[ddls]

I'm an experienced diver (PADI dive-master, ACUC instructor, IANTD gas-blender and normoxic trimix diver, TDI hypoxic trimix diver and dive-master) with several thousands of dives of which many at 100m+ (330 feet) depths. I lived in Dahab where for 16 months I assisted training technical diving instructors in the blue hole and at other less touristy dive sites around there. The blue hole is in not a particularly dangerous dive site, but its popularity attracts the most cocksure types trying to prove something. The main attraction at the blue hole is the arch. It's a 40 meter archway who's apex is at 56 meters and leads from the cylindrical blue hole out into "the blue", where depths are insane and theres nothing but water anywhere you look. It's very beautiful, especially while the sun is rising, as it faces East. 56 meters is not particularly deep, but it's deep enough for any beginner and even most advanced divers to get seriously narced (drunk on nitrogen). It's also _just about_ deep enough for the partial pressure of oxygen to reach a critical point where oxygen becomes toxic. When it does, your muscles try to burn off the excess oxygen and you temporarily lose control of them, they shake, so your chin and cheeks start to twitch and you can easily drop your regulator. When the twitching is over, you involuntarily take a deep breath...

If you're an experienced diver, know your narcosis limits, know your oxygen toxicity limits, know your air consumption, havn't had a drink the night before, and are physically fit, maybe you can pull it off without trimix. Otherwise, it's an absolute beginner's dive with the right gas blend.

[ghostpepper]

There's a short documentary about it on youtube where (IIRC) they claim most of the fatalities are people who try to swim through the arch on a single standard aluminum 80 tank.

[Stevvo]

A divemaster once told me that the best training he ever had for recreational diving was doing a technical diving course, because knowing your limits was no longer something optional.

The written test for open-water felt inadequate to me; I expect most reasonably smart people could pass it without any study, what with only needing 75% on multiple choice.

[Apofis]

Question: Why are divers injecting this trimix?

[ddls]

I don't recommend anyone "inject" trimix ;) But jokes aside : Trimix is an (artificial) gas blend that includes helium, which is an inert gas that your body doesn't metabolize (though their is still a lot of unanswered questions about its effects below 250 meters). Your body metabolizes gases (normally only Oxygen) according to their partial pressures in your body. The partial pressure of Oxygen at sea level is just 21% (there's an atmospheric pressure of 1 Bar and air is 21% Oxygen, 78% Nitrogen, and 1% Aragon and other weird stuff), but as you dive deeper into the water and the ambient pressure rises, so does the partial pressure of Oxygen and Nitrogen. Oxygen becomes toxic around (to be safe) 1.2 ppO2 and Nitrogen causes narcosis at a pp that varies greatly from diver to diver. As a rule of thumb, somewhere between 40 and 60 meters is where things get dangerous. So, to reduce the partial pressure of these gases, Helium is introduced, reducing the proportion of the metabolized gases in the total mix, and allowing the diver to go deeper before the critical thresholds are reached. To dive below a certain depth (about 60 meters), a mix is required where the proportion of Oxygen is less than the minimum 17% Oxygen at 1 Bar required for your body to function. We call these mixes "Hypoxic Trimix". A typical deep technical dive involves switching gases (different tanks) at least three times during the dive, and grabbing the wrong regulator is a deadly mistake. Technical diving is not for the distracted. It is, also, the slowest and most meditative rush you'll ever experience. Add overheads and caves and it really does feel like an excursion to an other planet. Speaking of space travel : more humans have been to space than below 200 meters.

[nradov]

That's mostly correct but your body doesn't metabolize oxygen based on the partial pressure. The actual rate of oxygen metabolism depends mostly on activity level regardless of depth or PPO2.

Your body can function pretty well at PPO2 levels less than 0.17 bar. People go to an altitude of 10000 ft / 3000 m all the time where PPO2 is only 0.14 bar.

[ddls]

0.17 is a rule of thumb we use in diving. There are many like it which allow a little leeway in case there's a slip-up. pp02 can surely go higher than 1.2 (I know personally) before causing problems in some people. That said, I bet if you're climbing at 0.14 bar that you've stashed a pony bottle of 100% oxygen somewhere in your kit, just in case, no ?

[nradov]

People live and work just fine at 0.14 bar PPO2, no supplemental oxygen required. Aerobic capacity will be reduced. 10000 ft is the usual limit for requiring supplemental oxygen in aviation.

[Anther]

Thank you for sharing your experience. It sounds like a fascinating subject.

[detaro]

Trimix is a different breathing gas mixture with added helium, to avoid the mentioned problems with too much nitrogen and too much oxygen.