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Altitude diving is one of those topics where a few simple rules are useful, but real understanding makes everything easi...
22/08/2026

Altitude diving is one of those topics where a few simple rules are useful, but real understanding makes everything easier.
Why do some tables start altitude procedures at 300 m, while others use 700 m?
What does your computer actually do when you set an altitude range?
Why does the same dive profile change when the lake is higher?
And what happens when you drive uphill after diving?
That is exactly what we work through in our SSI Altitude Diving Specialty.
SSI does not provide its own full course material for this specialty, so we built our own. At Punkfish Academy, the course includes our own explanations, examples, course material, and an exam, with the same approach we use for our decompression theory courses: clear physics, practical examples, and no unnecessary drama.
We also bring quite a bit of real-life experience to the topic. Our dive centre is based at around 700 m, while we dive in the Atlantic at sea level. For us, altitude after diving is part of normal operations, not just a theoretical chapter in a manual.
In the course, we look at pressure, altitude ranges, dive planning, computers, tables, no-decompression limits, supersaturation, and what changes when you dive in a mountain lake or move to a higher place after a dive.
If this series made you curious, the full Specialty is the next step.
More information here:
https://punkfish-academy.com/en/ssi-altitude-diving/

Altitude diving becomes much easier when you understand the pressure changes behind it — and that is exactly what we teach in the SSI Altitude Diving Specialty.
Punkfish Academy - Altitude and Diving 10/10

After looking at altitude, there is one small pressure fact that is just too nice to leave out: the air pressure around ...
20/08/2026

After looking at altitude, there is one small pressure fact that is just too nice to leave out: the air pressure around us is not fixed.
At sea level, standard atmospheric pressure is 1013 hPa, or 1.013 bar. On weather maps, pressure is usually shown in hectopascals, but the idea is the same: 1000 hPa = 1 bar.
In everyday weather, sea-level pressure often moves somewhere around the high 900s to low 1000s hPa. A calm high-pressure day may be above 1020 hPa. A low-pressure system may sit below 1000 hPa. In stronger weather systems, the numbers can move much further.
As a rough real-world range, 950 to 1050 hPa already covers a lot of ordinary weather in temperate regions.
Translated into diving language, that is about 0.95 to 1.05 bar.
So even the weather can move surface pressure by a few hundredths of a bar — the same order of magnitude as a few tens of centimetres of water column.
And then there are the extremes.
The highest sea-level air pressure record below 750 m is 1083.8 hPa. The lowest widely cited non-tornadic pressure measurement is 870 hPa, measured in Typhoon Tip.
That means the difference between extreme high and extreme low pressure is more than 0.2 bar — roughly the pressure change of more than 2 metres of water. We would not dive in such extreme weather conditions anyway, of course, but it is one of those facts that puts our perception of pressure changes at altitude into perspective.
For normal diving, we do not plan our dives from the weather report like this. But as a pressure fact, it is a beautiful reminder: the “surface” is not a perfectly fixed number. This is why in the Altitude Simulator you can even choose the weather-related pressure.
https://tools.punkfish-academy.com/altitude-simulator/en/

Weather can move surface pressure by measurable amounts — small compared with depth, but very real.
Punkfish Academy - Altitude and Diving 9/10

19/08/2026

Just a few moments with Punkfish Diving, I mean with us. Smiles, adventures, fun, entering from the shore, sometimes from a boat and always meet some special specials under water.

We love our La Palma
📷 .manuel

See you soon

Altitude is not only relevant when the dive itself happens in a mountain lake. It can also matter after the dive.Imagine...
17/08/2026

Altitude is not only relevant when the dive itself happens in a mountain lake. It can also matter after the dive.
Imagine a simple day of diving at sea level. You surface, pack your gear, and then drive uphill. The dive is over, but the pressure around you continues to change.
You can observe the effect of this change in our Dive Analyser:
https://tools.punkfish-academy.com/dive-analyser/en/index.html
Upload a profile, or choose one of our examples, and go to the last part of the tool.
Here you see the Surface GF at the end of your dive — the GF with 0 minutes waiting time. You can now change the waiting time before the ascent and change the target altitude. You can observe which GFs are reached when moving to a higher altitude, and how even a short surface time before moving up helps to control this.
The pattern is usually more useful than one single number: the higher you go, the sooner you go, and the more loaded the tissues are after the dive, the more visible the effect becomes.
This is also why driving uphill after diving should not be treated exactly like flying. The physics is related — ambient pressure decreases — but the pressure profile, timing, and altitude can be very different.
For us, the helpful question is not “mountain or no mountain?”
The helpful question is: How much pressure change, how soon after the dive, and after what profile?
There are more factors to take into account besides the GFs, but supersaturation is the most relevant risk factor. Read more here:
https://punkfish-academy.com/en/pass-rides-after-diving/

Driving uphill after diving is a pressure change after the dive. The size and timing of that change matter.

