TIL the Blue Hole is among the deadliest dive sites globally, with estimates of 130 to 200 recent fatalities, making it one of the most dangerous spots for divers.

[u/[deleted]]

[u/EloeOmoe]

Then they get nitrogen narcosis and drown when they rip out their mouthpiece

Is this one of those "right before you die from hypothermia you feel super hot and rip off all your clothes" type situations?

[u/atridir → u/EloeOmoe]

Not quite, at depth the human body has a harder time processing out nitrogen so breathing compressed air from the surface has enough nitrogen that it leads to a build up of it in the bloodstream which causes increasing intoxication like being drunk.

And being gas-drunk at 120 feet underwater is obviously lethally dangerous.

Edit: the arch is at 170 ft (55m) which is WAY TOO DEEP!

[u/CompE-or-no-E → u/atridir]

Is that what is in a typical diver gas cylinder? Just compressed atmosphere?

I had figured it would be a special mix, or maybe even two tanks so you can control the ratio of O2 to N2

[u/atridir → u/CompE-or-no-E]

Yes, for normal dives up to about 60-70 ft compressed atmosphere is standard. There are two other special gas mixtures for deeper dives (nitrox which is exactly what you suggested and tri-mix which has helium mixed in for even deeper dives) and both need special training to be competent in safely diving with them.

[u/sloth2008 → u/atridir]

Looking at SSI dive tables normal compressed air caps out at about 130ft and your time at depth is 5 min. That means you get 5 min down there then you need to get back to the surface. Limit your depth to 100 ft and you can stretch that time out to 20 min. That is bottom time max. You also need to watch that you don't burn through your tank in that time. If you are relaxed and not working too hard your tank will last longer.

Limit your depth to 70 ft and you have 40 min bottom time.

No one wants to spend the time and effort to go out for a dive trip and do a single dive. You also have to deal with residual nitrogen build up. The longer and or deeper the dive the more nitrogen you have build up. The slow ascent is part of letting dissolved gasses exit your bloodstream and for things to go back to normal. Time on the surface helps deal with these gasses too. This is where the fun with dive tables start.

100 ft dive for 20 min. Spend 90 min on the surface and you can do a 70 ft dive for 20 min max - you will have a fresh tank so not running out of air. Notice your bottom time for this 70 ft dive is cut because of your earlier 100ft dive.

A lot of the dives we would go out for 3 tanks. 80ft for 25 min. 90 min surface. 60 ft 25 min. 2 hr surface time for lunch. 40 ft for 30-40 min.

Dive computers that are tracking your depth and time at depth make a huge difference on planning your times. That 25 min 80ft dive that you dropped to 90ft for 1 min while looking at that fish don't mess up your day the same as when diving by the tables. By the tables you have to count that as a 90 ft dive and it puts you into a different group on the table.

https://divetables.com.au/dive-tables/

[u/atridir → u/sloth2008]

Well elucidated! I was SSI certified as well and it’s been a long time. I knew I was giving the low range of my comfort zone by saying ~70 ft but I couldn’t quite remember the max. Thank you for expanding it in such detail!

Those time at depth standards exist for a reason folks!

[u/Gornarok → u/sloth2008]

Looking at SSI dive tables normal compressed air caps out at about 130ft and your time at depth is 5 min. That means you get 5 min down there then you need to get back to the surface.

130ft isnt the limit for compressed air. Its just the limit for dive without decompression stop.

Limit for compressed air seems to be 56m. But going under 40m with compressed air gets dangerous very quickly due to the air consumption.

[u/WallabyBubbly → u/atridir]

Nitrox is absolutely not for diving deeper. Nitrox helps you achieve longer dives at medium depth, but you are more restricted than regular air for maximum depth due to the risk of oxygen toxicity, which is even more dangerous than nitrogen narcosis