Diving and altitude [Archive] - BSAC Scuba Diving Forums

chris b

07-03-2005, 19:21

richard2338

07-03-2005, 20:45

In theory, yes, I am fully aware and have read quite extensively on the subject however the following stats (rounded up and converted to Meters of sea water for clarity) make me wonder why we spend time on this.

Sea level = 1ata or 0M
Everest = 9000m or equivalent to ascending another 3M
Commercial airliner = equivalent to ascending another-0.3m
1000m hill= equivalent to ascending another 1m

Unfortunately, you may have made a few slight mistakes with your 'stats'. I understand that most commercial airliners (Boeing and Airbus) operate with a cabin pressure of between about 0.69 to 0.72 bar. So this represents a reduction in pressure from sea level of approx 0.3bar or about 3msw (metres sea water), not 0.3msw.

Mt Everest is approx 8800m high and the atmospheric pressure at this altitude is approx 0.33bar, hence this represents a reduction in pressure from sea level of 0.67bar or nearly 7msw.

Its important to think in terms of percentage pressure change...

Going from 10m underwater to the surface the pressure halves (2 bar to 1 bar = 1/2)
Hence, depth control in shallow water when decompressing is important.
But ALSO: Going from sea level into an airliner at altitude represents a pressure change of 1bar to 0.7bar = approx 1/3 less pressure. Quite significant also.

You wouldn't want to do your 6m (1.6bar) mandatory decompression stops in a third less pressure would you? This would be about 1 bar - in other words, the surface! Unless you think the 6m stops in the yellow zone of the BSAC '88 Tables are unnecessary as well? :-)

The percentage pressure change in going from sea level to what is effectively a partial vacuum at altitude is a little more significant than you might at first think.

Regards
Richard

David Walker

07-03-2005, 21:46

Why do we worry about flying after diving or driving over mountains?

I seem to remember someone once told me that it might be important if the plane had a problem and depressurised, which would rapidly bring on a bend.
Personally, if my plane depressurised at altitude I probably wouldn't be so worried about a bend as being sucked out of the side of the fuselage to fall 35,000ft!

To give a more sensible answer, I suppose its just how the tables were created. The pressure differential is obviously such that ascending quickly would cause a problem. Added to that is the fact that they have the traditional in-built conservatism so assume instantaneous ascents up to that height which I imagine adds to the limitations. In the end believe what you will. Most computers have no idea where they've been when they're not in the water, so completely ignore the fact that you may have just skiied down from the top of Everest in your dive kit and gone straight into a lake at the bottom. Does that mean that they don't see it as an important factor? Or is an inevitable fudge factor included somewhere so that they behave like everyone has just come down from the top of a mountain?

I see the BSAC tables as something you teach people within the bounds of the courses, after which they have very limited (if any) use. For that purpose I go with what the tables say. For the people in the club who don't dive on a computer they'll probably be training in Stoney to a max of 20m where its very difficult to get anywhere near deco when they're doing their 2 dives, air-pigging it all the way!

David

chris b

08-03-2005, 13:09

:=Sea level = 1ata or 0M
:=Everest = 9000m or equivalent to ascending another 3M
:=Commercial airliner = equivalent to ascending another-0.3m
:=1000m hill= equivalent to ascending another 1m

:=Unfortunately, you may have made a few slight mistakes with your 'stats'. I understand that most commercial airliners (Boeing and Airbus) operate with a cabin pressure of between about 0.69 to 0.72 bar. So this represents a reduction in pressure from sea level of approx 0.3bar or about 3msw (metres sea water), not 0.3msw.

I think I better run you through my basic maths so you can spot any errors?.because I still can?t see it.
1, Sea level is 1ata/bar the equivalent pressure change in fresh water is 10m.
2, Sea level to the edge of the atmosphere is 100,000ft or 30,480m. This is equivalent to that 10m of water in pressure terms of pressure change.
3, Divide the 30,480m by 10 = 3048m altitude for every 1m equivalent change in water pressure.

If the Ave pressure in a passenger airliner is 0.7ata/bar this would seem to be a pressure change of 0.3ata from sea level which is equivalent of a 3m depth change in water so yes, I stand corrected, I got my decimal in the wrong place and going in an airliner is the equivalent of changing depth by 3m.

What I actually meant was that ascending a 1000m hill is equivalent to 0.3m change in altitude (1000/3480m=0.328ata) And how long would it take to ascend that 0.3m equivalent depth? I am guessing it would be equivalent to a very slow ascent.

