Diving at altitude

Redetermine no-decompression limits (and deco)

As we have already seen, the air pressure in the mountains is lower than at sea level, falling by about 0.1 bar per 1000m. This has implications for the no-decompression limits.

For the question of whether supersaturation could lead to bubbles after surfacing, not only the pressure in the tissue is relevant, but also the overpressure in relation to the ambient pressure. Only when the overpressure becomes large enough will bubbles develop and perhaps problems as a result. Thus, if the ambient pressure is lower, the overpressure would be higher for the same saturation – therefore, the no decompression limits must be shorter.

In principle, you can imagine this quite simply: If I only have an ambient pressure of 0.9 bar at an altitude of 1000m, I double the pressure already at a depth of 9m instead of 10 as in the sea.

This difference can be captured – simplified – with a correction factor. This simply sets the current pressure in relation to the pressure at sea level.

This factor can be used to convert the actual depth dived to its equivalent at sea level: the depth times the correction factor gives the depth to plan no-decompression limits with.

Correction factor mountain lake diving with example

To make this a little easier on yourself, there are tables where this calculation has simply already been done. Here you can easily read how an actual dive depth translates into the theoretical depth that you must use to plan no-decompression time or decompression.

So, for example: If you plan a dive to 30m at 1200m, it’s like being at 34m. From here, you go into any no-decompression table and plan the dive as if you were at 34m – but stay at 30m.


Theoretical depth at different altitudes

In addition, there are tables that apply to a specific height range. The Bühlmann tables, as shown here as an example, are available for different altitudes up to 3500m.

Of course, you could take such a table before each dive, plan the dive with it, and then dive according to this plan. But because it’s complicated and error-prone, no one actually does it that way anymore. Why should you? After all, the dive computer can calculate this more accurately.

Dive to 30m for 20 min with EAN32, planned in Subsurface

In order for the computer to calculate correctly, you have to tell most models what altitude you are at. The pressure sensors measure the absolute pressure and then subtract the set surface pressure.

You can usually find an explanation in the manual of the computer, which height ranges you can set. In this example, a fairly typical division, you can see that the range up to 700m does not require any special adjustment. Above that there are three levels – higher than 3700m most computers do not foresee.

However, some computers can calibrate themselves: They measure the pressure before the dive, and are thus no longer susceptible to human error. You just have to make sure that the computer is turned on before the dive and can detect the air pressure.

Mares mountain lake steps

What the computers calculate are adjusted no-decompression limits or even adjusted deco. This is not based on voodoo, but on tissue pressures under the constant assumption that specific overpressures are tolerated. These are only reached more quickly at altitude.

Now, if in fact you just have to make sure that the computer is set correctly – how can you plan dives at altitude?

Nothing could be easier: in planning software such as Subsurface, you can simply enter the altitude at which you dive. Then you plan the dive as usual, but you will see deco stops displayed earlier than would be the case at sea level. If you want to stay within the no decompression limits, you have to shorten the time at depth accordingly.


Bühlmann and the Swiss mountains: Can the decompression model do it?

We have seen that in principle we simply set our computer in mountain lake mode, perhaps as a small additional safety factor more conservative – and can go diving with peace of mind.

Nevertheless, it is quite interesting to consider what adjustments might be necessary to make the decompression models still work at altitude. For that, the good news first: Actually, you don’t have to do anything special at all. The godfather of decompression, Albert Bühlmann, already included altitude in his models. No wonder, after all, he worked in Zurich and had the Swiss mountain lakes as a reference.

This graph represents the M-values, i.e. the critical supersaturation, in the Workman model (which has long been out of use) and in Bühlmann’s model. On the x-axis the ambient pressure, on the y-axis the gas pressure in the tissue, in the middle the line representing complete saturation. Above this is the M-line, the tolerable overpressure for any depth. As you can see here, this line goes to zero. So instead of taking the gauge pressure at sea level, you can take the one at slightly lower ambient pressure from the same model.
Since the M-line has slope slightly greater than one, so the overpressure at depth is always allowed to be slightly greater than when it reaches the surface, this gradient, the tolerated overpressure, becomes slightly less at altitude. This results in a difference that goes beyond conversion with a fixed correction factor.

