Deep dives on air?

[Peter69_56]

I wonder how having a DM on the surface will help anything going on below 40m though....

When you do the extended range course you learn a couple of truths.

First is that you have to deal with any issues at depth. Going immediately to the surface is generally NOT an option unless you are at extremely high risk of dying if you don't (say no gas), but knowing if you shoot to the surface you will probably die anyway (due to significant deco obligation, thus blood will froth.

Second is that you resolve the problem with what you have at depth, and dive conservatively with the thought of as if you are diving solo (although you are not).

Third is that at every atmosphere of depth any issues are multiplied in complexity due to the depth, thus (for example) if you are at 100 m any problems become 11 times harder (generally speaking).

The only way anyone at the surface may be of help is if you can send up a "yellow" SMB with attached slate describing the problem (say low gas and in considerable deco). Hoping that someone on the surface will know something is wrong is a false hope.

 

[Gareth J]

BSAC Dive Leaders are qualified to dive to 50m on air, ALTHOUGH it is now strongly recommended that Trimix is used below 40m.
The BSAC did for a time have an Extended Range Divers course, this allowed rich Nitrox mixes, accelerated decompression and air dives to 50m. This course has now been dropped and the Advanced Decompression procedures course is probably the closest, however the recommended air limit is 40m. As with most organisations, there are now a series of Trimix courses, from Sports mixed gas through to Advanced Mix gas.

I think the modern wisdom is that air diving to depths beyond 40m (some will argue shallower), is very risk due to the narcosis. Those of us who have done deep air diving would in general agree. The mitigation in the old days was practice and kit familiarity. i.e. muscle memory. When things did go wrong, working a solution was very difficult.
The main issue with insisting on the use of Trimix beyond 40m say to 50m, (Sports Mixed Gas), is the availability (possibly cost), of Trimix. I have certainly been places where getting O2 was difficult, Helium was none existent.

There is a significant difference in warmer clearer waters than the darker, colder poor visibility waters of the channel and North sea.

Gareth

 

[Storker]

I think the modern wisdom is that air diving to depths beyond 40m (some will argue shallower), is very risk due to the narcosis.

As a more "advanced" diver than I am, have you considered gas density at depths beyond 35-40m and the risk of insufficient CO2 ventilation due to that?
Gas density guidelines

There is a significant difference in warmer clearer waters than the darker, colder poor visibility waters of the channel and North sea.

...and the waters north of the North Sea ;-)

 

[Gareth J]

Storker

I think that when deep on air, breathing resistance was noticeable, which must have been down to gas density. In truth, the higher performing regulators made this less problematic than some of my earlier equipment. I do believe, historically, a lot of the deep water black out that used to occur was probably significantly influenced by CO2 issues. Of course high CO2 significantly also increases the risk of Narcosis.

One of the things you soon learnt was to take things slow and steady. The mantra STOP and BREATH if there where any issues was one taught to me early on.

UK diving generally suffers from a number of contributing Narcosis factors. Generally its cold, dark. Add the potential for very strong tides which increase work effort and the risks of narcosis even with very experienced divers is high.
That said I have done numerous dives in the UK where the visibility has been excellent and the light penetration superb, Hand Deeps in August was stunning even at 50m - you would have easily thought you where at 10 or 15m.
The off shore dives, although deep generally have far better water visibility and light penetration than the shallower inshore dives.

The big issue for me is its all well and good when things are going well its when you are dealing with problems that the issue of Narcosis really bites. The ability to think clearly is compromised. Task narrowing can increase the risk and danger significantly. This is without the problems of fear that narcosis increases and multiplies.

I can remember my first deep dive on a UK wreck using Trimix, I was stunned at the amount of fishing line on the wreck, something I had never 'seen' before on the numerous dives I had completed on her using air. That was a stark warning on how much you are compromised when breathing air at depth, rather than a less narcotic gas.

Gareth

 

[Storker]

UK diving generally suffers from a number of contributing Narcosis factors. Generally its cold, dark. Add the potential for very strong tides which increase work effort and the risks of narcosis even with very experienced divers is high.

I think I know what you're talking about. I've set my personal depth limit on air - or nitrox - at about 30m, since I often notice oncoming narcosis symptoms somewhere between 25m and 30m. Whether that's due to my narcosis susceptibility, my ability to notice oncoming narcosis or the conditions I usually dive under, I can't say. Probably it's a combination. I've broken my 30m limit in warmer, clearer waters without noticeable narc symptoms, and I've experienced both myself and my buddy being close to freaking out (the proverbial "dark nark") at less than 30m depth.

It's not possible to attribute these experiences to one factor only, but I believe that both conditions (cold, dark, limited visibility) and physical factors (gas density, insufficient CO2 ventilation) probably have contributed.

 

[Gareth J]

The problem with Narcosis is it is hugely variable, even with experienced divers.

I used to dive a lot with a buddy, Ian, we had done some deep and long dives together over the years. We did a club exped to Scapa Flow. We spent the week diving the WWII German High Sea Fleet in the flow. The last dive was on the James Barrie, which is a trawler that sank on the approaches, in about 40m-50m (140ft). It wasn't that much deeper than the Margraf that is nominally 42m, which we had dived earlier in the week. The big difference is that it is in the hugely tidal approaches.
We dived the wreck quite well, but I could tell Ian was a little unsettled through out. At the start of the ascent he failed to deploy a DSMB cleanly, and then indicated he was in trouble. I sort of dragged him up my SMB line, leaving the reel below me.
Once we hit about 30m he relaxed and was back to normal. We sorted my reel out, did the decompression required and a little bit more and finally got out.
On the boat he said he was 100% certain he was about to die, was convinced he was descending not ascending. As soon as we went through 30m his head cleared and he couldn't work out what had come over him.

Gareth