Punkfish Academy - Altitude and Diving 8/10

Gradient Factors can feel abstract until we remember what they compare.During a dive, the diver reaches a certain level ...
15/08/2026

Gradient Factors can feel abstract until we remember what they compare.
During a dive, the diver reaches a certain level of inert gas pressure in the different tissues. During ascent, this tissue pressure will gradually become higher than the ambient pressure, and the tissue becomes supersaturated.
The Bühlmann model gives us an upper limit for the tolerable inert gas pressure at a specific ambient pressure: the M-value.
The Gradient Factor tells us where the current tissue pressure sits between ambient pressure and that limit.
At altitude, the ambient pressure is lower. The M-value is lower as well. So the same tissue pressure means a higher Gradient Factor.
That is why Gradient Factors change when we move the same dive profile to a higher altitude.
The simulator makes this visible. Take one profile and look at the Gradient Factors at sea level. Then keep the profile unchanged and increase the altitude. The depth and time have not changed, but the pressure relationship has. You will see that your dive ends with a higher Surface Gradient Factor.
This is especially useful because it connects the theory to something many divers already use. GF is more than a setting in a computer menu. It is a way to describe how close a tissue compartment is to the Bühlmann limit.
And because that limit depends on ambient pressure, altitude naturally affects the result.
Try it slowly: same dive, different altitude, and watch the Gradient Factors move.
https://tools.punkfish-academy.com/altitude-simulator/en/

The same dive ends with higher GFs when diving at altitude — because supersaturation is relative to ambient pressure.

Punkfish Academy - Altitude and Diving 7/10

To get a better understanding of what happens at altitude, we created a tool to simulate dives at different elevations. ...
13/08/2026

To get a better understanding of what happens at altitude, we created a tool to simulate dives at different elevations. Try it here:
https://tools.punkfish-academy.com/altitude-simulator/en/
Altitude diving becomes much easier to understand when we keep the dive itself unchanged. So let’s start using the tool with one simple dive: choose a depth and a time, check your NDL or deco at sea level, and then change only one thing — the altitude of the dive site.
So, we move the lake up the mountain.
Start with something simple, for example an 18 m dive for 30 minutes at sea level.
Then move the same dive to 700 m, 1000 m, or higher.
The water depth stays the same, while the pressure profile changes. The dive starts at a lower surface pressure, reaches a slightly lower absolute pressure at depth, and returns to a lower pressure at the end.
That change is enough to move the decompression calculation. You can observe how the no-decompression limits become shorter, and, if it is a decompression dive, how your stops become longer.
You can now go through these changes for different depths, different times, and different altitudes, so you develop a better understanding of how big the effect is in different settings.

The Altitude Simulator shows how the same dive profile changes when the surface pressure changes.

Punkfish Academy - Altitude and Diving 6/10

To understand altitude diving properly, it helps to look at it through the Bühlmann model.The model tracks inert gas pre...
10/08/2026

To understand altitude diving properly, it helps to look at it through the Bühlmann model.
The model tracks inert gas pressure in different tissue compartments and compares that tissue pressure with the surrounding ambient pressure. From that comparison, it calculates how much supersaturation is still acceptable.
This limit is described by the M-value.
An M-value is the maximum allowed tissue pressure for a given ambient pressure. It has two parts: the ambient pressure itself, and an additional amount of tolerated supersaturation above that pressure.
At altitude, the ambient pressure is lower from the beginning. In the Bühlmann model, tissues are assumed to tolerate more supersaturation at higher ambient pressure, so the tolerated supersaturation becomes smaller when the ambient pressure is lower.
In practice, this can show up as shorter no-decompression limits, earlier decompression stops, or Gradient Factors that rise differently from what we would expect from the same profile at sea level.
The good thing about the Bühlmann model is that altitude was part of the story from the start. Bühlmann worked in Switzerland, with mountain lakes as a very real reference, and altitude decompression tables were tested extensively in the Swiss mountains. The pressure differences are calculated into the model and were checked in real dives.
The plot in this post is explained in more detail here:
https://punkfish-academy.com/en/saturation-and-desaturation/

At altitude, lower ambient pressure means lower M-values and a lower tolerated supersaturation.
Punkfish Academy - Altitude and Diving 5/10

