Nitrox and Diving at Altitude
The laws of Richard Boyle and John Dalton describe how air pressure and density, respectively, vary when we dive, and that of William Henry how gasses get absorbed in liquids and tissues under pressure. Nitrogen absorption and release rates affect the no-decompression bottom times and are therefore extremely important to divers. The whole pressure picture changes at higher elevations because the air is less dense at altitude.
We can all agree that the pressure at the surface, at sea level is one atmosphere, or 14.7 pounds per square inch. Course materials then show how a certain volume of air is compressed to half at a depth of 33 feet because the additional pressure of the water on top of us adds another atmosphere, or 14.7 pounds per square inch, for a total of two atmospheres, or 29.4 pounds per square inch. At 66 feet the same volume of air is subjected to three atmospheres and has compressed to one third of its original surface volume. Conversely, if you blow a certain amount of air into a balloon at a depth of 66 feet, that volume will double once you get back up to 33 feet and triples at the surface. This is an essential part of understanding diving physics.
However, does this hold true at altitude? Let's think this through with the example of one of my favorites places to dive, Flaming Gorge, which is at 6040 feet above sea level.It would seem that once you are fully acclimated to the altitude, theoretical and actual depth should be the same. At Flaming Gorge you start out at a surface air pressure of roughly 0.8 atmospheres (0.801)
US Navy Dive Manual 6
If you then dive down to 99 feet you would add another atmosphere's worth of pressure for each 33 feet, for a total of 3.8 atmospheres. Then you go back up to the surface where the pressure is once again 0.8 atmospheres. So the pressure difference between surface and 99 feet is three atmospheres. At sea level you would go from 1.0 atmosphere at the surface to 4.0 atmospheres at a depth of 99 feet, and then back up to 1.0 atmosphere, for the same pressure difference of three atmospheres. If anything, when diving in Flaming Gorge you have less total pressure on top of you at 99 feet than at sea level (3.8 ata instead of 4.0 ata) where you would reach 3.8 ata already at 93 feet. So why do the altitude tables show that 99 feet at 6000 feet corresponds to an ocean depth of about 125 feet and not 93?
While the calculation above is correct, it does not address the problem we're trying to solve. The problem is the absorption of nitrogen, and that means we need to think in terms of pressure ratios and not pressure differential. At sea level, the pressure doubles at 33 feet compared to the surface, triples at 66 feet, and quadruples at 99 feet. Now look at Flaming Gorge where the surface pressure is only 0.8 atmospheres. That corresponds to 26.4 feet of water. So at an altitude of 6000 feet, the pressure doubles at 26.4 feet, triples at 52.8 feet, and quadruples at 79.2 feet. This means that as far as nitrogen on gassing and off gassing goes, you need to divide actual depth by surface pressure to arrive at theoretical depth.
Equivalent Depth (fsw) =
Altitude Depth (ffw) x Pressure at Sea Level (ATA)
Pressure at Altitude (ATA)
At 6000 feet above sea level, four times surface pressure is reached at 79.2 feet whereas at sea level four times surface pressure is reached at 99 feet. Henry's law states, "At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid." The way gasses dissolve and expand in liquids and tissues in the body is very complex, but if for simplicity's sake we assume that nitrogen bubbles expand as we ascend to the surface, those bubbles will expand to four times their size from a 79.2 feet dive in Flaming Gorge and from a 99 feet dive at sea level. That is why we need altitude conversion tables (click here to download a .pdf altitude conversion chart.)
It should be obvious by now that altitude conversion tables are used to stay within safe decompression limits. So they handle the nitrogen side of things, but what about oxygen? That is an entirely different ballgame. As anyone who has taken an Enriched Air Diving, or Nitrox class knows, high oxygen partial pressure can become a problem. Oxygen toxicity can occur when oxygen partial pressure exceeds certain values. This can lead to convulsions and loss of consciousness, and thus quite possibly drowning. The recommended oxygen partial pressure limit is 1.4 ata during the working part of the dive. The 1.4 ata limit is used by Nitrox divers to compute the "MOD," or "Maximum Operating Depth" of a Nitrox mix. The formula used to compute the MOD is:
MOD = (1.4 / oxygen percentage x 33) – surface pressure in feet of water.
If we dive 32% Nitrox with a ppO2 of 1.4 at sea level, the MOD is 111.38 feet. But what would the MOD be if we dive 32% Nitrox at 6000 feet where the surface pressure is only about 0.8 ata? The answer is 118 feet! Yes, the MOD for diving EANx32 at that altitude is actually 6.4 feet deeper than at sea level. In fact, those 6.4 feet apply to all Nitrox percentages. That's because oxygen toxicity depends on pressure and not on pressure ratio.
As far as pressure goes, 99 feet in Flaming Gorge is only as much as 93 feet in the ocean. But as far as nitrogen uptake goes those same 99 feet are like 125 in the ocean.
If we dive air, oxygen toxicity is rarely an issue. At 6000 feet, an air diver would not reach the 1.4 ata partial oxygen pressure level until a depth of almost 200 feet, far deeper than the recommended recreational diving depth limit. For Nitrox divers, however, the MOD can become an issue. At 6000 feet in altitude, the MOD for a diver using 36% Nitrox is about 103 feet, and that is actual feet, not altitude adjusted feet.
What about altitude adjustment for Nitrox divers? Can they use the same altitude tables? Not directly. Nitrox divers know that in order to use regular dive tables, we need to first calculate the equivalent air depth, or EAD. The formula to compute EAD is:
EAD = [(1-fO2) x (depth+33)] -33
You could generate Nitrox Equivalent Air Depth tables in a spreadsheet program with this formula, and then use those values to generate a second table that shows Equivalent Air Depth for a given percentage Nitrox at a given altitude, and then use those twice adjusted depths to look up maximum no-decompression bottom times in standard dive tables. What you'll find is that using Nitrox at altitude almost cancels out the effect of altitude: At altitude the equivalent ocean depth is deeper than the actual depth as far as nitrogen goes. But with diving Nitrox the equivalent air depth is always shallower than the actual depth as far as nitrogen goes. Depending on the altitude, diving with a certain mix of Nitrox means you can use the standard no-decompression sea level air tables. For diving Flaming Gorge, for example, using 34% Nitrox gives you about the same bottom times as diving air at sea level.
Many divers rely on their personal dive computers for planning dives as an alternative to consulting dive tables. It is every diver’s responsibility to understand the information being provided, since a number of dive computers use different algorithms.
Disclaimer: This is informational only and should not be used for dive planning.