Oxygen

[cargorat767]

Hi guys and gals.

Here is a little something that puzzles me....

According to our table of usefull oxygen, the minutes of available oxygen increases with cabin altitude, which at all doesn´t make sense to me..

Does anyone have an answer to that?

Cheers..

[mad_jock]

Its because the volume required at altitude is the same as at lower altitude/sea level.

But because the pressure is less the bottle gas will expand to more volume than it would at sea level. So you get more time higher up.

Conversely when diving were the pressure increases by 1 bar every 10meters you go down the quanity of air you breath increases the deeper you go.

12ltrs 220bar at 15 meters will last me an hour to an hour 15 depending how much swimming I am doing.

At 40 meters it will last 20-30 mins.

[cargorat767]

Thanks.

Though I find it a bit peculiar that the same amount of air is now more worth with less ambient pressure...

[turbroprop]

Hi cargorat767

MAD_JOCK is spot on, however if you want to breath oxygen for extended periods at height ie military the the partial pressure of the oxygen comes into effect and some form of pressure breathing system is required.

So as M_J rightly says there is more volume, but the pressure is low. In Jocks diving case at 100ft he is breathing air in at 4 times the pressure that at the surface so his tank will only last approx a quarter of the time than near the surface. The body can only use a certain amount of oxygen so at depth there is a surplus whereas at height there is shortage.

The other quirk is that too much oxygen is poisonous and if Mad_Jock was to take a tank of pure oxygen down to 30metres it had the potential to kill him.

[mad_jock]

Its partial pressures again

O2 becomes toxic at about 1.6 bar which equates to 16 meters under water.

Once you get old and can't be bothered running the risk you don't push it passed 1.4 partial pressure. Which is 14 meters.

There is another exposure toxity which is a length of time exposed. But thats not a problem breathing it at above 10 000ft altitude because the PP is lowered.

Your cabin won;t go above 10 000ft which is normally about 0.7bar pressure.

So every time you breath in the pressure in your cyclinder will drop by 30% less than it would if you were breathing it on the ground. The volume of gas you breath in and out will be the same. Its just that there is only 70% of the number of molecules in that volume compared to at sea level. Once the number of molecules becomes low enough there isn't enough of them in each breath to keep you alive (this is technically aload of :mad: because its to do with pressure gradients and efficiency of the lungs and blood)

[The Terminator]

Small correction (or maybe not depending on the circumstances), Mad_Jock if you are breathing 100% oxygen it will be toxic at 6 meters, as you were at 1bar to begin with.

[mad_jock]

Your right enough its been a few years since I have dived on mixed and done multiple stage deco plans.

the mil go down to 8 meters.

All this gas stuff comes of a distubution curve.

The old tables all got developed using goats, extremely fit navy divers and also stuff that nobody likes to talk about from WWII.

Most of it doesn't take into account fat chuffers under pressure so they are progressively bring the time limits and also pressures down for the new population profile.