Diving at Altitude Part 2

By Dr. Sawatzky

When we dive at sea level, we assume that the person has not been under pressure greater than sea level for a period of time before the dive. If the person has been under increased pressure, they will have additional nitrogen in their body at the start of the dive and we must take that nitrogen into consideration. What I am talking about is simply repetitive diving where we use the "repetitive group" to account for the extra dissolved nitrogen from the previous dive. When we dive at altitude, the same concerns apply. The difference is that we will normally have spent days or months at the increased pressure (lower altitude) before the dive, not the minutes of bottom time during a previous dive.

To completely decompress from saturation takes several days. However, for practical purposes we use 24 hours because the vast majority of the excess nitrogen will have left the body during that time. Therefore, when diving at altitude we must either spend 24 hours or more at the altitude of the dive before diving or take the excess nitrogen into consideration. How we do this depends on the tables we are using. One very crude and conservative method is to add two repetitive groups for every 1,000 ft of altitude gained before the dive. For example, if we ascended from 2,000 ft to 4,000 ft altitude and wanted to do a dive within the first 24 hours, we would start the dive with a repetitive group of "D".

For the DCIEM altitude tables, you add 10 fsw to the depth of the dive before you look up the depth correction. For example, if you were doing a 100 fsw dive, 7200 feet above sea level, you would look up the depth correction for a 110 fsw dive if you were diving within 24 hours of arriving at altitude. In this case you would have to add 30 fsw to the depth and therefore, would use the 140 fsw decompression schedule (theoretical ocean depth). The Buhlmann altitude tables assume that you have just arrived from sea level and therefore they can be used without correction. For decompression computers and PC based decompression programs, you have to follow the directions for the specific computer or program you are using.

During the ascent part of a dive, you are off-gassing inert gas. At altitude, the rate of pressure change will be greater because you are returning to a surface pressure of less than one atmosphere (ATA). Therefore, you will have to ascend at a slower rate to keep the rate of pressure change the same as at sea level. Remember that the ascent rate to be used at altitude depends on the ascent rate of the sea level table being used.

For the same reason that ascent rates must be slower when you dive at altitude, your decompression stop depths must be shallower. The Buhlmann altitude tables have split the three metres (10 ft) stop into four metres (12 ft) and two metres (6 ft) stops. This does not become a major concern until serious decompression dives are done at significant altitudes. As before, if you are using a decompression computer or PC based decompression program, determine how it deals with this problem.

Descending to a lower altitude after an altitude dive is always a good idea as it reduces your risk of DCS. However, ascending to a higher altitude can be dangerous and certain precautions must be taken. The safest and easiest rule is to wait for 24 hours. Unfortunately, this is often not practical. Another safe and fairly easy rule is to calculate all of the dives done during the 24 hours before the ascent to higher altitude as if they had been done at the higher altitude. For example: You are living 3,000 ft above sea level and plan to dive in a nearby lake that is also 3,000 ft above sea level. However, to get to the lake, and more importantly to return home after the dive, you have to drive over a 5,000 ft pass. You would plan the dive as if the lake was 5,000 ft above sea level. As usual, if you are using a decompression computer or PC based decompression program, you must determine how it deals with this problem.

One change you might not think about is that neoprene is actually thicker at altitude. The gas bubbles in the neoprene expand as the atmospheric pressure is reduced. This has little effect during the bottom portion of the dive as the neoprene is crushed by the water pressure. However, if you are planning a 10 ft decompression or safety stop, you might want to start the dive wearing a couple more pounds of lead (or place a drop weight) so that you are not too buoyant at the end of the dive.

Altitude dives are usually in colder water than sea level dives and the ambient temperature can be extremely cold. Therefore, thermal protection before, during and after the dive is usually more important during altitude diving. Remember that if you are cold or working hard during the dive, you will want to be more conservative when you calculate your decompression requirements. Other considerations related to cold water diving are to ensure that your regulator is designed for cold water use, that your air is dry enough for the temperatures expected, and that you are trained and experienced in cold water diving.

Altitude dives are often conducted in very remote locations. Therefore you need additional emergency training and equipment. First Aid, CPR, Oxygen First Aid, and Life Saving/Scuba Rescue training are highly recommended. Oxygen kits for emergency oxygen use, first aid kits, backup diving equipment, reliable communications and personal protective gear will all need to be available at the dive site.

Another consideration is "hypoxia". At altitudes above five or six thousand feet, some individuals who travel directly from sea level and then engage in heavy physical labour will encounter problems because of the reduced oxygen tension in the air. This can be a significant problem after the dive. At higher altitudes, altitude (mountain) sickness and high altitude pulmonary/cerebral edema can occur. It is always a good idea to take a few days to acclimatize to higher altitudes before engaging in heavy physical work. It is also a great idea to maintain a high level of aerobic fitness, all the time.

We all know to use sun protection when we go to the beach, but the problem is much worse at higher altitudes where there is less air to block the UV rays. Remember to pack your sunscreen and UV blocking sunglasses when preparing for an altitude dive.
Skin problems are extremely common at higher altitudes because of dry air and wind. Lip creams and moisturizing creams are a good idea.

That pretty well completes our discussion of altitude diving. I strongly recommend you take a full altitude diving course before undertaking this highly (pun intended) enjoyable but more complex form of diving.