If it takes the average airliner, at least 3mins to climb to altitude (a bit longer actually, as they approach cruse altitude) this is 3mins to do the equivalent of a 3m ascent (on top of the time it took to get to the airport?etc) So I am really struggling to see where the danger is here.

As far as the other stats go?.

Mt Everest is approx 8800m high and the atmospheric pressure at this altitude is approx 0.33bar, hence this represents a reduction in pressure from sea level of 0.67bar or nearly 7msw.

That is the same as the change in the airliner cabin, but now it is 7m?..?

Everest = 8800m so divide by 3048m altitude (= to 1m of water) = equivalent 2.88m depth change. I can?t see where you got 7m from. Yes, the pressure has changed BY 0.33bar but not TO 0.33bar, it has change TO 0.67bar surely?

Going from 10m underwater to the surface the pressure halves (2 bar to 1 bar = 1/2)
Hence, depth control in shallow water when decompressing is important.

No disagreement there.

But ALSO: Going from sea level into an airliner at altitude represents a pressure change of 1bar to 0.7bar = approx 1/3 less pressure. Quite significant also

Well, no I can?t really see that, it is equivalent to a 3m depth change. After how long out of the water worst-case? 1H?

You wouldn't want to do your 6m (1.6bar) mandatory decompression stops in a third less pressure would you?

No but to be honest that is not what we were talking about. (Sorry if that seems rude, it isn?t my intention)

This would be about 1 bar - in other words, the surface! Unless you think the 6m stops in the yellow zone of the BSAC '88 Tables are unnecessary as well? :-)

Well, no because that wasn?t what I was talking about either. I don?t use the 88 tables because frankly, for a half descent dive (depth and length) I think they are dangerous??which seems widely accepted. I would personally do my safety stop at 12m, 9, and 6m?but that is also off topic, sorry.

The percentage pressure change in going from sea level to what is effectively a partial vacuum at altitude is a little more significant than you might at first think.

Well sorry, I still can?t see it; I think an equivalent of a 3m change at a controlled rate after what can logistically be a minimum of an hour at ?my last stop depth? is not really very significant

Thanks for your input Rich, I do appreciate it. I am just trying to really get my head around it to see what I have missed?but as I think you might have guessed, I still don?t see a significantly robust rational to this segment of what we teach. This has been around for so long it may perhaps be a worthy topic for debate.

It is a bit like the deepest dive first thing which all agencies have likewise adopted yet science is now telling us there is no empirical data to back at up as a recommendation.

Interesting stuff.

Chris B

Mike Halligan

08-03-2005, 16:05

I think I better run you through my basic maths so you can spot any errors?.because I still can?t see it.
1, Sea level is 1ata/bar the equivalent pressure change in fresh water is 10m.
2, Sea level to the edge of the atmosphere is 100,000ft or 30,480m. This is equivalent to that 10m of water in pressure terms of pressure change.
Yes, OK so far.
3, Divide the 30,480m by 10 = 3048m altitude for every 1m equivalent change in water pressure.
No. The relationship between air pressure and height AMSL is not linear. We are dealing with gravitational attraction between gas molecules and the Earth.

If the Ave pressure in a passenger airliner is 0.7ata/bar this would seem to be a pressure change of 0.3ata from sea level which is equivalent of a 3m depth change in water so yes, I stand corrected, I got my decimal in the wrong place and going in an airliner is the equivalent of changing depth by 3m.
What I actually meant was that ascending a 1000m hill is equivalent to 0.3m change in altitude (1000/3480m=0.328ata) And how long would it take to ascend that 0.3m equivalent depth? I am guessing it would be equivalent to a very slow ascent.
Wrong thinking, I'm afraid. Your 1000m hill is not equivalent to 1/30th bar. Also, we plan for instant conversion because we cannot know how powerful a vehicle takes you up that mountain road or how directly it approaches the pass.

If it takes the average airliner, at least 3mins to climb to altitude (a bit longer actually, as they approach cruse altitude) this is 3mins to do the equivalent of a 3m ascent (on top of the time it took to get to the airport?etc) So I am really struggling to see where the danger is here.
The average airliner doesn't matter. What matters is the actual initial rate of climb, generally by 350 to 650 m/min to the level at which pressurisation cuts in (1,500m in 2-4 mins). Subsequent climb to 10,000 m will be slower but once there will impose very significant gas gradients should the pressure hull be damaged.

As far as the other stats go?.
:=
:=Mt Everest is approx 8800m high and the atmospheric pressure at this altitude is approx 0.33bar, hence this represents a reduction in pressure from sea level of 0.67bar or nearly 7msw.
:=

That is the same as the change in the airliner cabin, but now it is 7m?..?

