Diving

[Skylark]

The issue of understanding basic physics and how the physiology of the body responds under pressure is all well and good but in this context is clearly becoming argumentative.

Like many others on here, I'm a diving instructor with a lot of dives under my belt.

Recreational diving is great fun and is very safe when conditions are appropriate and the divers are properly trained.

In my 20 years I've been involved with two incidents. The first was my buddy in the Red Sea; she suffured a burst ear drum in about 3 m of water. She was an experienced diver with a couple of hundred dives logged. She was completely disorientated by the incident and would have been inacapable, probably, of returning to the surface by herself. Just one of those things, it happens.

The second, I witnessed (and brought to the surface) a casualty who sadly didn't recover. He was a trainee in 4m of water.

I'm not going to preach, it's your life.

Under a keel at 2m you can easily become disorientated. While scrubbing, the viz will probably reduce to almost zero. You may be hyperventilating because of the work being done, this may make you feel light headed. Despite being at only a couple of metres, you will use a quite remarkable quantity of air (one of the reasons I'm against diminutive scuba devices as I've posted earlier). As others have suggested, your mask may start to leak or become dislodged and all of a sudden you're in trouble.

Take it or leave it, get trained. Simples

 

[tcm]

Physics: If the poster who thinks that being couple of metres down is like 250kilos on your chest would like to try the same - make really sure you can escape the weight! If 250kilos is ok (really?)...then try the 750kilos, yeehah! Quite a sizeable motorbike eh? Not too much further down before you get an equivalent weight of a car sitting on your chest. Of course, it's not *really* like this... because the analogy is total rubbish.

Latin : google "if in doubt anglicise" and/or use a z, lots of examples.

Other exmples: she had 200 dives - but *still* got all disorientated in 3m when summink went wrong. Sounds almost an argument against training! Cos surely the training should have kicked in the *moment* something went wrong.. and it didn't. Heyho.

Summary: With some common sense and care you'll be absolutely fine using scuba gear under the boat. I do it, loads of people do it. You'll also be fine jumping out of a taxi. However, a good friend somehow broke his leg whilst getting out of a taxi... but that doesn't mean he lacked training either. It means that sh!t happens.

BUT: This does not mean that all training is rubbish either - far from it.

HOWEVER : the notion that you need a full training course££ before even putting on a stab jacket and breathing compressed air at 2metres depth otherwise ooer massive danger is simply not true. Because pretty much the very first thing they do on a scuba course is erm, give you the gear and sit you at the bottom of the swimming pool. Which is all that the OP wants to do.

 

[fishermantwo]

Interesting how the trained scuba divers can screw up another thread. For some reason or other some one comes along in thread number 5 and mentions scuba. The original question was how to rub the slime off the side of a yacht. How many professional hull cleaners around the world use scuba?
Most people can snorkel and free dive several feet. The tricky part though is holding your breath and working under a boat, especially a work boat like mine. That can be disorienting and dangerous, especially at night. The answer is of course is a hookah unit. Widely used around the world except it seems in the UK.
My advice to the original poster is to buy a good face mask and spend up big on one of the new breed of snorkels. Short set of fins and dive belt etc. See how it goes and check out what the local hull cleaners do and go from there.
With my yacht I find its easier to nudge the keel into the sand on a nice shallow area and work standing up with goggles and snorkel.

 

[Alcyone]

Physics: If the poster who thinks that being couple of metres down is like 250kilos on your chest would like to try the same - make really sure you can escape the weight! If 250kilos is ok (really?)...then try the 750kilos, yeehah! Quite a sizeable motorbike eh? Not too much further down before you get an equivalent weight of a car sitting on your chest. Of course, it's not *really* like this... because the analogy is total rubbish.

Latin : google "if in doubt anglicise" and/or use a z, lots of examples.

Other exmples: she had 200 dives - but *still* got all disorientated in 3m when summink went wrong. Sounds almost an argument against training! Cos surely the training should have kicked in the *moment* something went wrong.. and it didn't. Heyho.

Summary: With some common sense and care you'll be absolutely fine using scuba gear under the boat. I do it, loads of people do it. You'll also be fine jumping out of a taxi. However, a good friend somehow broke his leg whilst getting out of a taxi... but that doesn't mean he lacked training either. It means that sh!t happens.

BUT: This does not mean that all training is rubbish either - far from it.

HOWEVER : the notion that you need a full training course££ before even putting on a stab jacket and breathing compressed air at 2metres depth otherwise ooer massive danger is simply not true. Because pretty much the very first thing they do on a scuba course is erm, give you the gear and sit you at the bottom of the swimming pool. Which is all that the OP wants to do.

I see you gave up on the maths then TCM. Didin't like what it was telling you eh?

 

[lw395]

Interesting how the trained scuba divers can screw up another thread. For some reason or other some one comes along in thread number 5 and mentions scuba. The original question was how to rub the slime off the side of a yacht. How many professional hull cleaners around the world use scuba?
Most people can snorkel and free dive several feet. The tricky part though is holding your breath and working under a boat, especially a work boat like mine. That can be disorienting and dangerous, especially at night. The answer is of course is a hookah unit. Widely used around the world except it seems in the UK.
My advice to the original poster is to buy a good face mask and spend up big on one of the new breed of snorkels. Short set of fins and dive belt etc. See how it goes and check out what the local hull cleaners do and go from there.
With my yacht I find its easier to nudge the keel into the sand on a nice shallow area and work standing up with goggles and snorkel.

