UKC

I was just watching a docu on diving in the Amazon, it gets very deep, but not as deep as free divers go. A couple of them had to go on oxygen and were at risk from surfacing too quickly. I seem to remember when I did some diving about 40 years ago always being told to come up at the rate of the pea sized bubbles.
I could probably google this and get loads of technical info, but my question to you Blue is how do free divers manage to ascend so fast? Is there any danger of 'bends' when free diving or is it just because they don't stay down very long?

In reply to Al Evans:

I have no experience of this... but I think it is to do with the time spent down. A free diver presumably won't spend very long down there.

In reply to Al Evans:

I would also assume that it is to do with the pressure at which they inhale the air. When you inhale at 40 metres the pressure of the air in your lungs and therefore going into your blood stream is not the same as at the surface. I believe it's to do with this but not dived for a while and would have to check my books.

In reply to puppythedog:

95% correct. The other 5% of correctness is in the second post.

i.e. mostly, freedivers can't succumb to the bends because the air pressure in their lungs never exceeds 1 atmosphere (whereas a scuba diver at 40m would take a breath of compressed air that is regulated to equal the surrounding water pressure, i.e. 5 atmospheres).
So the freediver at 40m can continue to hold that breath all the way to the surface and their lungs (which have been compressed at depth) will expand as the air expands, but will expand only to 1 atmosphere.
If the scuba diver holds that breath and ascends to the surface, the air in their lungs will want to expand to five times its volume. The lungs can't accommodate this so they would tear. Therefore the scuba diver should be exhaling (or indeed continue to breathe normally). Even at that, the scuba diver must be careful to make a slow ascent because some of the air would (due to time at depth under compression) have made its way into the bloodstream in solution or as tiny tolerable bubbles. I'm not sure why, but it just does. When pressure reduces, that air in the blood expands. This causes the "bends". So a slow ascent allows that air in the blood to return to the lungs.

Top level freedivers who are diving deeper than around 80m can experience this due to the time they spend under a lot of pressure. 80m+ dives tend to take more than 3.5 minutes, and for two of those minutes they are deeper than 40-50m. This can cause air to be absorbed into the blood.

They don't make a slow ascent, given that they are holding their breath, but after surfacing and recovering they dive back down to around 15m and breathe compressed air and make a slow ascent like a scuba diver. This tends to solve the problem as the air in the bloodstream stays absorbed for the short surface interval. It's a bit like a micro version of a scuba diver going to a decompression chamber.

In reply to Blue Straggler:

Interesting stuff. Just to chime in on a couple of things as a physicist...


My first comment is a bit of a pedantic one, doesn't change the intent of what you wrote, but the air pressure in their lungs *does* go up, to e.g. 5 atmospheres. However since you only had one lungful of surface-pressure air to start with, the volume goes down and the number of molecules stays (near enough) the same. i.e. you don't have "more" nitrogen to cause problems. Contrast this with taking a lungful of 5 atm scuba air at 40m depth, and you will have 5x as much nitrogen in your lungs, i.e. 5x as much to get busy dissolving into your bloodstream, and more importantly this is constantly being replenished with every breath you take, giving yet more to cross into your bloodstream


This is all because at higher pressure *more* nitrogen can (and will) dissolve in water than at 1 atm. In scientific terms the partial pressure of nitrogen is higher ("there are more molecules about"). When you ascend to lower pressure, the water can't hold it any more, and it comes out of solution. This is exactly the same effect as opening the top of a bottle of fizzy pop - you release the pressure and the gas comes out of solution.

In reply to jonny taylor:

Good stuff and not pedantic at all...thanks for elaborating on my rushed and slightly sloppy response! I even contradicted my own opening statement. Freedivers CAN succumb to the bends but it's much rarer. We more commonly get a mild narcosis

In reply to Blue Straggler:

after surfacing and recovering they dive back down to around 15m and breathe compressed air and make a slow ascent like a scuba diver. This tends to solve the problem as the air in the bloodstream stays absorbed for the short surface interval. It's a bit like a micro version of a scuba diver going to a decompression chamber.

I didn't know that, thanks all for answering my question.

In reply to Blue Straggler:




They do? I'v never heard of this, and I dont see how it would work. The diving manuals I'v read explicitly say that ISNT a suitable way to deal with a rapid ascent or suspected DCI. where did you hear about this? I'm very curious and happy to find out I'm wrong!

In reply to pebbles:

where did you hear about this? I'm very curious and happy to find out I'm wrong!

Blue Straggler is a free diver, that's why I directed my question at him, he should know.