Everest = 8800m so divide by 3048m altitude (= to 1m of water) = equivalent 2.88m depth change. I can?t see where you got 7m from. Yes, the pressure has changed BY 0.33bar but not TO 0.33bar, it has change TO 0.67bar surely?
No, low pressure, thin air at almost 9000 m looks right to me.

:=Going from 10m underwater to the surface the pressure halves (2 bar to 1 bar = 1/2)
:=Hence, depth control in shallow water when decompressing is important.
No disagreement there.
:=But ALSO: Going from sea level into an airliner at altitude represents a pressure change of 1bar to 0.7bar = approx 1/3 less pressure. Quite significant also
Well, no I can?t really see that, it is equivalent to a 3m depth change. After how long out of the water worst-case? 1H?

:=You wouldn't want to do your 6m (1.6bar) mandatory decompression stops in a third less pressure would you?
No but to be honest that is not what we were talking about. (Sorry if that seems rude, it isn?t my intention)

:=This would be about 1 bar - in other words, the surface! Unless you think the 6m stops in the yellow zone of the BSAC '88 Tables are unnecessary as well? :-)
Well, no because that wasn?t what I was talking about either. I don?t use the 88 tables because frankly, for a half descent dive (depth and length) I think they are dangerous??which seems widely accepted. I would personally do my safety stop at 12m, 9, and 6m?but that is also off topic, sorry.

:=The percentage pressure change in going from sea level to what is effectively a partial vacuum at altitude is a little more significant than you might at first think. The body doesn't deal in percentages, it feels pain when you abuse it and doesn't feel pain when you don't. Empirical data has been converted to tabulated suggestions of what seems not to do harm.
Well sorry, I still can?t see it; I think an equivalent of a 3m change at a controlled rate after what can logistically be a minimum of an hour at ?my last stop depth? is not really very significant
Why then do divers get bent crossing the Cairngorms on their way back from Scapa? If it isn't significant, then that shouldn't happen - but it does.

Thanks for your input Rich, I do appreciate it. I am just trying to really get my head around it to see what I have missed?but as I think you might have guessed, I still don?t see a significantly robust rational to this segment of what we teach. This has been around for so long it may perhaps be a worthy topic for debate.

It may also be a reason why so many divers and caisson workers now survive.

It is a bit like the deepest dive first thing which all agencies have likewise adopted yet science is now telling us there is no empirical data to back at up as a recommendation.

Hmmm, Science, that's all that hypothesis and theory stuff isn't it? Doesn't usually seem to hold up as well as practical experience.

We didn't know why tailplanes were falling off or why windows fell out, until quite a few did and we then looked into the wreckage. No-one knew the science of the deep stall until too late, and that's the problem with science. Give me engineering any day ;-)

HTH

Mike

chris b

08-03-2005, 17:33

Hi Mike, thanks for all this, it is a whole other perspective that I had not considered.

:=1, Sea level is 1ata/bar the equivalent pressure change in fresh water is 10m.
:=2, Sea level to the edge of the atmosphere is 100,000ft or 30,480m. This is equivalent to that 10m of water in pressure terms of pressure change.

Yes, OK so far.

:=3, Divide the 30,480m by 10 = 3048m altitude for every 1m equivalent change in water pressure.

No. The relationship between air pressure and height AMSL is not linear. We are dealing with gravitational attraction between gas molecules and the Earth.

Could you give me an idea of where I could find out what the curve actually is? (preferably in lay person language!) and I would have thought (but could be wrong) at the bottom end of that curve (say the first 9000m were we are talking about) it woud be reasonably linear(for practical terms)?

Wrong thinking, I'm afraid. Your 1000m hill is not equivalent to 1/30th bar.

Could you expand on that Mike? I'd be interested to know if it isn't what it is equivalent to.

Also, we plan for instant conversion because we cannot know how powerful a vehicle takes you up that mountain road or how directly it approaches the pass.

I of course was guestimating how long it takes to climb ?a reasonable hill;? say through Snowdonia after a dive in 'one of the welsh slate mines' :-) I guess instant conversion is used for the simple reason you state, it can?t be said for certain that any rate of climb is 'the minimum' however neither is ?instant conversion? possible I suppose...

:=If it takes the average airliner, at least 3mins to climb to altitude (a bit longer actually, as they approach cruse altitude) this is 3mins to do the equivalent of a 3m ascent (on top of the time it took to get to the airport?etc) So I am really struggling to see where the danger is here.