M Values Workman and Bühlmann

The model could now be used to calculate dives at any altitude. Despite the good empirical validation of the model even in mountain lakes, this is nevertheless not a good idea: at extreme altitudes, the model probably reaches its limits, and factors outside diving play a major role. In the area where people are fine without significant altitude adjustment, you can probably rely on the descompression models. When additional factors come into play, dives become experimental: there is simply no large database on dives at 4000m. Whoever dives there always does a bit of experimenting on himself. Great caution is then appropriate.

Beyond no-decompression Limits: Potential risks

Now, however, something else comes into play when diving in the mountains: Often you are only ascended directly before the dive, so you still have a higher nitrogen saturation in your body than the environment. You are already decompressing before the dive.

These are not enormous quantities, and during the trip and preparation, some desaturation / equalization already takes place. Nevertheless, when diving close to the limits, it may be wise to plan somewhat more conservatively than at sea level to compensate for this effect.

To have a rough idea of what kind of “presaturation” we are talking about here: Suppose you drive from 0 to 1000m. With a rounded 80% nitrogen in the air, our body would be completely saturated with 0.8 bar nitrogen at sea level. If you now ascend to 1000m, the pN2 (nitrogen partial pressure) is only 0.72 bar. All tissues are therefore slightly oversaturated for the time being. Suppose we were to beam up to the mountain and then immediately jump into the water, what constitutes this presaturation? We have a relative overpressure of 0.08bar pN2 – this corresponds to the difference that one meter of water depth makes. Not much, but enough to shorten the dive time a little.

This fact is often presented as having to be dealt with in the same way as a previous dive. That doesn’t get to the heart of the matter. Although we already have a slight overpressure in the tissues, what we do not have are bubbles that could well be there from a previous dive.

Diving a little more conservatively is also important for other reasons: for one thing, as we saw at the beginning, our bodies react to altitude. With relatively small altitude differences such as 1000m, this is only a very small effect, but it also means a certain amount of stress. And: mountain lakes are usually cold, and decompression becomes less efficient when you are freezing. These are also good arguments for taking it easy.


Another quite important factor, not directly related to altitude, is the water temperature, and also the ambient temperature on land. Lakes are often very cold, so you need proper cold weather protection – dry suit, thick hood, gloves, maybe even a heated vest.

All this makes the dive more strenuous, which is another reason to prefer setting the limits a little safer. Furthermore, as already mentioned, the cold can have an influence on the quality of the decompression.

As you freeze toward the end of the dive, your body begins to restrict blood flow and concentrate blood in the center of your body. This means that nitrogen from the periphery, from the poorly perfused tissues, is not removed as well. Decompression becomes more inefficient, risk increases. While it has not been conclusively proven exactly what effect cold has on decompression, and there are some rather contradictory studies. Nevertheless, one large study from the U.S. Navy in particular provides quite relevant data.

Gerth, Wayne et al: The Influence of Thermal Exposure on Diver Susceptibility to Decompression Sickness. NEDU. 2007

Here divers were exposed to very different temperatures in the water (but in a pressure chamber under controlled conditions), and in different pairings: Very warm or very cold during the bottom time, and both again during the decompression phase. This results in four variants: warm/warm, warm/cold, cold/warm, and cold/cold. All dives went to 40m, surfaced according to Navy tables as one would after a 70 minute dive. Here, the length of the dive was increased slowly at first in different rounds of the experiment, and not continued when too many DCS cases occurred.

What happened:

In the group that was warm during the saturation phase but cold during decompression, so many cases occurred even with relatively short dive times that longer times were not tested at all.

Thus, it seems reasonable to assume that cold at the end of the dive carries with it an increased risk of DCS. Besides general comfort, this is definitely a reason to dress warmly enough, and approach dives in the cold more cautiously.

DCS cases at deco in cold water, NEDU
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