Diving at Altitude Part I
By Dr. Sawatzky
Most divers dive in the ocean, or in rivers and lakes not too far above sea level. However, all of Saskatchewan, Alberta, most of British Columbia and much of the US Northwest are high enough above the ocean that the altitude of the dive must be taken into consideration. In this and the next column, I will explore the many physiological changes that occur at altitude and explain how and why we have to alter our dives and dive planning.
Atmospheric Pressure
Atmospheric pressure is caused by the weight of the air. At sea level, the weight of the atmosphere is 14.7 pounds per square inch (1,033 grams per square centimeter). This is equivalent to the weight of 33 feet (10 meters) of sea water, 34 feet of fresh water, 760 mm of mercury (Hg), 29.92 inches of Hg, 1.013 Bar, 1013 millibars, or simply one Atmosphere (ATA). As we ascend to higher altitudes, the weight of the atmosphere decreases as shown in Table 1.
Altitude (feet) Pressure Error (ffw)
mm Hg PSI ATA
0 760.0 14.70 1.000 0.0
1,000 732.9 14.17 0.964 1.2
2,000 706.7 13.67 0.930 2.4
3,000 681.2 13.17 0.896 3.5
4,000 656.4 12.70 0.864 4.6
5,000 632.4 12.23 0.832 5.7
6,000 609.1 11.78 0.801 6.7
7,000 586.5 11.35 0.772 7.7
8,000 564.6 10.92 0.743 8.7
9,000 543.3 10.51 0.715 9.7
10,000 522.8 10.11 0.688 10.6
11,000 502.8 9.73 0.662 11.5
Table 1 Pressure Variations with Altitude
Determining Dive Depth at Altitude
This seemingly simple task is actually quite complex. Depth gauges are usually absolute (air plus water) pressure gauges that assume the pressure due to the atmosphere is one ATA. At altitude, the pressure due to the atmosphere is less than one ATA and therefore the depth gauge will read "shallow" (the actual depth of the dive will be greater than the reading on the depth gauge). The amount of this error is shown in the last column of Table 1 (roughly altitude in thousands of feet plus one). For example, if you were doing a dive 5,000 ft above sea level, you would have to add six ft to the reading on your depth gauge to determine your actual depth. If your gauge was reading 50 ft, you would actually be at 56 ft. If it was reading 100 ft, you would actually be at 106 ft. This correction only applies to gauges that do NOT adjust or compensate for altitude. Some gauges can be adjusted to "zero-out" at altitude. For these gauges, you must set the gauge so that the needle points to zero prior to each dive.
Most modern electronic gauges (dive computers) sense the change in altitude (atmospheric pressure) and adjust themselves so that they read the depth accurately. Some however require that you enter the altitude. As there are so many different electronic depth gauges and computers on the market, it is imperative that you determine exactly how the one you are using responds to altitude. A computer that does not adjust for altitude should NOT be used on an altitude dive.
Another (the simplest) way to determine depth is to use a weighted line. You drop it to the bottom, mark the water surface, pull the line up and measure the depth. Unfortunately, at many dive sites this is not practical.
Altitude Decompression
Standard decompression tables/equations calculate the partial pressure of inert gas in several theoretical tissue compartments. They contain pre-determined assumptions about how much supersaturation each compartment can tolerate and set the no-decompression times and decompression stops so that these limits are not exceeded. These limits are based on the ratio of the partial pressure of inert gas in the compartment to the ambient pressure. At altitude, the surface pressure is less than one atmosphere. Therefore, the assumed maximum partial pressures of inert gas that can be tolerated in each tissue compartment at the end of the dive have to be lowered.
One solution to this problem is to simply calculate the theoretical ocean depth (TOD) where the ratio between the inspired inert gas partial pressure and the atmospheric pressure would be the same as at the altitude of the dive. This is very easy and can be done using the following equation: TOD = (1/altitude pressure in ATA) X actual dive depth. For example, if you were planning a dive to 70 fsw at a site 6,000 ft above sea level, the TOD would be: TOD = (1/0.801 ATA) X 70 fsw = 88 fsw. Therefore, you could simply use the 90 ft schedule from any sea level decompression table (remember that your actual maximum depth is going to be 70 fsw). This concept was first used by Jon Pegg (1965) but was popularized by E.R. Cross in 1970. These "corrections" to sea level tables for altitude diving are widely used.
Unfortunately, life is not so simple. The basic problem is that using decompression equations based on diving data to generate altitude tables involves extrapolating these equations (curves) through the zero point on the graph. This assumes altitude DCS is the same as diving DCS, an assumption that is probably not true. In addition, there is some evidence that the tolerated tissue inert gas to ambient pressure ratios are different at different altitudes.
Simple altitude DCS occurs when a person ascends to altitude and gets bent (typically over 18,000 ft above sea level). This is the same as ascending from a saturation dive. The difference is that after a saturation dive, the diver stays at surface pressure indefinitely. When a person goes to high altitudes, it is usually only for a short period of time (minutes to hours), and then they descend. The equations that have been developed to predict the risk of DCS at altitude are very different from those used to predict DCS after saturation diving, and those are very different from the equations used for bounce diving. An altitude dive is usually a combination of decompression from saturation, plus a bounce dive, where the diver returns to a surface pressure of less than one atmosphere.
We can make some general comments about altitude decompression. The theoretical ocean depth calculations are relatively crude approximations. Nevertheless, they can be used for altitude dives well inside no-decompression limits with a maximum depth of 100 fsw because these types of dives have a very low risk of DCS on virtually all sea level tables. A better solution would be to actually calculate altitude tables using the decompression equations, adjusted for reduced atmospheric pressure. Altitude tables were actually calculated using the DCIEM model, adjusted for altitude. A table of corrections were then developed, based on the calculated altitude tables, to allow the sea level tables to be used. This is still a relatively crude technique and untested for the DCIEM model. It should only be used for altitude diving with no or minimal decompression (a very safe part of the DCIEM sea level tables).
Altitude tables developed by Dr. Buhlmann of Switzerland (up to 3,800 meters, 12,500 ft above sea level) and those derived from them are relatively safe to use at altitude. When it comes to decompression computers, they must be designed to be used at altitude. Your last option is to use PC based programs to develop custom altitude decompression profiles.
I am out of room in this column. Next time we will consider several other aspects of altitude diving including altitude acclimatization, altered ascent rates, altered decompression stop depths and what to do when you are going to change your altitude after the dive. Until then, have fun diving but remember to avoid altitude diving until you have been properly trained to do so.