The average airliner doesn't matter. What matters is the actual initial rate of climb, generally by 350 to 650 m/min to the level at which pressurisation cuts in (1,500m in 2-4 mins). Subsequent climb to 10,000 m will be slower but once there will impose very significant gas gradients should the pressure hull be damaged.

Well here I am a bit sceptical. First off, the rate of equivalent pressure change still seems consistent with a 'reasonable' ascent, secondly the practical reality of getting to an airport is going to take some time at ?the previous stop depth? and thirdly, I would imagine (I certainly don't know) that pressure hull damage that had a significant enough effect on pressure difference to effect a bend would likely necessitate either oxygen masks dropping or a return to airport? Either way, getting a skin rash would not be highest on my list of worries!

:=Everest = 8800m so divide by 3048m altitude (= to 1m of water) = equivalent 2.88m depth change. I can?t see where you got 7m from. Yes, the pressure has changed BY 0.33bar but not TO 0.33bar, it has change TO 0.67bar surely?

No, low pressure, thin air at almost 9000 m looks right to me.

Could you tell me what you based this on Mike as for the sake of my education (!) it would be good to see how you arrived at the statement. The manual tells me ?this is how it is? but doesn?t tell me why?.which is the reason I started this thread.

:=:=Going from 10m underwater to the surface the pressure halves (2 bar to 1 bar = 1/2)
:=:=Hence, depth control in shallow water when decompressing is important.
:=No disagreement there.
:=:=But ALSO: Going from sea level into an airliner at altitude represents a pressure change of 1bar to 0.7bar = approx 1/3 less pressure. Quite significant also
:=Well, no I can?t really see that, it is equivalent to a 3m depth change. After how long out of the water worst-case? 1H?
:=
:=:=You wouldn't want to do your 6m (1.6bar) mandatory decompression stops in a third less pressure would you?
:=No but to be honest that is not what we were talking about. (Sorry if that seems rude, it isn?t my intention)
:=
:=:=This would be about 1 bar - in other words, the surface! Unless you think the 6m stops in the yellow zone of the BSAC '88 Tables are unnecessary as well? :-)
:=Well, no because that wasn?t what I was talking about either. I don?t use the 88 tables because frankly, for a half descent dive (depth and length) I think they are dangerous??which seems widely accepted. I would personally do my safety stop at 12m, 9, and 6m?but that is also off topic, sorry.
:=
:=

:=:The percentage pressure change in going from sea level to what is effectively a partial vacuum at altitude is a little more significant than you might at first think. The body doesn't deal in percentages; it feels pain when you abuse it and doesn't feel pain when you don't. Empirical data has been converted to tabulated suggestions of what seems not to do harm.

The reason I am pursuing this is that empirical data doesn?t not seem to be readily quoted and I would suggest (because I don?t have any empirical data) that people getting bent due to driving over a mountain or flying after diving is incredibly rare, additionally, I would also suggest that perhaps in these instances, there could well be other more pertinent reasons for the bend in the first place?..

:=Well sorry, I still can?t see it; I think an equivalent of a 3m change at a controlled rate after what can logistically be a minimum of an hour at ?my last stop depth? is not really very significant

Why then do divers get bent crossing the Cairngorms on their way back from Scapa? If it isn't significant, then that shouldn't happen - but it does.

That it happens I don?t doubt, (as I have no evidence either way) but one wonders if we attributing symptoms/incidence to the wrong cause

:=
:=Thanks for your input Rich, I do appreciate it. I am just trying to really get my head around it to see what I have missed?but as I think you might have guessed, I still don?t see a significantly robust rational to this segment of what we teach. This has been around for so long it may perhaps be a worthy topic for debate.
:=
It may also be a reason why so many divers and caisson workers now survive.

Well as I said at the start, it would be good to see the evidence to support that

:=It is a bit like the deepest dive first thing which all agencies have likewise adopted yet science is now telling us there is no empirical data to back at up as a recommendation.
:=
Hmmm, Science, that's all that hypothesis and theory stuff isn't it? Doesn't usually seem to hold up as well as practical experience.

You?re talking about the 88 tables again aren?t you? ;-) Seriously though, the deepest dive first advice being 'de-bunked' was based on actual observation of real life dives (study carried out by DAN I believe)

We didn't know why tail planes were falling off or why windows fell out, until quite a few did and we then looked into the wreckage. No-one knew the science of the deep stall until too late, and that's the problem with science. Give me engineering any day ;-)

I think I?d agree...or woodwork

Cheers

Chris B

HTH

Mike

Mike Halligan

08-03-2005, 19:25

Chris,
:=:=1, Sea level is 1ata/bar the equivalent pressure change in fresh water is 10m.
:=:=2, Sea level to the edge of the atmosphere is 100,000ft or 30,480m. This is equivalent to that 10m of water in pressure terms of pressure change.
:=:=3, Divide the 30,480m by 10 = 3048m altitude for every 1m equivalent change in water pressure.
:= No. The relationship between air pressure and height AMSL is not linear. We are dealing with gravitational attraction between gas molecules and the Earth.
Could you give me an idea of where I could find out what the curve actually is? (preferably in lay person language!) and I would have thought (but could be wrong) at the bottom end of that curve (say the first 9000m were we are talking about) it woud be reasonably linear(for practical terms)?