When we dive at sea level, we usually start the dive at about 1.013 bar of atmospheric pressure. To keep things simple, ...
08/08/2026

When we dive at sea level, we usually start the dive at about 1.013 bar of atmospheric pressure. To keep things simple, let’s call that 1 bar.
Under water, pressure increases by roughly 1 bar every 10 metres, so a dive to 20 m at sea level happens at about 3 bar absolute pressure: around 1 bar from the atmosphere, plus about 2 bar from the water column.
At altitude, the water column behaves in the familiar way. Ten metres of water still add roughly 1 bar of pressure. The dive simply starts with less atmospheric pressure above the surface.
At 1000 m altitude, atmospheric pressure is about 0.899 bar, so the same 20 m of water gives us about 2.9 bar absolute pressure.
Decompression models work with pressure. They track how much inert gas is dissolved in the tissues, expressed as a pressure, and they compare that tissue pressure with the pressure around the diver.
That comparison is where supersaturation comes in.
Saturation tells us how much inert gas is dissolved in the tissues.
Supersaturation tells us how high that tissue pressure is in relation to the ambient pressure around the diver.
And this is the key idea for altitude diving:
Supersaturation is always relative to ambient pressure.
If the surrounding pressure is lower, the same tissue pressure represents a higher level of supersaturation.
So a tissue compartment can contain the same amount of nitrogen, expressed as the same pressure in bar, and still sit closer to its limit at altitude because the surrounding pressure has changed.
The gas is the same. The water is the same. The reference pressure is lower.
For more background on diving at altitude, you can read more here:
https://punkfish-academy.com/en/diving-at-altitude/

At altitude, the same tissue saturation leads to a higher supersaturation because the ambient pressure is lower.

Punkfish Academy - Altitude and Diving 4/10

Altitude can sound like a big change: 300 m, 700 m, 1,800 m — those are large numbers. But as divers, we experience much...
05/08/2026

Altitude can sound like a big change: 300 m, 700 m, 1,800 m — those are large numbers. But as divers, we experience much larger pressure changes under water all the time.
That is why it helps to translate changes in atmospheric pressure into something divers already know well: metres of water.
At sea level, atmospheric pressure is about 1.013 bar.
At 700 m, it is about 0.932 bar.
At 1000 m, it is about 0.899 bar.
So the pressure drop from sea level to 700 m is about 0.08 bar.
That is roughly the same pressure change as going 80 cm shallower under water.
From sea level to 1000 m, the pressure drop is about 0.11 bar.
That corresponds to just over 1 metre of water.
How relevant is this pressure difference? Most divers have seen this before: two computers on the same dive do not always show exactly the same depth. One may say 10.0 m, another 10.7 m or 11.0 m. That does not usually worry us, because we understand that pressure sensors and depth calculations have tolerances.
The interesting part is the scale: the pressure difference between sea level and 700 m is in the same order of magnitude as less than one metre of water.
And that brings it back to us: our base at Punkfish is around 700 m above sea level. In pressure terms, that means roughly 0.08 bar less than at sea level — about 80 cm of water column.

700 m sounds impressive — until you translate it into pressure: about 80 cm of water.

Punkfish Academy - Altitude and Diving 3/10

As divers, we already know that air has weight. A scuba cylinder is slightly lighter at the end of a dive, because some ...
02/08/2026

As divers, we already know that air has weight. A scuba cylinder is slightly lighter at the end of a dive, because some of the gas inside has been used. Breathe 100 bar from a 12-litre cylinder, and you have used roughly 1.4 kg of air. So yes: air may feel invisible, but it has real mass.
Atmospheric pressure comes from the weight of the atmosphere — the weight of all the air above us.
Pressure is force per area. At sea level, the column of air above us pushes down on every surface. That “push” is what we call atmospheric pressure. At sea level, it is about 1 bar.
A useful image is this: imagine a tiny square on the ground, 1 cm² in size. Above that square is a column of air reaching up through the atmosphere. The weight of that air column creates the pressure at the bottom.
As we go higher, there is less air above us, so the pressure drops.
But it does not drop in a perfectly straight line. The air near the ground is compressed by the air above it, so it is denser. Higher up, the air is thinner. That is why the pressure curve becomes flatter with altitude.
For diving discussions, a simple rule of thumb is often close enough: about 0.1 bar less pressure per 1000 m of altitude. It is not exact, but it is a useful way to picture what is happening.
This is the starting point for altitude diving: in the mountains, or when moving to a higher place after a dive, the surrounding pressure is lower than it was at sea level.

At altitude, there is less air above us, so the pressure is lower.

Punkfish Academy - Altitude and Diving 2/10

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