Diving Nitrox At Altitude

[sig]

I received my nitrox certification about two months ago but I am still unclear on one thing. The LDS I certified at says that since I dive at an elevation of 6300ft I should adjust my nitrox limits for a theoretical altitude. So at a actual depth of 110ft at this elevation I would actually be diving at a theoretical depth of about 137ft. If I figured my maximum mix for actual depth, then using a mix of 37% would put me at a maximum 1.6 ata. If I figured my mix by theoretical altitude though my max mix would be 30%.

Speaking to another LDS, they said I only need to figure the actual depth into the equation and near as I can tell my computer which is supposed to automatically adjust for altitude does not change what it suggest for maximum operating depth at different altitudes.

If anyone can give me a definitive answer I would appreciate it. I would like to have the benefits of diving with a higher mix but being that pressure is increased at altitude in my mind I can see why the lower mixture would be required to prevent oxygen toxicity. Any links to this question would also be appreciated.

Thanks

 

[-=>Larry<=-]

Hi Sig,
I'm posting mostly so I can see the authoritative answer when it goes up - BUT - here's my take as a physicist:

The "Diving at Altitude" concerns & rules deal primarily with pressure diferential when you come up - the table jigger is [in my interpretation] a way to keep that pressure diferential appropriate when you are faced with reduced pressure at the surface
BUT -
The PO2 limitation for NitrOx is based on immediate partial pressure as the concern - so - depth/mix alone are the determining factors for this one.

That should leave you using the NitrOx tables as usual for O2 depth/exposure - and expecting a higher effective load when looking at the nitrogen side of the exposure.

Cheers

 

[roakey]

Sig,

Larry nailed it. When you compute your FO2 simply ignore altitude. For O2 you’re worried about an absolute PP of O2, so altitude doesn’t enter the picture. Well, it does, but it makes you conservative in two ways. First, you don’t have 1ATM on top of the water (assuming your numbers are correct, at the altitude you’re talking about you have 110/137 = .8 ATM) and two, if you use 33 ft = 1 ATM, you’re also being a little conservative because fresh water is 34 feet = 1 ATM.

Not a lot of conservatism, but some.

There’s a lake here in Colorado I dive a lot that’s at the 10k level. My buddy scubaturek and I have noticed (note that we don’t do this, but it was an interesting revelation) that we could switch to 100% O2 at 30’ safely in this lake. Because 30 / 34 = .9 ATA and the pressure at 10k feet is .7 ATA so .9ATA + .7ATA = 1.6ATA which is an acceptable resting exposure for O2 (note there’s no conservatism in here at all).

A comment about “best mix”: Don’t bother. Scubaturek said it best: Your gas is just another piece of equipment, and you should know your equipment well. What’s the MOD of 32%? Easy, 130ft. 36%? Easy again, 110ft. How about 26%? I have no idea, let me get out my calculator, a luxury I don’t have at depth, and figure it out.

My standard Nitrox gasses are air (very rare these days), 36%, 50% and 100%. Since they’re only four gasses I deal with, I know them well. If I lived at sea level I’d use 32% instead of 36%.

Keep it simple, including your gasses. Sounds like 32% for your dive would be perfect, since that gives you an O2 PP of 1.4 ATA at 111ft (actually a bit less than 1.4, but again, don’t worry about it).

As you no doubt have discovered, Nitrox is a wonderful altitude gas (I keep telling the shops around here that they should “push” it as such ). You stated that your 110-foot dive has an EOD (Equivalent Ocean Depth) of 137 feet. By using 32% Nitrox, that translates to an EAD of 114 feet, or right about back where you started (well, OK, you’d have to use the 120 foot table, so you’re not exactly back where you started).

