Trivia Question: Where Would You Be At 1/2 Ata?

[DeepTechScuba]

Walter, it seems to me from your observations, that we could adapt our dive tables to any altitude, then, by bearing in mind at what depths we effectively multiply our ambient pressure on the surface at.

Looking first at CNIDAE's altitude chart, pick an altitude, any altitude. Lets say, 9000 ft.

9000 ft = 21.38 inches of Hg

At this altitude, this by definition then becomes 1 ata.

But we need to figure out how much fresh water equals 1 ata at this altitude.

So, 21.38 / 29.92 x 14.7 psi = 10.5 psi of atmosphere above us.

Now we have to figure out how many feet of fresh water equals 10.5 psi. Thats easy too:

10.5 / 14.7 x 34 ft = 24.3 ffw per ata at 9000 ft altitude

Now we can use any friendly dive table that we like for any gas taht we have a dive table for, and we can convert.

So, at the surface of our high altitude mountain lake, the ambient pressure will be 10.5 psi and the equivalent of 1 ata for us before we begin to scuba dive.

At 24.3 ft the pressure will double, to the equivalent of 2 atas at this height.

at 48.6 ft it will triple, to the equivalent of 3 atas.

at 73 ft it will be like 4 atas at sea level. So 73 ft depth at 9000 ft altitude equals 4 atas, the equivalent depth of 100 fsw at sea level.

Now, lets do it all over for 18,000 ft!

CNIDAE's chart for 18,000 ft shows an ambient pressure of 14.94 inches of Hg, which we already know is 1/2 of 14.7 psi, or 7.35 psi.

34 ffw x 7.35 / 14.7 = 17 ffw per equivalent ata at this altitude.

So a dive to 17 ft at an altitude of 18,000 ft is like a dive to 33 fsw at sea level.

Here are the other equivalents:

17 ffw is equivalent to 33 fsw at sea level
34 ffw is equivalent to 66 fsw at sea level
51 ffw is equivalent to 99 fsw at sea level

So the answer is YES you CAN SCUBA DIVE at 18000 ft altitude. its just that all of you equivalent depths are twice what they would be if you were diving at sea level.

How interesting!

Thanks for the seed crystal in your post, Walter, that led to this crystallization of understanding.

Now I do not know what is so hard about that to understand for G. Irvine, such that he "ignores" altitude when he dives. He better not dive too high up, if he always ignors it.

 

[cnidae]

Walter once bubbled...
That will get you bent.

"The pressure is not a result of the atmosphere's above you."

Pressure is the weight of the atmosphere. When we dive, it is also the weight of the water.

"The pressure is determined only by the amount of pressure you apply from your zero point (surface)."

There is no "zero point." At sea level, we are under 14.7 PSI pressure. Our bodies (assuming we've been at that pressure a while) are saturated with N2 at 14.7 PSI. If we descent into the water, the pressure increases. The deeper we go, the more the water above us weighs and the more pressure we are under. As we increase the pressure by diving deeper, more N2 can dissolve into our bodies. If, for example, we were to dive from the surface at sea level to 68 ffw (66 fsw), we would triple the pressure from 14.7 PSI to 44.1 PSI. The longer we stay at that depth, the closer we approach nitrogen saturation at 44.1 PSI. The tables are designed to limit our time at depth to allow us to safely return to 14.7 PSI. They are not designed to allow us to safely return to the surface unless the surface is 14.7 PSI.

If, for example, we were to dive from the surface at an altitude of 8000 ft to 68 ffw (66 fsw), we would almost quadruple the pressure from 10.9 PSI to 40.3 PSI. A similar change occurs at sea level when we dive to 100 ft. We will need to return safely, not to 14.7 PSI, but to 10.9 PSI. Logic would indicate when diving at 68 ft, we should use 100 ft on the Sea Level dive tables. When I check the Sea Level Equivalent Depth table, I find the sea level equivalent depth for 70 ft at an altitude of 8,000 ft is 100 ft. Amazing!

So how come I've never felt better after a dive if I exceeded the NDL time? In reality I should of felt like shhht

 

[Charlie99]

cnidae once bubbled...
Tell me how my logic is flawed.

I'm trying to figure out what your training agency told you, and why.

