high altitude diving

[bleeb]

A slightly different but related matter:

I'm visiting friends in Bali & staying at their house in the mountains.
My friends are also keen on going diving with me, but they haven't done it since they moved to Bali last year.
Now my question: is there any serious risk of getting bent after 2 dives to let's say 30m & then going back "home" afterwards which happens to be at an elevation of 500m?

Thnx for your input!

The short answer is that it's marginally possible, especially if you're near NDLs, or have done a fast ascent.

One set of recommendations is the NOAA Ascent to Altitude Table. (Note: Be careful to use the NOAA dive tables on the same page, and not someone else's.) As you can see, for a 500 m ascent (rounded up to 2000'), most 30 m dives are unlikely to be an issue, but the longer ones might require a bit more surface interval before ascending.

From my non-expert reading of past SB discussions, long shallow dives might actually be more likely to benefit from a bit of an SI before ascending. Most tables seem to be based primarily on one or a few theoretical compartments, while in reality long dives are more likely to fully load faster tissues, but that's where even a relatively short SI before ascending would unload them adequately.

 

[Bob DBF]

...and controversial. Believe it or not, some people argue there is no need to factor altitude into dive planning at all.

Rather than go directly to:

Nutjobs

What is the reasoning for disregarding the altitude? I could understand it if the dives were planned so conservative as to make the altitude irrelevant but, can't think of any other reason to ignore the difference in pressure at altitude.


Bob
-------------------------------
I may be old, but I’m not dead yet.

 

[boulderjohn]

What is the reasoning for disregarding the altitude? I could understand it if the dives were planned so conservative as to make the altitude irrelevant but, can't think of any other reason to ignore the difference in pressure at altitude.

Their reasoning is as simple as saying that they don't think altitude makes enough difference to need to be considered, and the system they use is conservative enough to deal with any minor differences related to altitude. That is the position of the UTD agency specifically. I don't know who else shares those beliefs on an official agency basis.

 

[Bob DBF]

Their reasoning is as simple as saying that they don't think altitude makes enough difference to need to be considered, and the system they use is conservative enough to deal with any minor differences related to altitude. That is the position of the UTD agency specifically. I don't know who else shares those beliefs on an official agency basis.

Thanks. I know from personal experience, which means nothing except to me, that being close to NDL and ignoring altitude can put you in quite a jackpot.


Bob
-------------------------
Hindsight is 20/20

I may be old, but I’m not dead yet.

 

[Dr Deco]

Absolutely. Being close to the NDL and ignoring altitude (reduced ambient pressure) is not a good procedure. The systems are based, in part, on pressure ratios [tissue to ambient]. Ignoring altitude upsets this concept.

 

[boulderjohn]

Absolutely. Being close to the NDL and ignoring altitude (reduced ambient pressure) is not a good procedure. The systems are based, in part, on pressure ratios [tissue to ambient]. Ignoring altitude upsets this concept.

You are referring to no decompression diving in your reply. The agency mentioned above says that altitude does not have to be considered for any diving, including decompression diving. Their argument is that there is not enough difference between sea level and altitude diving to require any adjustment to decompression profile.

Can you point the way to data, studies, etc. that would help one evaluate this belief?

 

[Jax]

I am so confused . . .

On the one hand, if you're at 5000' and you go to 100 ft, you leave the surface at .83ATA, gain 3 more ATA, and then return to the surface . . . so your net change is still 3 ATA, so one might think, use the same tables.

However, the tables were developed with someone going down to a depth and ongassing a certain amount of nitrogen, then coming up through a range of pressures to a surface pressure of 1. If you dive the same pressures, and you load 'close to' the same nitrogen, but as you come up you are stopping at less pressure gradients, and coming out a a lesser pressure gradient, it sure seems like the off-gassing could get fast enough to cause problems.

Confusing . . . Especially since this on- and off-gassing physiology is such an exact science . . .

 

[boulderjohn]

I am so confused . . .

On the one hand, if you're at 5000' and you go to 100 ft, you leave the surface at .83ATA, gain 3 more ATA, and then return to the surface . . . so your net change is still 3 ATA, so one might think, use the same tables.

However, the tables were developed with someone going down to a depth and ongassing a certain amount of nitrogen, then coming up through a range of pressures to a surface pressure of 1. If you dive the same pressures, and you load 'close to' the same nitrogen, but as you come up you are stopping at less pressure gradients, and coming out a a lesser pressure gradient, it sure seems like the off-gassing could get fast enough to cause problems.

Confusing . . . Especially since this on- and off-gassing physiology is such an exact science . . .

It's a bit more complicated than that. If I really understood it all, I wouldn't be asking these questions. But I can give you an example of the effect of altitude using Boyle's Law to track a bubble as it ascends from a depth of 4 atmospheres gauge pressure to the surface, using your .83 ATA for the altitude example.

The bubble (assume a beginning volume of 1) will grow in accordance with the standard formula:

P(1) X V(1) = P(2) X P(2)

Change in Volume at Sea Level
4 atmospheres gauge = 5 ATA

5 X 1 = 1 X V(2)
5/1 = V(2)

So we see that the bubble will expand to 5 times its size, as explained in every OW class.

Change in Volume at 5,000 feet
4 atmospheres gauge = 4.83 ATA

4.83 X 1 = 0.83 X V(2)
4.43/ 0.83 = V(2)
5.82 = V(2)

At 5,000 feet, the bubble will expand to nearly 6 times its original size, compared to 5 times that size at sea level.

The closer you are to the surface, the greater the difference between the pressure gradients at sea level and the pressure gradients at altitude. Emerging from the water at altitude creates a greater gradient than emerging from the water at sea level.

 

[Dr Deco]

Hello Jax:

It looks exact because of the numbers. There is an apparent certitude that does not exist in reality. People vary considerably in their gas exchange characteristics and their propensity to form the micronuclei that can develop into decompression bubbles. There is really a physiological variation.

To allow for this variability, table designers add safety margins. For example, the NDLs are shortened considerable with respect to bottom times. Because of these margins, it is possible to extend some bottom times or reduce some decompression times for short dives without encountering DCS problems. Some luck is required.

Except for simple dive scenarios or small altitude increases, tables developed for sea level would be less reliable at altitude.

I do not know of any data that would show that altitude is of no consequence.

Dr Deco :doctor:

 

[boulderjohn]

I do not know of any data that would show that altitude is of no consequence.

Actually, that is not what I asked for. I am looking for any data that can lead one to form a conclusion one way or the other.