Roak

 

[sharpenu]

It is my understanding that the astronauts have something close to the same problem. You see, at 1ata the space suit's joints are too stiff and difficult to bend. I am told that they spacewalk at a reduced pressure to allow ease of movement. This places them at risk of the bends. This was not a problem until the Apollo fire in which three astronauts were killed in a fire which occured in a pure O2 atmosphere. The solution has been to have the astronauts "decompress" by breathing 100% O2 by mask for approx 2 hours before the walk. They then breathe a reduced pressure of O2 at 100% to achieve the required PO2. My point is NASA should have something on this phenomenon. The usual disclaimer: If I have been misinformed, my apologies feel free to give me the correct info...

 

[roakey]

You're right, sharpenu, astronauts breathe 100% O2 before a walk to avoid the bends. But I took sig's post as a question about the other side of the equation, the O2 exposure side rather than the N2 side on which altitude DOES have a profound effect.

Most all the agencies have an altitude course, which covers diving air at altitude sufficiently. However, PADI gets quite confused when it comes to Nitrox at altitude, suggesting that you base your O2 % on the EOD, not the actual depth. IANTD has it right when it states to base the mix on the actual depth, not the EOD.

The physics behind the O2 limits and the N2 limits are completely different.

Roak

 

[sharpenu]

Sorry, misunderstood the question. (Water on the brain?) There are several ways you can compute this. For the nitrogen, you should use either an altitude dive table. Try:
http://www.cosimo.com/amnavalt.html

Or you can compute your EAD using a table like this:
The standard disclaimer being that these remarks are only a GUIDELINE and should not be a substitute for proper instruction from an apropriate agency.//

1000 1.037
1500 1.056
2000 1.075
2500 1.095
3000 1.116
3500 1.136
4000 1.157
4500 1.179
5000 1.202
5500 1.224
6000 1.248
6500 1.272
7000 1.295
7500 1.321
8000 1.346
8500 1.372
9000 1.399
9500 1.427
10000 1.453
10500 1.479
11000 1.511
11500 1.541
12000 1.572
Table From: NOAA Diving Manual (1991 Edition, page 10-27).

Just look up the altitude in the column on the left (ALWAYS round up) and then multiply your depth by the column on the right. This gives the depth you use on the sea level table. You should also consider yourself in a repetitive dive status when arriving at altitude. Advance two letter groups for each 1000 feet of altitude or fraction thereof. Arriving at 2200 feet would place you in letter group F. After the apropriate interval, you will offgas the nitrogen from the lower altitude and may dive as if it were a first dive.

To compute PO2, instead of adding 1ata for surface pressure, you would add the figure below: (round altitude up)
Altitude Pressure(ata)
0 1.000
1000 0.964
2000 0.930
3000 0.896
4000 0.864
5000 0.832
6000 0.801
7000 0.772
8000 0.743
9000 0.715
10000 0.688

PO2=(P+(ffw/33))(EANX%)
Example:EANx32 at 100ffw in a lake @2000ft alt:

PO2=(.93+(100/33))(.32)
= (3.96)(.32)
PO2= 1.26atm

Hope this helps... (Once again please notify me of any errors in calculation or information.)

AND... THis is provided for informational purposes ONLY and is not to be considered instruction or as a substitute for instruction. Consult your LDS for apropriate instruction.

 

[sig]

Thanks for all the help

 

[Bigtop]

I have a question from another angle. My sons and I live at 10,000 ft and we are completing our Rescue Diver and Enriched Air/Nitrox certification in Hawaii. Because we live at 10,000 our bodies have acclimatized to the high altitude in that we carry approximately 1/3 as many red blood cells than a person who lives at sea level. Since the red blood cells carry the O2, should we be worried about the Nitrox calculations or the algorithms in our dive computers? Would we be more susceptible to oxygen toxicity?

 

[Gene_Hobbs]

Hi Bigtop,

The increased red cells will increase O2 carrying capacity but most of us have saturated red cells all the time. The oxygen in your blood at pressure is higher for everyone and not impacted in any way by the number of red cells. No worries from living at higher altitude at all.

Take care and have fun!

 

[Bigtop]

Thank you