Diving the Dahab Blue Hole on Trimix

I'd been diving at the Dahab Blue Hole a few times, but with the intention of diving the arch, I decided to enroll in a TDI Trimix course with Team Blue Immersion in Dahab. First, though, let's explain, in simple terms, why trimix is a good idea for this dive. A term familiar to all tec-trained divers is Maximum Operating Depth (MOD), which means the maximum depth at which a gas mix can safely be used. Any deeper and the partial pressure of oxygen (or pO₂) exceeds a safe limit, which simply means that by exceeding the given MOD of a gas mix, a diver risks acute oxygen toxicity. The standard gas mix for scuba divers, which we call air, contains approximately 21 percent oxygen, and has a MOD of 184 feet (56 m). Nitrogen narcosis is also a problem, caused by the increased concentration of nitrogen in our tissues the deeper we go. A dive to the arch, deeper than 184 feet, is therefore beyond the limits of a standard oxygen mix.

So, how does trimix make diving deeper possible? As you may have guessed, "trimix" means three mixes, which is to say three breathable gases in your tank. Along with the standard nitrogen and oxygen, helium is added to the blend to create trimix. The mixture contains lower oxygen and nitrogen percentages, thereby reducing narcotic effects and increasing MOD. Helium makes a good breathing gas because it's less narcotic than nitrogen due to its low density, which also makes it easier to breathe. And, as an inert gas, it doesn't interact with any other chemicals. On the downside, it can reduce body temperature.

So, in order to enjoy the beauty of the famous arch at 197 feet (60 m), I needed to take a trimix course. The TDI course has the following prerequisites:

• Minimum age 18
• Minimum certification of TDI Advanced Nitrox and Decompression Procedures Diver, or equivalent
• Proof of 100 logged dives

As anyone who has completed a well-taught tec course will know, the beginning of the course is always a mixture of in-the-water drills, repeated until perfect, and dry-land theory, which includes a lot of math. Don't let that discourage you, however, as it's fairly simple. During the course, students refresh their tec knowledge and learn a few new things as well. The course covers the following, among other topics:

• Gas planning based on equivalent narcotic depths
• Nitrogen and helium absorption and elimination, CNS and OUT limits, isobaric counter diffusion.
• Decompression gas choices
• Emergency and contingency planning (equipment failure, omitted decompression, etc.)
• Decompression diving procedures
• Proper trim, buoyancy and finning techniques
• Management of multiple decompression/stage cylinders
• Emergency procedures (equipment failures, catastrophic gas loss, omitted decompression, navigational errors, injured/unconscious diver, etc.)
• Equipment considerations, cylinder labeling, analyzing trimix nitrox and mixes, and gas-blending procedures

The average day during the five-day course meant skills and drills in the morning, theory and diving planning in the afternoon, a beer over dinner, and early to bed. Many people find the rescue-diver course tiring as it's quite physical at times, but the trimix course added mental challenges to the mix as well.

The training leads up to the final dive of the course, a 197-foot (60 m) trimix dive through the arch at the Blue Hole. With the excitement building, the evening before was dedicated to planning out the dive, writing up the plan on our slates and wet-notes, analyzing the gas blends, labeling tanks, and preparing equipment for our early-morning dive. The 20-minute drive to the site gave us the opportunity to go over our dive plan and mentally prepare. Earlier is better when it comes to the Blue Hole, as there are fewer snorkelers and other divers, and the light gives the arch an eerie, deep-blue color as it comes into view. Our dive plan gave us three minutes to reach 197 feet from the signal to descend, and what an amazing three minutes it was, spent in the trim position, free-falling through the water as the arch came into view and we started to slow the descent rate. Coming to a stop and holding position at 197 feet, we entered the arch. With our plan allowing for 14 minutes of bottom time, we could complete a swim-through of the arch and take a good look around, catching some larger fish silhouetted in our torch beams. As the last minute approached, we prepared ourselves for the ascent and the start of our decompression. As I still had one vacation day left, we dove the arch again the next day from another entry point outside the Blue Hole, but this time I was a fully qualified TDI Trimix diver.

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