Search me, Guv. Someone is bound to know whether gravitational attraction is a square or cube rule, I've a fancy it is cubic. Either way, it is far from linear.

:=Wrong thinking, I'm afraid. Your 1000m hill is not equivalent to 1/30th bar.
Could you expand on that Mike? I'd be interested to know if it isn't what it is equivalent to.
No I can't. You used a linear relationship and I'm sure it is not linear, but then I'm more than happy to accept that.

Also, we plan for instant conversion because we cannot know how powerful a vehicle takes you up that mountain road or how directly it approaches the pass.
I of course was guestimating how long it takes to climb ?a reasonable hill;? say through Snowdonia after a dive in 'one of the welsh slate mines' :-) I guess instant conversion is used for the simple reason you state, it can?t be said for certain that any rate of climb is 'the minimum' however neither is ?instant conversion? possible I suppose...
But your 'reasonable hill' is in fact hundreds of metres, all the more dangerous if your dive was at sea-level, and you merely pass through.

:=:=If it takes the average airliner, at least 3mins to climb to altitude (a bit longer actually, as they approach cruse altitude) this is 3mins to do the equivalent of a 3m ascent (on top of the time it took to get to the airport?etc) So I am really struggling to see where the danger is here.
:=The average airliner doesn't matter. What matters is the actual initial rate of climb, generally by 350 to 650 m/min to the level at which pressurisation cuts in (1,500m in 2-4 mins). Subsequent climb to 10,000 m will be slower but once there will impose very significant gas gradients should the pressure hull be damaged.
Well here I am a bit sceptical. First off, the rate of equivalent pressure change still seems consistent with a 'reasonable' ascent, secondly the practical reality of getting to an airport is going to take some time at ?the previous stop depth? and thirdly, I would imagine (I certainly don't know) that pressure hull damage that had a significant enough effect on pressure difference to effect a bend would likely necessitate either oxygen masks dropping or a return to airport? Either way, getting a skin rash would not be highest on my list of worries!
Quite. Decompression of the airliner is catastrophic, but then you would have knowingly presented yourself to fly with a pre-disposition to greater than normal harm should that occur. I think the airline might prefer to know that before accepting you for carriage.

The mid range of initial ascent rates I quoted (500m/min) is around 18mph, vertically. i.e. 1/3rd loss of ambient pressure in 3 minutes and a heck of a lot faster than any legal road user crossing mountains. Once you reach pressurisation altitude, further ascent is irrelevant.

:=:=Everest = 8800m so divide by 3048m altitude (= to 1m of water) = equivalent 2.88m depth change. I can?t see where you got 7m from. Yes, the pressure has changed BY 0.33bar but not TO 0.33bar, it has change TO 0.67bar surely?
:=No, low pressure, thin air at almost 9000 m looks right to me.
Could you tell me what you based this on Mike as for the sake of my education (!) it would be good to see how you arrived at the statement. The manual tells me ?this is how it is? but doesn?t tell me why?.which is the reason I started this thread.
I think Messrs Hilary and Tensing would explain better than I, but any Alpinist will have the data. Suffice to say the air is thin and the pressure is lower, proved by the facts that altitude sickness is well documented and water boils at less than 100degC - so you can't get a decent cup of tea in the Southern Alps!

:=:=:The percentage pressure change in going from sea level to what is effectively a partial vacuum at altitude is a little more significant than you might at first think.
The body doesn't deal in percentages; it feels pain when you abuse it and doesn't feel pain when you don't. Empirical data has been converted to tabulated suggestions of what seems not to do harm.
The reason I am pursuing this is that empirical data doesn?t not seem to be readily quoted and I would suggest (because I don?t have any empirical data) that people getting bent due to driving over a mountain or flying after diving is incredibly rare, additionally, I would also suggest that perhaps in these instances, there could well be other more pertinent reasons for the bend in the first place?..
One might suggest that the rarity of the occurence is a commentary on divers' regard for those risks. Accepted wisdom has served us well for centuries, ignore it as you wish - but please don't ask the NHS to fund the consequences.

