Altitude Diving: How to Plan a Dive in a Mountain Lake

At what altitude does a dive count as an altitude dive?

Somewhere between 300 m and 700 m (1,000 and 2,300 ft), depending on whose rules you follow. PADI and Suunto draw the line at 300 m. The US Navy corrects every dive above 1,000 ft (305 m), and dives deeper than 44 m from 300 ft (91 m). Bühlmann's air tables and the DECO 2000 table are calculated to be read unchanged up to 700 m.

Below the line, the lower air pressure is treated as too small to matter. Above it, your table or computer has to know the altitude. Our altitude diving calculator uses the same bands: no correction up to 300 m, the DECO 2000 table read with your real depth up to 700 m, and above that a table made for the altitude, a corrected sea-level table or a computer set for altitude.

Why lower air pressure shortens your no-stop time

Because what matters on the way up is the pressure drop relative to the surface, and at a lake the same water sits under less air. Richard Vann explains it in DAN's Alert Diver: the relative difference between the air pressure and the pressure underwater grows, so any given depth has a bigger effect than at sea level, and altitude dives have shorter no-decompression times. The StatPearls review on diving at altitude adds that the probability of decompression illness rises in proportion to the drop in air pressure.

The drop is large. The standard atmosphere gives about 795 hPa at 2,000 m (6,562 ft), and Shearwater's manual rounds it to about 800 mbar, roughly a fifth less than the 1,013 hPa at sea level. In the Bühlmann table values Beat Mueller presented at the Bühlmann Memorial Symposium in 2019, the 18 m no-stop limit is 55 minutes at sea level, 44 minutes on the 701-2,500 m table and 32 minutes at 4,500 m.

How to use a dive table at altitude

There are two ways. One is a table calculated for the altitude, such as Bühlmann's 1986 set with separate cards for 0-700 m and 701-2,500 m, or the mountain-lake versions of DECO 2000. Taucherpedia says a special altitude table is needed above 700 m. The other is the Cross correction, published by E. R. Cross in 1967 and 1970 and used by the US Navy, which converts the lake dive to a deeper sea-level dive you can read on a normal table.

The two routes do not give identical limits. A 1995 review by Egi and Brubakk found that constant-ratio versions of the US Navy schedules, the family the Cross correction belongs to, came out more conservative than Bühlmann's altitude tables at higher altitudes, longer bottom times and deeper dives.

The Cross correction multiplies your depth by sea-level pressure divided by the pressure at the lake. The result is the sea-level depth that gives the same pressure ratio when you surface. The US Navy manual sets out the steps:

• Correct the depth: real depth × sea-level pressure ÷ lake pressure. The Navy's example is 60 fsw (18 m) at 5,000 ft (1,524 m), which becomes 72.1 fsw (22 m). Entered as 18.3 m at 1,524 m, our calculator gives the same 22.0 m.
• Read the table at that equivalent depth, rounded up to the next row as usual.
• Correct any stops the other way: table stop × lake pressure ÷ sea-level pressure. A 20 fsw (6 m) stop at 5,000 ft moves up to 16.6 fsw (5.1 m).
• Strictly the ascent rate should be corrected too, but the Navy says this third correction can safely be ignored.
• For a repetitive dive, look up the residual nitrogen time at the equivalent depth as well.

Worked example: a 20 m dive in a lake at 2,000 m

Take a fresh-water lake at 2,000 m (6,562 ft), a planned maximum depth of 20 m (66 ft) and a sea-level table with a 3 m (10 ft) safety stop. Our altitude diving calculator gives the numbers below. Each step uses unrounded values, and the results are shown to one decimal.

On a table in 3 m steps, 25.5 m is read on the 27 m row, where the same dive in the sea would sit on the 21 m row. The US Navy table works in feet: 25.5 m is 83.6 ft and falls on the 90 ft row, with a 33-minute no-stop limit. The same 20 m in the sea is 65.6 ft, read on the 70 ft row with 48 minutes. The lake takes almost a third off the no-stop time.

The ascent rate is the correction the Navy lets you skip; the calculator shows it so you can see its size.

Dive computers at altitude: automatic or manual?

Most current models detect altitude by themselves, but some need a setting, so check the manual. DAN says the same in its report on a diver bent at a lake at about 1,800 m: many computers adjust automatically, but not all.

Automatic models read the air pressure around the time you switch them on. Shearwater's Perdix 2 samples it every 15 seconds while off, keeps a 10-minute history, takes the lowest value as surface pressure at switch-on and does not update it until the next switch-on. Mares says the Genius senses ambient pressure every 20 seconds even when turned off, and Garmin's Descent G1 uses its barometric sensor. Suunto's Zoop Novo has three manual altitude settings, and Suunto warns that the wrong one gives erroneous dive and planning data.

The limit is what the computer cannot know. DAN's 2020 answer to a reader puts it plainly: a computer that samples pressure at the dive site senses only where you are, not where you travelled from, and may not account for the ascent. So if your computer fixes its surface pressure when you switch it on, do that at the lake, not in the valley, and allow for the drive up yourself.

