Altitude diving

At a glance

How Altitude Diving Works

When a diver is at sea level, the atmospheric pressure pushing down on the water's surface is approximately 1 bar (1 atmosphere absolute or ATA). This pressure determines the partial pressure of nitrogen in the air we breathe. As a diver descends, the ambient pressure increases by 1 bar for approximately every 10 metres (33 feet) of depth. This pressure gradient dictates how quickly nitrogen dissolves into the body's tissues.

At higher altitudes, the atmospheric pressure is lower. For example, at 3,000 metres (10,000 feet) above sea level, the atmospheric pressure is closer to 0.7 bar. This means that a diver starts with less ambient pressure. When diving, the total pressure at any given depth will be less than at sea level, but the *relative* pressure change during descent and ascent is what matters for nitrogen loading and off-gassing. The lower surface pressure effectively makes the pressure gradient larger for a given depth, causing tissues to absorb nitrogen more quickly and off-gas more slowly during ascent.

To compensate for this, divers must adjust their dive planning. This typically involves using specialised altitude dive tables or setting a dive computer to an 'altitude' mode. These adjustments either treat the dive as if it were deeper than its actual depth or shorten bottom times and extend decompression stops, effectively reducing the nitrogen load or extending the off-gassing period to mitigate the increased risk of decompression sickness.

Diving at Altitude vs. Sea Level

What Altitude Diving means for your liveaboard trip

For the vast majority of divers, altitude diving is unlikely to be a direct concern during a liveaboard trip, as these vessels operate exclusively at sea level. However, understanding the principles of altitude diving is still relevant for two key reasons: pre- and post-liveaboard travel and the concept of relative pressure changes.

Firstly, if you are planning a liveaboard trip to a destination that requires you to fly into a high-altitude airport (e.g., Quito, Ecuador, before a Galápagos liveaboard, or Mexico City before a Baja California liveaboard) and you plan to dive immediately upon arrival, or if you plan to visit high-altitude attractions after your liveaboard, you must be aware of the implications. Just as flying after diving poses a risk due to reduced cabin pressure, diving at altitude requires careful consideration of nitrogen off-gassing. Ascending to a high-altitude city after a sea-level dive trip has similar physiological effects to a fast ascent or flying too soon.

Secondly, the principles of altitude diving highlight the critical importance of understanding pressure changes in relation to nitrogen management. It reinforces why ascent rates are crucial, why safety stops are recommended, and why no-fly times are strictly enforced after diving. All these measures are designed to manage the body's nitrogen load and prevent decompression sickness by controlling pressure changes. Therefore, while altitude diving itself might not be part of your liveaboard itinerary, its underlying concepts are fundamental to safe diving practices at any depth or elevation.

Global High-Altitude Dive Locations & Notable Lakes

Common misconceptions

Myth: Altitude diving is only for extreme adventurers. Fact: While some high-altitude dives are challenging, the core definition applies to any dive above 300 metres (1,000 feet). Many popular recreational lakes and reservoirs worldwide are at elevations requiring altitude adjustments, making it relevant for a broader range of divers.

Myth: My dive computer automatically adjusts for altitude. Fact: While many modern dive computers have an 'altitude mode' or automatically detect altitude, it is crucial to verify that your specific computer has this feature and that it is correctly activated before diving at elevation. Always check your manual and consider redundancy.

Myth: You just need to dive shallower at altitude. Fact: Diving shallower helps reduce nitrogen absorption, but it's not the only factor. The lower ambient pressure at the surface still alters the pressure gradient and off-gassing dynamics. Proper altitude adjustments, which may include shorter bottom times, longer safety stops, or specific dive table modifications, are essential for safety regardless of depth.

Myth: The risks of altitude diving are the same as sea level diving. Fact: The primary risk of decompression sickness is significantly amplified during altitude diving if no adjustments are made. The reduced surface pressure means that the body off-gasses nitrogen more slowly on ascent to the surface, and the relative pressure changes are greater for a given depth, increasing the likelihood of bubble formation.