Crossing mountain passes after diving
Off home: How high should you go after diving?
Another question that often arises in connection with diving at altitude is the area around the mountain lake. It is not uncommon to have to go a little higher after the dive, over a pass, to get back home. At what point can this pose an additional risk?
Very often, another ascent after diving is seen as a big risk, kind of like getting right into an airplane. However, there is no evidence of a demonstrable risk, and when one considers the pressure differences that are being talked about, a small further increase seems quite unproblematic. If you have to go from 1000m to another 1500m and then down into the valley, you accept another pressure difference of about 0.05 bar. Under water, this would be just 50cm additional water column, a pressure difference that can also occur as a measurement inaccuracy. And: The range in which a table or a setting on the computer is valid covers e.g. 700 to 1500m altitude. If it doesn’t matter whether I dive at 800 or 1,500 m, why should it be so critical to change altitude after the dive?
Supersaturation at increasing altitude
We’ve already seen that pressure decreases with increasing altitude—but slowly. 1,000 m corresponds to one metre of water column—at first that doesn’t sound spectacular. And it isn’t, but the tolerated supersaturation also becomes smaller as pressure drops. That’s why every additional reduction in pressure has an effect.
The graphic shows tissue supersaturation after a dive that ended with a GF of 50—a normal, gentle dive. The black bars mark where the GF was at sea level; the bars then show the GF after you’ve been beamed to 700 m. Unrealistic, yes—but it can illustrate the impact of an altitude change. And you can see: it’s not dramatic, but large enough that you might want to keep a slightly bigger margin to the limits if you know you’ll be going up soon after diving.
To take a closer look, we’ve built our own little tool: an altitude simulator. It’s part of our Dive Analyser—you can use real dive profiles to see what happened during the dive, and simulate what happens to tissue loading if you ascend to up to 3,000 m with or without a break.
More than just the model
Small altitude differences are certainly not something that, on their own, can cause major problems. Whether you still have to cross a mountain pass after a dive in an alpine lake, or you gain a few hundred metres of elevation after diving on a steep island like La Palma (or Bali) – the effect isn’t dramatic. You should be aware of it, plan your dives accordingly and conservatively, and if in doubt have a coffee at sea level first—but our decompression models account for altitude, too.
Still, we shouldn’t lose sight of two factors that go beyond supersaturation alone:
Bubbles continue to develop after the dive, even more so than underwater. Most bubbles can be measured 30-45 minutes after the dive, only after that they slowly decrease. If you reduce pressure even further just when most bubbles are developing, that could theoretically “break the camel’s back.” If you were already close to a DCS hit, the drive up could trigger that small additional bubble growth that ultimately pushes you over the line into an accident.
DCS symptoms usually appear only after the dive, very often within the first hour. If you’re driving uphill, it doesn’t make things any better – but suddenly you might suspect the ride itself of having caused the DCS. This uncertainty can be avoided: If you have concerns about whether everything went well, there is nothing to be said against a break at sea level.
Dive center at 700m, diving in the sea?
As Punkfish, we naturally have a very particular interest in the topic of altitude diving. We dive in the sea—but our base and our accommodations are up in Fuencaliente, at 700m. And none of us are particularly risk-seeking—quite the opposite: organizing safe dives is extremely important to us.
There’s no contradiction. Our shore dives have such gentle profiles that the small altitude difference after packing up really isn’t a problem anymore—and we know after which dives we’d rather stop somewhere down below for a bit.
You can find more about this in a blog article.
