood pCO[sub]2[/sub] falls below a certain level, your breathing reflex no longer works.
LL
quesar said "Wouldn’t you have to multiply that by the mass ratio (gas X’s mass/ total mass) of the gas whose partial pressure you are calculating? "
Not according to Dalton’s Law…Total Pressure of a gas mixture = the sum of the Partial Pressure of each gas.
P total = P1 + P2 + P3 + …Pn
Example:
For 100 PSI of air with an oxygen content of 20%, the Oxygen partial pressure is .2(100 PSI)= 20 PSI.
Maybe you were thinking of the “Mole Fraction Relation” which I don’t remember much about.
Isn’t O1 extremely poisonous? I can’t do subscripts, but I mean molecules comprising a single atom of oxygen rather than the normal two atoms.
[QUOTE]
*Originally posted by LazarusLong42 *
**
A clarification if you will allow a small hijack:
Actually, breathing pure oxygen over a number of hours will not harm you, either. I am going from memory here, so forgive me if my numbers are not exact. The shuttle astronauts, in preparation for an EVA, breath pure oxygen to remove some of the nitrogen from their body. Since their space-suits are only pressurized to about 4 psi, they would get “the bends” if they did not prepare this way. I believe their space suits are supplied with only oxygen as well. (Not sure on that, though.)
I seriously doubt this would be NASA’s proceedure if it were harmful to the astronauts, especially considering Apollo 1’s mishap with a pure oxygen environment.
Regards.