Nitrox Diving at Altitude / Flying After Diving

By Dr. Sawatzky

The real fun when using nitrox at altitude is determining your decompression schedule (as you might have guessed). One ingenious technique is to calculate the nitrox mix which will result in the theoretical ocean air depth (TOAD) being the same as the actual depth of the dive at altitude. You have to think about that last sentence a bit. When diving at altitude, the equivalent ocean depth is always deeper than your actual depth at altitude. When diving nitrox, your equivalent air depth is always shallower than your actual depth. At a specific altitude, a specific nitrox mixture will result in these two calculations cancelling out (the theoretical ocean air depth will be equal to the actual depth at altitude). Therefore, if you use the correct nitrox mixture at altitude, it is the same as diving air at sea level as regards decompression. Then you can use your air computer (assuming it reads an accurate depth by adjusting for atmospheric pressure) or air tables on the nitrox altitude dive. The only additional calculation, as per any nitrox dive, is the maximum operating depth to prevent O₂ toxicity.

This technique is really a double conversion (unless you like math, you might want to skip this section). First you calculate the theoretical ocean air depth: TOAD = (1/altitude atmospheric 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 TOAD would be (1/0.801 ATA) X 70 fsw = 88 fsw. You then have to calculate the nitrox mixture that would give an equivalent air depth (EAD) of 70 fsw for a dive to 88 fsw: FN₂ = (EAD + 33) X 0.79 / (D + 33). In the above example: FN₂ = (70 + 33) X 0.79 / (88 + 33) = 0.67. Therefore, you would use a nitrox mixture with 33% O₂ (or more) on this altitude dive to 70 fsw and follow the sea level 70 fsw schedule (or use an air computer). In addition, the MOD must be calculated and a maximum pO₂ of 1.5 or 1.4 ATA is recommended. Table 1 gives sample values for this calculation with the additional conservatism of adding in the depth gauge correction and using the TOAD correction at 130 fsw (from IANTD's Altitude Diving Manual by Gary Taylor).

A second way to determine your decompression schedule is to calculate the TOAD for the nitrox being dived at altitude and use the resulting air decompression table. This is similar to the calculations above but the equations are flipped around. The equivalent air depth for the nitrox is: EAD = {[(FN₂)(D+33)]/0.79}-33. The theoretical ocean air depth is then: TOAD = (1/ATA at altitude) X EAD. For example, if we were diving nitrox 40 at an altitude of 6,000 ft, to a depth of 60 fsw, the EAD would be {[(0.60)(60+33)]/0.79}-33 = 38 fsw. TOAD = (1/0.801) X 38 = 47 fsw. Therefore, on this nitrox 40 dive to 60 fsw at an altitude 6,000 ft above sea level, we would use the 50 ft sea level schedule. Remembering again to stay well inside the no-decompression limits.

For both of these methods of determining your nitrox altitude decompression schedule, there are several limitations and other factors that you must take into consideration (see previous two columns). To quickly review they are altitude ascent rates, rounding rules (next deeper depth, next longer bottom time, next higher altitude), no decompression stops, max depth 130 fsw, pre and post dive altitude adjustments, adjustments for stresses and all the standard nitrox limitations.

The third option to determine your decompression schedule is to use special nitrox altitude tables.

Fourth, you can use PC based decompression programs that take nitrox and altitude into consideration to generate your own altitude nitrox tables. Three common programs are PRO Planner, Abyss, and Voyager. This is an excellent technique when you know your dive profile in advance. Remember that you always have to calculate tables for unexpected deviations from your dive plan.

Finally, you can use one of several currently available nitrox dive computers that also make the proper altitude adjustments. This is really the best way to go as you have true, real time, multi-level diving. Just remember that the only realistic backup for this kind of computer and diving is another identical computer.

To finish this discussion on altitude diving and using nitrox at altitude, I have to emphasize that these columns are NOT a replacement for full nitrox and altitude diving courses. Please take the relevant courses to ensure that you have the required knowledge and skills to do this type of diving safely.

Flying After Diving

Flying after diving is really just ascending to altitude after a dive. Therefore, many of the same techniques and concepts we covered in the previous discussion apply.

When we surface after a dive we have extra inert gas, usually nitrogen, dissolved in our tissues. The body will tolerate a certain ratio between the partial pressure of the inert gas in the tissues and the surrounding or ambient pressure. When we ascend to altitude, the ambient pressure is reduced and therefore, the ratio between the partial pressure of the inert gas in the tissues and the ambient pressure is increased. If this pressure ratio exceeds some preset limit, we risk developing DCS. There are several techniques to attempt to deal with this problems.

One technique is to calculate all dives within 24 hours of ascent to altitude as if they had been done at that altitude. Therefore, on your Caribbean dive vacation, you could calculate the dives on your last day of diving as if they had all been done at an altitude of 8,000 feet above sea level (the maximum cabin altitude in commercial jets).

Another technique is to wait until your repetitive factor (DCIEM tables) has decreased to 1.0. Another technique is to wait until your repetitive group (most other tables) has decreased to at least a "D" and preferably, an "A".

An easier and more reliable rule is to wait at least 12 hours before ascending to 8,000 ft (commercial jet) after a single no-decompression dive, 24 hours after multiple dives or decompression dives, and at least 48 hours after any diving before ascending to altitudes higher than 8,000 ft. Most modern dive computers will also calculate how long you must wait before flying in commercial jets. Conversely, after a week or so of hard diving, why not take the last day of your vacation off to do that shopping/sight seeing you have been putting off?

This concludes the discussion on altitude diving and flying after diving. It is a bit more complex and challenging than sea level diving, but it opens up a whole new realm of dives to be done.