How does altitude affect scuba diving limits?

NOAA's weather station records put O.R. Tambo International at 1,694 metres above sea level. At that height the standard atmosphere gives a surface pressure of about 0.83 bar, around 18% less than at Sodwana. That missing pressure changes the arithmetic of every dive. An 18-metre dive near Johannesburg's altitude plans like a sea-level dive to about 22 metres, so no-decompression limits are shorter. A table or computer set for sea level will underestimate the nitrogen you carry out of the water and eat into a safety margin you can't see.

Nothing about the water warns you. A highveld dive looks and feels like any other, which is why altitude mistakes tend to stay hidden until someone reads the computer's settings. The risk sits in the maths of the dive, in how the computer is set up and in any drive or flight between altitudes.

Why does altitude change a dive?

Decompression depends on pressure ratios, not on the depth number alone. Your tissues take on nitrogen at depth and have to release it against the pressure waiting at the surface. At altitude that surface pressure is lower, so the same depth creates a bigger ratio between bottom and surface. Writing for DAN, Richard Vann explains that this makes any given depth count for more, which is why altitude dives have shorter no-decompression times. Why that extra nitrogen can form bubbles is covered in our guide to how nitrogen supersaturation causes the bends.

The standard fix dates from E.R. Cross's work in 1967 and 1970. Multiply the planned depth by sea-level pressure divided by the pressure at altitude, and you get a deeper "theoretical depth" to plan with. The figures below use NASA's standard atmosphere model, and real weather shifts them a little from day to day.

Elevation	Surface pressure	An 18 m dive plans like
Sea level	1.01 bar	18.0 m
300 m	0.98 bar	18.7 m
1,000 m	0.90 bar	20.3 m
1,694 m (O.R. Tambo)	0.83 bar	22.1 m
3,000 m	0.70 bar	26.0 m

The same physics moves the biggest pressure change closer to the surface. At sea level, pressure doubles at about 10 metres. At 0.83 bar, surface pressure doubles after about 8.4 metres of fresh water. That's where ears, BCD volume and ascent control see their largest proportional change, a couple of metres shallower than the 10-metre figure from sea-level training.

At what altitude do altitude procedures start?

PADI defines altitude diving as diving higher than 300 metres above sea level. The US Navy tables are stricter, applying corrections from 300 feet, according to DAN. Suunto's Zoop Novo manual starts at PADI's figure. Its manual altitude setting offers three bands: 0 to 300 metres, 300 to 1,500 metres and 1,500 to 3,000 metres. A diver using one at Johannesburg's elevation needs the top band, then has to change it back before a coastal trip.

How do dive computers handle altitude?

Many computers measure absolute pressure, and DAN notes that they can be rezeroed for the surface pressure at altitude. How they do it varies by brand, which is the part divers tend to skip in the manual. The differences below come from the manufacturers' own documentation.

Shearwater's Perdix sets its surface pressure when you turn it on, using the lowest reading from the previous 10 minutes. It keeps that value until the next turn-on. Below 965 millibars it forces the altitude setting to Auto, so a Perdix switched on in Johannesburg can't be left on sea level.

Scubapro's Galileo 2 checks atmospheric pressure every 60 seconds and sorts the result into altitude classes. Suunto's Zoop Novo, as above, relies on the diver picking the right band. A computer that works out altitude for itself removes one error. A manual setting leaves it with you.

Depth display is a separate setting from altitude. Shearwater notes that fresh and salt water differ in density by about 3%. A computer set for salt water therefore shows a shallower depth for the same pressure. Inland dives happen in fresh water, which makes the salinity setting worth checking alongside altitude. Comparing computers on algorithm and settings is covered in our guide to what a dive computer's algorithm changes.

Do you need to wait after arriving at altitude before diving?