How long to wait after driving up to the lake

The US Navy says 12 hours. Its manual gives that as the time the body needs to equilibrate at altitude, and a dive within 12 hours of arrival counts as a repetitive dive, with the trip up as the first dive. Suunto's manual asks for at least 3 hours. Bühlmann's 1986 tables took a third approach and were calculated for a diver who arrives and dives straight away.

The Navy gives the trip a repetitive group by altitude: A for 1,000-2,000 ft, E for 6,000 ft (1,829 m) and G for 8,000 ft (2,438 m), which then shrinks with time at altitude. In the manual's example, a diver flown by helicopter to 6,000 ft starts a 100 fsw (30 m) dive 90 minutes later. The group has dropped from E to D, the equivalent depth is 130 fsw, and 11 minutes of residual nitrogen time go on the bottom time. Vann's example for 8,000 ft is 40 minutes added to an immediate 60 ft dive, and only 8 minutes after waiting 6-8 hours.

Bühlmann's assumptions are specific. The 0-700 m table assumes a diver adapted to sea level who dives at once at 700 m. The 701-2,500 m table assumes full adaptation at 700 m and a one-hour drive to 2,500 m before an immediate dive. A same-day trip from the coast to 2,500 m is outside what either table assumed.

Thinner air is the other thing to adjust to. Vann notes that most altitude diving takes place below 8,000 ft, which healthy people generally tolerate well, and that acute mountain sickness can develop with sudden exposure higher up. The Navy warns that altitudes above 10,000 ft (3,048 m) can put serious stress on the body and must be reached slowly. Two Swiss divers who made 18 dives at 4,400-4,780 m on Mount Kenya in 1988 had no DCS but reported exhaustion from the lack of oxygen.

Depth gauges and fresh water

A gauge or computer that zeroes itself at the lake needs no depth correction, and the Navy names a gauge you can re-zero at the site as the preferred method. Most sealed mechanical gauges cannot be reset and read low throughout an altitude dive. The Navy adds 1 fsw for every 1,000 ft of altitude to their readings, about 0.3 m per 305 m.

Sources treat fresh water differently. The Navy assumes every dive is in sea water and makes no salinity correction. Vann explains why that works: depth gauges measure pressure, so their reading can go straight into a sea-water table, although the real depth in fresh water is a little greater. The StatPearls review, by contrast, includes water density in its equivalent sea depth. Our calculator follows the Navy, which leaves a small margin when you enter the fresh-water depth.

On a computer, the water setting changes only the displayed depth. Shearwater's manual says fresh and salt water differ in density by about 3%, its default EN13319 setting sits between them, and the setting has no effect on decompression calculations, which use absolute pressure. In the worked example, 20 m of lake water shows as 19.5 m on a computer set to salt water.

Going higher after an altitude dive

What counts is how much higher you go after the dive. The US Navy's table for ascent to altitude after diving is set out by the increase in altitude, so the drive back down to the valley, which raises the pressure around you, is not part of it. A pass above the lake is. DAN Europe treats a post-dive climb of more than about 700 m as carrying the same risk as flying.

The Navy table grows quickly with height. A diver in group G needs no wait for a 4,000 ft (1,219 m) climb, 1 hour 19 minutes for 5,000 ft and 9 hours 13 minutes for 8,000 ft. The Navy also notes that no surface interval is needed before a commercial flight if the dive site is at 8,000 ft or higher, because the flight then brings an increase in pressure, not a decrease.

Other guidance is simpler and stricter. Taucherpedia asks for at least 24 hours between the last dive and a flight or a mountain pass. Cabin altitude, road trips and the consensus waiting times are covered in our guide to flying after diving.

What altitude trials found, and where to add margin

Altitude procedures rest on a small number of field trials, and those trials had cases. In 1969 the Swiss Army dived Lake Silvaplana on the French GERS tables and 2 of 8 divers had severe DCS, though Mueller notes it is not known whether the tables had been adapted for altitude. A Boğaziçi University programme in Turkey later tested no-stop limits for 3,500 m in a lake on Mount Kaçkar.

A computer that never alarms is no guarantee either. In a DAN case report, a 48-year-old diver dived to 20 m for 40 minutes at a lake at about 1,800 m (6,000 ft), with three stops on the way up and computers that showed no omitted decompression. About 45 minutes after surfacing she had numbness and tingling in her right foot, and she was treated in a hyperbaric chamber.

If symptoms appear after a lake dive, give oxygen and call local emergency services and the DAN Emergency Hotline (+1-919-684-9111), which DAN staffs 24 hours a day. Our guide to decompression sickness symptoms and first aid covers what to look for. Vann's advice for altitude diving is to have oxygen for first aid and a clear plan for evacuation to a medical facility with a hyperbaric chamber before the dive. DAN's extra measures for altitude divers:

• Keep normal hydration.
• Use nitrox whenever possible.
• Avoid or reduce repetitive dives.
• Ascend slowly, hand over hand on an ascent line.
• Avoid or reduce hard work in the water and after the dive.
• Carry enough oxygen for the journey to the nearest emergency department, which in this case was 1.5-2 hours away.