Our problem is that the BSAC '88 and all other tables before and after relate what appears not to have harmed the great majority of divers. That is the nature of empirical derivation and the theories worked out for calculation, recalculation and argument are just that, theories which purport to explain observations.

When we get the builder's plans for human physiology, we will know the true answers. Unfortunately, we will then be dead, and unable to apply them.

HTH

Mike

richard2338

08-03-2005, 19:31

Could you give me an idea of where I could find out what the curve actually is? (preferably in lay person language!) and I would have thought (but could be wrong) at the bottom end of that curve (say the first 9000m were we are talking about) it woud be reasonably linear(for practical terms)?

Mike is quite right, the reduction of air pressure as you increase in altitude is not linear. Here are some nominal values for air pressure at different altitudes:

0m = 1 bar
1000m = 0.87 bar
2000m = 0.76 bar
3000m = 0.67 bar (10000ft - approx equivalent to commercial airliner cabin pressure)
4000m = 0.58 bar
5000m = 0.51 bar
8800m = 0.30 bar (Mt Everest)
10000m= 0.26 bar
20000m= 0.07 bar

Regards
Richard

chris b

09-03-2005, 20:45

Mike is quite right, the reduction of air pressure as you increase in altitude is not linear. Here are some nominal values for air pressure at different altitudes:

0m = 1 bar
1000m = 0.87 bar
2000m = 0.76 bar
3000m = 0.67 bar (10000ft - approx equivalent to commercial airliner cabin pressure)
4000m = 0.58 bar
5000m = 0.51 bar
8800m = 0.30 bar (Mt Everest)
10000m= 0.26 bar
20000m= 0.07 bar

Regards
Richard

Thanks Rich, that's great

So this would mean:

Alt-Press-Equivillant depth change in water:

0m = 1 bar 0m
1000m = 0.87 bar +1.3m
2000m = 0.76 bar +2.4m
3000m = 0.67 bar +3.3m (commercial airliner cabin pressure)
4000m = 0.58 bar +4.2m
5000m = 0.51 bar +4.9m
8800m = 0.30 bar +7m (Mt Everest)

This is why I think we are over emphasising the altitude issue.

I am not saying that it should not be in the manual or lectures, but I am saying that we should perhaps
A, not give it such a disproportionate amount of emphasis
B, Consider giving students some background information (such as the above) rather than say "just do X"
I never did find out why diving after flying is such a bad thing either....as it would seem that worst case (IE jumping out of plane into sea) we would effectively be finishing the dive at a lower altitude/pressure than we started it at....which actually seems a rather good thing!

Best

Chris B

Dave Woodward

09-03-2005, 21:49

Not into doing the maths right now, but is it more to do with the difference? If you say, at sea level 20 m to surface is 3 times pressure 3 bar to 1 bar.

Now at 1000m, this is 2.87 to .87, which equals 3.298 time surface pressure.

At 2000m, this is 2.76 to .76, which equals 3.63 time surface pressure.

So for the altitude, you could have the equivalent of a 23m dive at 1000m, or 26m dive at 2000m.

Is that a difference?

(is that right?)

Dave

:=Mike is quite right, the reduction of air pressure as you increase in altitude is not linear. Here are some nominal values for air pressure at different altitudes:
:=
:=0m = 1 bar
:=1000m = 0.87 bar
:=2000m = 0.76 bar
:=3000m = 0.67 bar (10000ft - approx equivalent to commercial airliner cabin pressure)
:=4000m = 0.58 bar
:=5000m = 0.51 bar
:=8800m = 0.30 bar (Mt Everest)
:=10000m= 0.26 bar
:=20000m= 0.07 bar
:=
:=Regards
:=Richard

Thanks Rich, that's great

So this would mean:

Alt-Press-Equivillant depth change in water:

0m = 1 bar 0m
1000m = 0.87 bar +1.3m
2000m = 0.76 bar +2.4m
3000m = 0.67 bar +3.3m (commercial airliner cabin pressure)
4000m = 0.58 bar +4.2m
5000m = 0.51 bar +4.9m
8800m = 0.30 bar +7m (Mt Everest)

This is why I think we are over emphasising the altitude issue.

I am not saying that it should not be in the manual or lectures, but I am saying that we should perhaps
A, not give it such a disproportionate amount of emphasis
B, Consider giving students some background information (such as the above) rather than say "just do X"
I never did find out why diving after flying is such a bad thing either....as it would seem that worst case (IE jumping out of plane into sea) we would effectively be finishing the dive at a lower altitude/pressure than we started it at....which actually seems a rather good thing!