Yes, if you've arrived from much lower down. A diver who lives at sea level carries nitrogen balanced to sea-level pressure. Drive or fly up, and the nitrogen that no longer matches the lower pressure behaves like residual nitrogen from an earlier dive. Vann's DAN article uses the US Navy example of a diver arriving at 8,000 feet: they start in repetitive group G. An immediate dive to 60 feet would carry 40 minutes of residual nitrogen time, which falls to 8 minutes after waiting 6 to 8 hours.

After about 12 hours at altitude, Vann writes, the excess nitrogen has washed out. Scubapro builds the same idea into the Galileo 2. It shows an adaptation time after a pressure drop and treats any dive during it as repetitive. Johannesburg residents who dive near home don't carry this problem, since their tissues already match highveld pressure. Visitors who fly in from the coast and head straight for the water do.

Is driving home to Johannesburg after diving like flying?

Physically, yes. The pressure drop is the same whether it arrives by road or by air. DAN Southern Africa lists Sodwana as about seven hours by car from Johannesburg, and that drive ends roughly 1,700 metres higher than the dive. DAN's flying-after-diving guidelines were written for cabin altitudes between 610 and 2,438 metres, and Johannesburg sits inside that band.

Those guidelines set a minimum surface interval of 12 hours after a single no-decompression dive. After multiple dives or multiple days of diving, the minimum is 18 hours. DAN hasn't published a separate rule for driving. Scubapro's manual, though, flags its prohibited-altitude display as important for divers who drive over a mountain pass to get home. A Sunday-morning dive followed by an afternoon drive home to Gauteng is worth factoring into the dive plan, even though no published rule covers the drive. The reasoning behind those numbers is set out in our guide to flying after diving and how long to wait.

What should a highveld diver check before the next dive?

Start with the computer, because it carries most of the altitude work once it's set up correctly. Confirm whether it adjusts for altitude automatically or needs a manual setting, and check the salinity setting for fresh water. Turn it on at the surface, not in the water. If you've come up from the coast within the last 12 hours, plan the first dive as a repetitive one. Raise it in the dive briefing too. A buddy whose computer still thinks it's at sea level will surface with different numbers from yours.

Sources

Frequently Asked Questions

Can you use a mechanical depth gauge at altitude?
It's not ideal. DAN notes that most pneumatic depth gauges can't be reset to zero at altitude and aren't recommended for altitude diving. If one is used, DAN's correction is to add 1 foot for every 1,000 feet of altitude. Near Johannesburg, that's about 1.7 metres.

Does the weather change altitude calculations?
Slightly. Scubapro's Galileo 2 manual points out that barometric pressure changes with the weather. Its altitude classes are defined by pressure switch points rather than exact elevations. A computer that measures surface pressure picks up those shifts, while a table built on a fixed elevation can't.

What are the highest altitude dives on record?
DAN reports that some of the highest altitude dives were made at 12,500 feet, about 3,800 metres, in Lake Titicaca on the Peru and Bolivia border. Jacques Cousteau dived there first in 1968. At that elevation the standard atmosphere gives a surface pressure of roughly 0.63 bar.

Is altitude sickness a concern for divers in Johannesburg?
DAN notes that most altitude diving happens below 8,000 feet, about 2,400 metres, and that healthy people generally tolerate those altitudes well. Johannesburg sits well below that line. Acute mountain sickness becomes a consideration at higher elevations, where DAN flags acclimatisation as an issue.

Does altitude change nitrox's maximum operating depth?
Oxygen partial pressure depends on absolute pressure, and at altitude there's less of it at any given depth. That puts the sea-level MOD for your mix on the conservative side in a highveld dam or quarry. The nitrogen side still needs altitude corrections, so nitrox doesn't remove the need for an altitude-aware plan.

This is general information, not a substitute for dive training, a dive medical or advice from a doctor trained in dive medicine. In an emergency, contact DAN Southern Africa on +27 82 810 6010.

Reviewed September 2026. Devon Ford runs Scuba Evolution, a PADI 5 Star Dive Centre in Glenvista, Johannesburg.