partial pressure of O2 & altitude

[kkoski]

Dr Deco,

It is well known that when diving at altitude you must use the theoretical ocean depth, not the actual depth, for the purposes of nitrogen management.

What about for oxygen toxicity?

My reasoning is:

1) The pressure of the amount of water above you (50' or whatever) is the same no matter what altitude you are at (as long as the density of the water is the same, fresh, or salt or...).

2) The pressure difference you experience between your depth and the surface does change based on your altitude. The atmospheric pressure is less at altitude than sea level, meaning the pressure differential is greater between a given depth and the surface (compared to sea level). This pressure difference is equal to the difference between the pressure at the theoretical ocean depth at sea level and the atmospheric pressure at sea level.

3) Oxygen doesn't care about the pressure difference. The danger in oxygen is exposing yourself to too high of a partial pressure, indicated by the pressure of the water above you (your depth).

4) The actual depth should be used for the calculation of oxygen partial pressure. Of course if you use the theoretical ocean depth you are adding a level of safety.

Make sense?

Katrina

 

[roakey]

Hi Katrina, welcome to the file!

I'm obviously not Dr Deco, but I'm "up the road" from you in Colorado Springs and do a lot of altitude diving. You're correct, O2 dosage is absolute, so you do not apply any altitude correction to it.

The mechanics are completely differnt between N2 and O2 limits. With N2 you're worried about not exceeding a certian N2 tension in your tissues as you surface, and when you're diving at altitude you're surfacing to a lesser pressure than at sea level, so you have to adjust your depths accordingly.

What's interesting is that there's one place where we dive often that's at 10k feet. At that altitude we could in theory go on pure O2 at 30'!

Roak

Ps. You do anything with the VLA?

 

[VTWarrenG]

The generally accepted figures of relevance are 1.4 ATA and 1.6 ATA. The body can usually handle 1.4 ATA of PPO2 during the working portion of a dive without incidence. The body can usually handle 1.6 ATA of PPO2 during the resting (deco) portion of the dive. These figures are in Atmospheres Absolute. They do not depend on altitude (or any other figure -- hence the term 'absolute').

However, the depth at which those ATA figures are reached DOES depend on altitude. Use the formulas or an 'equivalent depth at altitude' chart to compute the pressure-depth (in ATA) of your intended bottom time, and use that pressure-depth to figure your oxygen exposure.

Your #2 is wrong. The difference in pressure between 34 ffw and the surface is always exactly 1 ATA.

- Warren

 

[AquaTec]

let me ask a question, i don't have the exact details but bare with me


If 18,000 ft above sea level equals 1/2 atmosphere
then would it not be true that at that same altitude you would need to dive say 15 feet to equal 1 ATA

after that every 34ffw would equal an additional ATA

I guess what I am trying to say is that if the surface of the water does not equal 1 ATA then it should hold true that you need to descend to a certain depth to get tp 1 ATA, after that you would then add 34ffw for each ata

I don't know, I am just posing the questions to all you guys to debate and see what comes of it.

 

[VTWarrenG]

AquaTec, you are correct.

Every 34 ffw adds one ATA. If your surface pressure is 0.5 ATA, then you will be at 1 ATA at 17 ffw.

- Warren

 

[AquaTec]

Warren

So how do we calculate ATA at altitude above the surface.
ie 10,000 ft equal ? ATA

 

[roakey]

Originally posted by AquaTec
So how do we calculate ATA at altitude above the surface.
ie 10,000 ft equal ? ATA

You asked, don't shoot the messenger

Actual ATA = e**(-0.0381* Altitude in feet / 1000)

So for 10,000 feet that would be e raised to -0.381. I don't have my scientific calculator handy.

Roak

 

[Dr Deco]

Dear Katrina:

You are correct in assuming that at altitude, the absolute pressure is less and therefore there exists a margin of safety with regard to oxygen usage. If you were high enough, and diving with no air pressure above you, 33 FSW would be only 1 ATA. It is, of course equal to 2 ATA at sea level when normal atmospheric pressure has been added.

As is true will all decompression (oxygen or air), you do yourself a favor by maintaining some activity when in the water and performing the stop. This would consist of simply moving your arms and legs in a gentle fashion.

When one is on the surface, some mild activity is suggested to maintain the circulation. This would not be activity such as lifting, straining, or climbing ladders. Sleep is not good during the surface intervals (night time is a different case) since that reduces the blood flow to the lowest amount possible and nitrogen washout is at its lowest.

Dr Deco
:doctor:

 

[kkoski]

Thanks to everyone who posted!

Roak, I did my masters thesis mapping a spiral galaxy with the VLA You don't live in Socorro for the diving!

-Katrina

 

[Dr Deco]

Dear Katrina:

Scuba diving and radio astronomy. That is a seldom-seen combination.:star:

Dr Deco