Best

Chris B

john williams

09-03-2005, 23:02

The important thing is the difference in pressure (or the pressure gradient) between us and outside of us - and not the equivalent msw.

As has been said
20m = 3 bar
10m = 2 bar so a 50% reduction
0m = 1 bar so a 100% reduction - to the point we have been designed to work at.

We know can dive to 6m all day and not get DCI when we return to the surface (we can get CAGE if we hold our breath on the way - but wont get DCI from a release of saturated nitrogen in our tissues)

So if we are saturated at 1.6bar and can come back to the surface safely. It is safe to assume that a 1.6:1 ratio of inside to outside is safe.

any further drop in the outside pressue has a HUGE impact upon that relationship.(as well as the fact that our bodies have not been designed to work at less than 1bar)

You drop to 0.8 outside pressure and that makes the ratio 1.6:0.8 or 2:1 (that's like spending all day at 10m and coming directly to the surface). How many missed stops is that?

This is like going over a hill of around 1500m

Drop it to 0.6 bar (what has been quoted as cabin pressure) and you get about 2.4:1

Which is like spending all day at 14m and then coming straight to the surface. How many missed stops is that?

However - if we are at Curent Tissue Code A before we fly - and therefore saturated at 1.0 bar - this differential becomes 1:0.6 which is equivalent to 1.67:1...amazing!...this appears to be the same margin of safety that we consider appropriate for coming back to the surface.

So flying - even in a pressurised aircraft with anything other than total desaturation is exactly like intentionally missing stops!

Should there be a presurisation problem and the cabin pressure is reduced to not much more than 0.2 bar (aircraft tend to fly at more than 10000m) and you are into a pressure gradient of 8:1 - which is bye-bye diver! Most non-divers would suffer from DCI in this case - but you are right ...the prime danger here is not DCI - it is the ground! But why do you think they provide oxygen? It's not just because there's not much of it to breathe up there ...it's also to deal with the DCI people get when the cabin depressurises.

************************************************** *********

About this "instantaneous change" malarky. That's easy:

Any change is provocative - we know that because we understand that any saw-tooth profile dive is provocative (and as instructors we worry about going up and down all day with trainees).

Few mountain passes are straight up and down - there are little hills that we go up and down on the way.

If we spend an hour packing the boat away and hitching it up then we "could" count an hour of surface interval before we set off. However - physical activity is also provocative (just look at the effect packing the boat away would have on a can of coke in your pocket whilst you did it!).

A can of coke is simply a volume of fluid supersaturated with a gas - exactly like a diver supersaturated with nitrogen.

So we take worst case scenario:
we recognise that you have probably off gassed since reaching the surface - but we don't know how much, and we don't know how "well shaken" you are (coke can analagy).

Let's say - to keep it safe that you have not off-gassed at all (can't be worse than that after all)

At what point on your journey does the change to altitude become significant? It's not at any given altitude, nor is at any given time - so we cannot offset any time against the equation - so we go worst case scenario again and ask the question. We also have to factor into the equation that atmospheric pressure is rarley exactly 1 bar at sea level, and rarely what is forecast, and that the pass will not be at the same point in the weather system as we are even if we can accurately measure atmospheric pressure at sea level after our dive.

So even the question cannot be asked with any degree of certainty/accuracy:
"If we were instantaneously transported there now - would it be safe?"
If the answer is "No!" then we wait quietly (probably in the pub) until the answer is "Yes!".

Or...we plan our diving so that when we ask that question we have a guaranteed Current Tissue Code that allows us to make a safe instantaneous transition to the altidue at the pass.

HTH

John

iain aitchison

10-03-2005, 18:27

OK I've got another one :-)

I was recently delivering a SD lecture, specifically ST4 regarding altitude diving and I was really struggling to see the practical point of it. I think I must be missing something (again) as I have never got this.

Why do we worry about flying after diving or driving over mountains?

In theory, yes, I am fully aware and have read quite extensively on the subject however the following stats (rounded up and converted to Meters of sea water for clarity) make me wonder why we spend time on this.

Sea level = 1ata or 0M
Everest = 9000m or equivalent to ascending another 3M
Commercial airliner = equivalent to ascending another-0.3m
1000m hill= equivalent to ascending another 1m

As for diving at altitude, surely as we are ascending to the same ambient pressure we left (which is a tiny difference as illustrated above) then this is again a moot point?

Lastly, flying after diving. The manual tells me we should use specific tables but does not give even an abbreviated rational as to why. I would have thought that we were, if anything in the bonus situation of being 'super de-saturated' after our 0.3m equivalent depth change. Certainly other factors such as hydration and fluid retention in the tissues may well have a detrimental effect but the altitude change......?

