We plan to conduct several dives to the SS Tahoe encompassing a variety of different depths and decompression profiles. Our team will be utilizing a variety of gas mixes during the dive with "Trimix" (a mix of oxygen, helium and nitrogen) as the gas we will breathe while on the wreck itself. During our ascent, we will breathe a variety of gases starting with another trimix, moving to air enriched with oxygen and finally to pure oxygen. In order for the divers to conduct the proper rate of ascent, make the proper stops for decompression and select the appropriate gases to breathe, they will be utilizing Departure, customized decompression software. Departure was designed by Altitude Concepts located at, and doing high altitude research in, Lake Tahoe for over 15 years. For more on Departures deep-stop algorithms, altitude capabilities and customization features, visit Departures website.
As mentioned earlier, ascending safely back to the surface is quite a complex task. Decompression tables and schedules for sea level in and of themselves are very sophisticated and detailed. Heighten that with the reduced atmospheric pressure present at altitude and the extremes of 39-degree water temperatures and our task becomes much more intricate. Altitude and cold water must be considered when developing decompression schedules for these dives as they will lengthen the decompression times when compared to dives of the same physical depth at sea level. As an example a 100-foot dive in Lake Tahoe is equivalent to a 128-foot dive in the ocean. Thus, and depending upon what dive table you use, a 100-foot dive at Lake Tahoe has a no-stop time limit of 10 minutes where a 100-foot dive at sea level allows you 25 minutes before having to make stops prior to surfacing. Therefore during New Millenniums expedition, a dive to the stern of the SS Tahoe in 463 feet of fresh water is equivalent to 584 feet in the ocean.A look at one of New Millenniums planned expedition dives: First, the team uses ultra conservative settings due to the complexities mentioned above. This requires a bit more gas but stop diving at this altitude has never been conducted and so we are presented with a variety of unknowns. Unknowns that are basically eliminated by implementing our conservative form of dive planning. On one of our expedition dives, the team will descend to the middle of the SS Tahoe (a depth projected to be about 415 feet) and move shallower toward the bow and then back up. We are planning a total bottom time (time from the second we begin our descent until we begin our ascent to our first decompression stop) of 10 to 12 minutes. In selecting our bottom gas - 11% oxygen, 63% helium and 26% nitrogen many factors were considered. Factors such as oxygen content, narcosis levels, and decompression obligations of the gasses were taken into account. Our approximate ascent time, after the 10 to 12 minutes spent dropping down and exploring the SS Tahoe, will be between 2 and 3 hours with our first decompression stop as deep as 270 feet. Quite a commitment just to see a sunken ship. However, the excitement and pride resulting from seeing this ship for the first time in 60 years is worth every second of our delay back into earths atmosphere. Now, one might inquire as to why the various gasses and their percentages were selected. Well the latter is determined by much theory and gas law calculations to detailed to explain here but choosing the various gasses is quite easy to explain. Nitrogen in the air you breathe is narcotic under pressure. Keep in mind that at 375 feet, the pressure upon the diver is 12 times greater than the pressure of the atmosphere at the surface. Therefore, we reduce the levels of nitrogen and replace it with helium. Why? Helium does not have the narcotic effect. But for every give you must have a take. While our thoughts will stay focused, the increased helium may tend to increase our total decompression time when compared to nitrogen. Simply put, we would rather take longer coming up than risk making a fatal mistake while at depth due to a mental imparity. Once on the surface, the team will have a minimum 48-hour surface interval prior to the next expedition dive.
In simple terms, New Millennium has a tremendous amount of
complexity, coordination and organization ahead. Stay tuned
For more detailed info on decompression diving, you can read Ascending From A Dive.