In reply to pebbles:

Sorry, I don't know the details as I am not a hardcore deep freediver so I've not bothered to learn the technical aspect of this. But it is done, I've seen it - that's where I "heard" about it.
It's commonplace to the extent that if entering a depth competition and planning to make dives deeper than - I think - 60m, the competitor has to pay an additional fee for the scuba tank. I think 60m is a bit shallow for this requirement but I guess the organizers prefer to play it safe. Or I misread and it's more like 80m.

In reply to Blue Straggler:

ok fair do's!

In reply to Blue Straggler:





It's not to do with the expansion of the air (that would cause lung rupture rather than the bends), but in fact due to the partial pressures of the gases. Gases at higher partial pressures dissolve in liquids (ie. blood and lymphatic fluid etc) more readily than at lower pressures such as on the surface. So the the longer spent at depth where the partial pressures are higher, the more gas dissolves into your body's fluids. If you surface too quickly these gases come out of solution as bubbles (causing the bends as air bubbles can travel to your brain in capillaries and block them) rather than coming out through the blood vessels in your lungs.

In reply to crayefish:


That's what I said ("the lungs can't accommodate this so they would tear") but thank you for elaborating on my sloppy comment that "some of the air would have made its way into the bloodstream..."


Here is a very useful article, there is some freedive-specific jargon in there but the gist is clear.

http://www.freediving.biz/education/DCSapnea.html

In reply to Blue Straggler:

Ah ooops, I missed the tear comment! But just wanted to clarify the bit about gas dissolving.

Being a scuba diver, I can't tolerate all this relaxed sloppiness from you free divers Nothing to do with jealously that you can go deeper! lol

In reply to crayefish:

Last year was the first time I dived deeper (43m) on a freedive than my max scuba depth had been, I was well chuffed innit.
Haven't even had a reg in my mouth since summer 2000

In reply to Blue Straggler:

Bastard! lol. I can only dream of that until I can afford to take up tec diving (as if I need ANOTHER stupidly expensive hobby in my life). Though did pop down to 41.5m once chasing after a thresher shark... towards the end of the dive while on nitrox 32. Gulp! Needless to say I had to do an unplanned deco stop and my 02 alarm was telling me I was dead

In reply to Al Evans:

I recall a story that was on telly.
These two divers had been exploring a long cave system, (picture a flooded 100m deep shaft - the numbers might be wrong but its just a sketch, and at the bottom there's a mile of horizontal shaft)

The logistics of taking bottles and dropping them off at preset points along the system would tax even Bonington, but over several weeks they ventured further and further.

To cut a long story short, one day they arrived back at the foot of the shaft, and realised they did not have enough air for even one of them to proceed slowly up the shaft without dying.

So they had a think. Eventually, one guy shot to the surface, screamed 'BOTTLES!' and immediately descended to the foot of the shaft again.

A support guy was at the suface, so he started chucking air bottles down the shaft, which they then used for the ascent, which took a few hours.

If anyone knows any links to the story, I'd be grateful.

In reply to crayefish:

My 42m scuba involved chasing a shark too. It was just on air, no alarms, standard deco according to computer and common sense.
Are we no longer supposed to dive to 42m on plain air? I'm somewhat behind on scuba no-nos, having lost interest in the activity!

In reply to Fredt:

Cool story!

I used to be a shellfish diver on the Cornish Coast and a friend who I worked with for a short period told me a good one as well:

On a scallop boat he had previously worked on, he was out one day acting as surface support for one of the dive crew (mainly collecting the lift bags full of scallops as they came to the surface etc).

He picked a bag out of the water and discovered that it contained no scallops but instead the diver's tanks and regulator setup.

He'd had a malfunction with some part of his kit which had rendered it unusable, and instead of rising to the surface on a single breath and risking DCI etc. had strapped his tanks to a lift bag and was holding his breath on the seabed!

They got him back safe although I believe with a mild case of the bends...

In reply to Blue Straggler:



Sharks and mantas are the universal reason for everyone going beyond the recommended limits

42m is fine on air - I think around 50m is the limit if you assume 1.4 is the max PP of O2, not withstanding getting narked. Though I know a Cuban instructor who did 70 on air (monitoring free diving funnily enough!). If you stick to the 1.4 limit then you should not go below 30m on Nitrox 32, though I usually dive to a 1.6 limit unless I am hungover; even then around 38m or something is the max before your comp thinks you're going to die. Though some US navy divers go up to 2.0 though I wouldn't recommend that as you are literally on the edge of death. lol