Thoughts appreciated.

Best

Chris B

Hi Chris, forum readers,

The altitude issues are relevant for a number of reasons and become more so as the changes in altitude become more severe.

For many dives in the UK, even at freshwater sites the changes are still relatively theoretical, for others using tables or computers in say an alpine or expedition situation, the issues become very relevant indeed.

A significant number of BSAC members live in places where these issues become relevant for their regular diving.

For any dives where the pressure is not as at sea level the changes must be compensated for. This is typically either by tables in altitude bands(Buehlmann), ambient pressure bands (BSAC 88), mathematical correction back to theoretical sea level depths (PADI RDP or wheel) or depth corrections to use the sea level table (BSAC/RNPL 1972, DCIEM). Most dive computers work in altitude groups or steps (here similar to tables)rather than with continuous changes to give a bit more safety and to allow some margin for the effects of weather (which can be say equivalent to 400 m altitude just on its own). A good example of continuous altitude compensation is the VR3 (necesary here to the pressure activation of the computer).

All these issues become increasing significant as the altitude
differences become more severe.

For a dive at say 3000m, the local ambient pressure is ca. 0,7 bar against ca. 1,0 at sea level. This would mean after a dive the diver must decompress at the end of the dive to 0,7 bar rather than 1,0 bar i.e. increase the decompression by a factor of over 1,4 or over + 40%. Example: comparing some deco times for say BSAC level 1 against level 4 for the same dive will confirm the an increase in deco requirements. The stops are done in shallower water too, as the altitude increases. On BSAC level 4 the stop depths change accordingly. Some computers including the Aladin will show a final stop depth of 2 m rather than 3 m at this sort of altitude, for the same reasons. Some tables including Buehlmann will do the same. These shallower stop depths are due to the more severe changes in pressure near the surface on altitude dives. Also because of this again the ascent rates become more critical and are generally slowed down further(PADI 9 m/m instead of 18 m/min, BSAC 88 a minute from the now shallower stop to the surface.

Examples of pressure changes:

1) dive to 10m at sea level 1 bar surface means 2 bar at depth (pressure has doubled)

2) dive to just 7m at 3000 m, altitude 0,7 bar surface means 1,4 bar at depth (pressure has doubled). At 10 m the pressure would be 1,7 bar so the change is not just doubled, it has increased by a factor of ca. 2,4.

At 3000m a 1,4 bar pp02 would be reached on pure oxygen at 7m rather than 4m, which is a factor of about 1,75 difference.

Diving straight away on sudden arrival at this altitude (cable car, helicopter) would mean that the diver has a significant overload of excess nitrogen from the lower altitude -equivalent to a fairly serious previous (sport)dive - even if the diver has not been diving at all. A computer not automatically compensating for the altitude will completely miss this, simply because the computer programme cannot compensate for it, which would then lead to inadequate decompression. Conversely even after a relatively serious (sport)dive at altitude, an immediate return to sea level will work against the desaturation time, leaving a computer user (Aladin, VR3 or similar) with little or no residual desaturation time on arrival back at sea level.

Here again it is the proportional differences which are the key to understanding the issues of altitude.

Inadequate hydration and/or acclimatisation will add to any decompression problem.

I dive regularly at 1000-2500 m altitude and I can confirm all these effects become very real indeed.

Even at 2000 m when climbing or skiing the first effects of the thinner air can be felt and that is without humping your gear to an altitude lake.

The thing about flying is comparable but a little different.

Immediately after a dive you have normally decompressed to allow you to (only just) safely surface at whatever altitude you were diving at. Immediately after the dive it is just that there is absolutely no reserve. If you had done a safety stop you would have a tiny theoretical reserve.(If you are lucky enough to have a VR3 you can follow all of this on the tissue graphs). If you then immediately go in a commercial jet you will expose yourself to typically 2500 m altitude or 0,75 bar ambient pressure shorty after taking off. As you would have then not allowed yourself to desaturate a bit in the time between finishing you dive and flying, the excess nitrogen still in you will be simply too much and you would probably get serious symtoms of decompression sickness as further nitrogen is released from the tissues as your body tries to continue the
decompression process to get you down to be clear at new requirement of 0,75 bar. If you wait before flying, after some time depending on how much diving and at which altitude, there will eventually be enough reserve to allow a resonable change like this to safely occur within a reasonable time.

Please Email me if I have missed something.

Iain Aitchison
BSAC First Class Diver / BSAC Advanced Instructor