They must have had a reason for explicitly telling you to ignore altitude. (Is this an accurate portrayal of the instruction or not???) It sounds like you covered a lot of deco theory in that class and I'm sure it was touched upon since you were intending to dive at altitude.

You can find Kevin's post at rec.scuba. Be prepared to wade through a lot of off topic junk. Below are the his initial trip report, the link the to "we covered it. ignore altitude" post and for the convenience of lurkers, the entire text of the "ignore" post.

GUE rec triox trip report

"Ignore the altitude" post

> > > Would you mind explaining the dive to 120' for 20 minutes?
> >
> > 120' on 30/30 triox. EAD for 120' on 30/30 is 120 * .8 = 96 feet.
> >
> > NDL for 100' on the air table is 20 minutes.
> >
> > Kevin Metcalfe
>
>
> Was there any consideration or discussion about NDL or depth adjustments for
> driving up and diving at 6,200' altitude?

Yes. Ignore the altitude. It doesn't matter.

Kevin Metcalfe

 

[jrmy_1]

Doppler once bubbled...
OK so at 18,000 feet, atmospheric pressure has dropped to about half of what it is at sea level... so what would your depth guage show if you dove to 34 feet in a pool up there?

What would the MOD of an EAN50 be at that altitude?

Regards

Doppler

I am going to have to say the MOD for EANx 50 would be about 90 fsw at PPO2 equal to 1.6 and 80 fsw at PPO2 equal to 1.4 and the depth guage would show about 17 feet if not zeroed prior to entering.

 

[Charlie99]

cnidae once bubbled...
So how come I've never felt better after a dive if I exceeded the NDL time? In reality I should of felt like shhht

Do proper deco from a deco dive and you will feel fine. One should know what sort of dive one is doing, though.

You can get away with very simplified procedures, .... most of the time.

DCS is probablistic and one can surface with significant deco obligation and still have less than 1 in 20 chance of being bent. For example, in the BR Wienke article I reference below, the 5% probability of being bent "NDL" for 80' is 60 minutes, 1% probability "NDL" is 15 minutes (no stop, 60fpm ascent).

For many years, it was just assumed that you couldn't get bent on a single 72 or whatever was the common cylinder.

---------

As you peruse the rec.scuba thread you may find some vague references by a GUE instructor to RGBM, B.R. Wienke and veiled assertions as to modern deco theory saying altitude corrections are not needed.

I haven't seen anything by BRW that supports that. When I went to find the bend probability numbers above, I stumbled over these comments by BRW: "The sought ratio constancy, R, at altitude induces a necessary scaling of actual depth to equival ent sea l evel depth (ESLD) for table entry, while all times remain unchanged. Actual depths at altitude are multiplied by factors, á , called altitude correction factors, which are just the ratios of sea level atmospheric pressure to altitude atmospheric pressure,
multiplied by the specific density of fresh water (0.975)."
from B.R. Wienke, Modern Decompression Algorithms: Models, Comparisons, and Statistics.

I'm sure GUE will provide other quotes from BRW if this is not an accurate reflection of his current thinking.

 

[Walter]

DIVING AT ALTITUDES ABOVE 10,000 FEET IMPOSES SERIOUS STRESS ON THE BODY AND IS STRONGLY DISCOURAGED

 

[DeepTechScuba]

There is no way that I would ever want to haul all my gear up higher than 10,000 ft anyway.

Lake Tahoe is high enough for me. My Suunto Vyper has a high altitude adjustment for Tahoe. No problem. No-brainer that way, if you can dial in an adjustment to your computer.

 

[Charlie99]

cnidae once bubbled...
So how come I've never felt better after a dive if I exceeded the NDL time? In reality I should of felt like shhht

I just noticed in Kevin's trip report: "For the second dive, my buddy was tired so it was just me and Andrew with Marc Hall doing video. "

Curiouser and curiouser.

 

[cnidae]

Walter once bubbled...
DIVING AT ALTITUDES ABOVE 10,000 FEET IMPOSES SERIOUS STRESS ON THE BODY AND IS STRONGLY DISCOURAGED

Why?

 

[cnidae]

Charlie99 once bubbled...
I just noticed in Kevin's trip report: "For the second dive, my buddy was tired so it was just me and Andrew with Marc Hall doing video. "

Curiouser and curiouser.

LOL