Altitude Diving Question

[MyronGanes]

When diving at higher altitudes, pressure at a given depth will be less than the pressure at sea level right? Water doesn't compress so any reduction in atmospheric pressure results in reduction of ambient water pressure.

When converting actual depth at altitude to theoretical depth at sea level the book always adds depth.

For exampleDiving 30 feet at 5000 feet altitude is equivalent to 36 feet at sea level.

As I understand it this is to illustrate that water pressure in both cases is equal.

So this doesn't make sense since 30 feet at 5000 feet will have less pressure than 30 feet at sea level. 36 feet at sea level will have even more pressure?

 

[knotical]

Good question – it shows that you are thinking about these things.

You are correct that the atmospheric pressure is less at elevation. For example, at 5000 feet it’s approximately 0.83 atm. So at any given depth the absolute pressure will be about 0.17 atm less than at sea level (assuming fresh water in both cases).

However, since the adjustments are usually made for the purposes of preventing DCS, we are primarily interested in the ratio between the pressure at depth and that at the surface.

Bubbles are more likely when you emerge from water at elevation – for the same reason that water boils at lower temperature – there is less atmospheric pressure.

At 5000 feet, with an ambient surface pressure of about 0.83 atm, you will add one (local) atmosphere for every 33 X 0.83 = 27 feet (approximate) of depth.

 

[tbone1004]

this one is a bit difficult for divers to grasp for some reason.

You have two forces to consider when under the water, atmospheres absolute, and atmospheres gauge. ATM/ATA. Your decompression tables are based on ATA because that is what your body feels at all times, your gauges read ATM which assumes you are at sea level. This is why your SAC calculations are always ATM+1, or depth+34 for FFW.

When diving at altitude, you have less absolute pressure being exerted on you, but your gauges can't compensate for that so what happens is they ignore the first few feet of water until they hit what they think is "0 ft". 5000ft is roughly .8 atm pressure. To get to the 1.0atm your gauge needs to read "0", you have to descent 2m, or almost 7ft. This is water depth that is not being compensated for by your depth gauge, but needs to be factored into your decompression schedules because your decompression and NDL's is based on you ascending to 1ATA, not .8ATA. This relationship is non-linear and cross referencing the tables is good. Navy has a really good table for all of this that is good to keep around if you're altitude diving regularly, otherwise the mental math can get interesting.

 

[tursiops]

Not quite.
The purpose of the Theoretical Depth is so you get your nitrogen management correct.
If you ascend from 68 ffw (3 ATA) at sea level, you are decreasing the pressure by 2 ATM, from 3 to 1, a fractional change of 3/1 = 3.
But if you ascend from 68 ffw at 5000 ft altitude, where the atmospheric pressure is just 0.83 ATM, then you are ascending from 2.83 ATM to 0.83 ATM, a fractional change of 2.83/0.83 = 3.4. The deco calculations actually depend on that fractional change, not the absolute change. So, back to sea level: how deep would you need to be to have a fractional change of 3.4? You'd need to be at 2.4 ATM depth, or 3.4 ATA, or about 82 ffw.

One way to think about this is using the Equivalent Air Depth concept from your Nitrox class. Whatever your real depth is, your EAD is shallower, because you have less N2 in your mixture, and the effect of less N2 is as if you were using air at a shallower depth.

For altitude diving, whatever your real depth is, your theoretical depth is deeper, because you are going through more (fractional) pressure change to get to the surface.

In fact, the EAD calculations and the theoretical depth calculations will cancel out for the right mixture at the right altitude. At 6000 ft, 34% Nitrox will just about balance out the altitude effects on N2 absorption and release. Where I used to dive in West Virginia at 3400 ft, 28% was the right mix.

Additional: tbone is correct that many SPGs will read incorrectly as to your actual depth. But this is a separate issue from how your actual depth relates to theoretical depth. Hey, this is why there is an Altitude Diving course!

 

[MyronGanes]

Great explanation. I was thinking in terms of absolute pressure changes and didn't think to pay attention to the pressure gradient.