I am sure we all have a friend or two that come off having an attitude. They may have their own set of rules that they feel apply to them. It is, do it their way or do not do it at all. In some ways, it is a good way to explain altitude diving. Altitude Diving is Diving with an Attitude. When you are diving at an altitude of 1,000 feet / 300 meters or more, the rules are different. Some of the rules you learned and have been following since you became a diver are applied differently and it is critical that you adjust your diving to meet the new conditions. Also, at higher altitude, there are other risks that need to be considered.

What is Altitude Diving?

Simply stated altitude diving is any dive done above the elevation of 1,000 feet / 300 meters above sea level. The diver training programs assume that you are diving at or near sea level. In those conditions, we start our dives at 1 ATM and as we descend we increase the pressure around us. Before we descend at sea level the air above us weights about 14.70 pounds per square inch. In salt water the water pressure that surrounds us increases 1 ATM (14.7 pounds or 1,013.25 millibars) for every 33 feet or 10 meters, fresh water is about 3% less. There are slight variations due to weather, however, there is very little impact. All of our dive tables use these “standards” as the starting points for the calculations needed to create your recommended NDL times.

When we start our dive, nitrogen in our system is in equilibrium with the outside pressure. As we change our depths during diving, the nitrogen in our system changes due to the influence of pressure on our body. The dive tables are designed to give us the information needed to return to the surface at a specified ascent rate to allow our body the time necessary to bring the nitrogen near to the levels that are acceptable to the body.

When we are diving at altitude, the starting and finishing atmosphere pressures are different. If we are diving at a nice lake sitting at 1,000 feet above sea level the ATM is not 1 ATM but has already dropped to .96 ATM. That not much of a difference but if you extend that to 10,000 feet your now at .69 ATM and that can make a substantial difference. The ten thousand foot mark is the maximum elevation still considered safe for recreational divers.

Surface Interval Without Diving

Imagine that you are on a business trip to San Francisco have the weekend off and plan on taking a drive up to Lake Tahoe. You are thinking it is only a 3 and half hour drive, so you can leave early in the morning and get there for a few dives in the afternoon. That is great, but, when you arrive and start your dive planning you need to calculate in your residual nitrogen, 12 pressure groups worth. When you move from one elevation to a higher one, the air pressure decreases just as it does when you are ascending whilst scuba diving. Your body has to release the gasses.

When you plan your dive, you need to add two pressure groups for every 1,000 feet difference in elevation. San Francisco is near sea level while Lake Tahoe is slightly over 6,000 feet above sea level. You can adjust your pressure group by taking into consideration the time between your arrival and the start of your dive. It will take about 6 hours for your body to naturally adjust to the point where you do not need to add the pressure groups. If you were going to a location with an 8,000 feet or more difference you must wait 6 hours before diving. USN tables suggest a 12-hour delay.

Lake Powell scuba diving altitude diving
Lake Powell 3,700 feet ASL. Photograph by Bernard Spragg

Theoretical Depth

The change in the air pressure also impacts on the pressure on you as you dive. It is almost like a reversed Enriched Air Nitrox’s equivalent depth. The theoretical depth depends on the elevation you are starting at. Let’s compare diving at 100 feet at different elevations. When you dive to a 100 feet/ 30 meters depth at 1,000 feet/ 300 meters elevations your theoretic depth is 103 feet/ 32 meters, not a big difference. It will have only a few minutes difference to your NDL time.

However, lets start a 100 foot/30 meter dive starting at 9,000 feet/ 2,700 meters. Your theoretical depth is now 140 feet/ 42 meters which is beyond the depth listed as the maximum for recreational scuba diving. If you were diving from a 10,000 foot / 3000-meter elevation you would exceed maximum depth at 90 feet/ 30 meters. If you do not adjust your dive time to include the theoretic depths you could be increasing your risk of DCS.

One of the key elements in becoming a deep diver included more in-depth training for nitrogen narcosis. Nitrogen narcosis might not be experienced by an Advanced Open Water diver who only dives to 100 feet normally. However, put that diver down 100 feet starting at 9,000 feet elevation and they will experience it to some degree.

Mountain Ailments

There are a few items that also needs to be considered when diving at altitude. Hypoxia is a medical condition where your body is not getting the amount of oxygen it required. At higher elevations, the air is less thick, so there are fewer molecules of oxygen for your body to absorb. This means your body has to work harder to get the oxygen you need. Your body will adjust to the air but it will take a few days. So until it does adjust you should limit your efforts. Move a little slower, carry less weight on each trip.

Mountain sickness is not related to diving but can be a major impact on your dive. The medical term is hypobaropathy, and it is sometimes called altitude sickness or “the altitude bends”. While it normally does not occur below 8,000 feet/ 2,400 meters it can. Many people to travel up to 8,000 feet will experience it. Once above 10,000 feet/ 3,000 meters, about 75% of people will experience mild altitude sickness. Many of the symptoms are similar to DCS. If you are diving in these higher altitudes, it is best to allow your body a few days to acclimatize before diving.

Getting Started

There are many great places to dive in the world that are significantly above sea level. Many of these are clear mountain lakes. If you are diving for the first time at altitude, consider taking the time to get certified for altitude diving.

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