For reasons I don't understand, the HSE don't like hookah set ups. Since the HSE is ourr commercial dive regulating body, pro hull cleaners don't use them here. I guess the HSE are more in tune with the requirements of north sea oil than with hull cleaning.

As for nudging the keel into the sand and standing up, excellent idea, but a little difficult in most UK locations due to the tide going in or our too much in the time it takes to clean a hull. Worth bearing in mind though if you can time it on the tide.
One technique for smaller boats, which sounds a bit silly but has its uses, is to get in the water alongside the boat wearing long woolly (football) socks and rub under the hull with your legs. I did say silly! You just need socks that go over your drysuit.... fender sox?

 

[tcm]

Fishtank maths for numpties

"I see you gave up on the maths then TCM. Didin't like what it was telling you eh?"

No, not at all! Your maths was so laughably rubbish and roundly destroyed elsewhere tat i'd left it out. But here goes.

You are alone in thinking that being a foot underwater "is like having a fishtank on your chest" or that "being three feet underwater is like having three fishtanks on your chest".

Those fishtanks were (you seemed to agree) 1 metre x .5 x .5 and hence contained 250kg of water. So a metre down it's like 750kilos, or so you think? And presumably 10mtres down it's like 7.5 tonnes and 40metres down is like 30 tonnes on your chest. I know quite a few people who have dived to 40metres, but none of them would have survived having the weight of 20 large cars on their chest.

We can make the fishtanks smaller if you like, perhaps just a foot cube - about a tenth the weight - so that being 40metres down is now like having just two large cars on your chest. Hm, i don't think so. No aspect of diving is like having a fishtank on your chest.

However, i can see that you're wedded to the idea of fishtanks on chest to illustrate pressure underwater. I wonder if you could possibly pack it in? It's definitely driving me further and further away from the idea of training - there a risk that the instructor will come out with this sort of rubbish.

I think you should restrict your mathematics to counting the flippers - try get that right before having a go at me...

 

[GarethW]

I would suggest popping along to your local BSAC club, explaining the problem. Offer them a day out on the boat in exchange for doing a dive to clean the hull.

Divers mostly love boats, and a chance to have a day sailing would be grabbed up, plus if you offer to provide lunch and a few beers you would be mobbed.

As a diver and resident in Holy loch with my 20' baby yacht. I can state I like red wine and polish lager (in no way suggesting I could be bribed to do a bit of hull cleaning )

 

[lw395]

It would be a bit like that, if your lungs were at surface pressure.
Pressure itself is not the issue, it's differences and changes in pressure.
Usually the issue is reducing the pressure, by surfacing. Whereas your lungs can be compressed without damage, they don't stretch very well. Hence snorkelling is pretty safe.
Your ears can be hurt either way.

The Victorians used to demonstrate this kind of thing by putting mice in bell jars and pumping the air out. I think the RSPCA would be banging on your door if you did that now, it wasn't good for the mice.

Other practical considerations are:
even a small current is a hard swim if you get separated from the boat
You need ropes to hang on to, which may bring dangers of entrapment
If you're on a pont
Make sure nobody is gonna start the engine, turn the wheel or use the bog.
Hitting your head on the prop when a wave moves the boat can hurt.
Getting back on the yacht can be hard, maybe best to de-kit in the water and get into the inflatable.

Take care, try not to damage yourself anywhere it's going to inconvenience me

 

[Skylark]

Other exmples: she had 200 dives - but *still* got all disorientated in 3m when summink went wrong. Sounds almost an argument against training! Cos surely the training should have kicked in the *moment* something went wrong.. and it didn't. Heyho.


Dear TCM, if you're having a pop at me I'd be grateful that you didn't. I stated clearly that it was an episode of burst ear drum. Disorientation is a consequence of such a barotrauma. You conclusion serves only to show your lack of knowledge on the subject matter.

I first registered to use this forum when I needed some help and advice. I’ve needed advice on a number of occasions since and I’ve never failed to be impressed, often humbled, by the quality of information flow. Many, many people give up their hard-earned experiences so freely to help others.

Most often, when entering a new port, there’s almost always a smiling face ready and willing to help take lines etc to make you feel warmly welcomed. One of the major attractions to sailing as a recreational pastime for me is the friendly and helpful people you meet along the way.

Sadly not true in all cases. This thread in particular seems to have brought out the rude, arrogant, argumentative, supercilious and, for me, most unwelcome replies so I’ll do myself a favour and not log-in for a while until the thread is dead and buried.

 

[Alcyone]

"I see you gave up on the maths then TCM. Didin't like what it was telling you eh?"

No, not at all! Your maths was so laughably rubbish and roundly destroyed elsewhere tat i'd left it out. But here goes.

You are alone in thinking that being a foot underwater "is like having a fishtank on your chest" or that "being three feet underwater is like having three fishtanks on your chest".

Those fishtanks were (you seemed to agree) 1 metre x .5 x .5 and hence contained 250kg of water. So a metre down it's like 750kilos, or so you think? And presumably 10mtres down it's like 7.5 tonnes and 40metres down is like 30 tonnes on your chest. I know quite a few people who have dived to 40metres, but none of them would have survived having the weight of 20 large cars on their chest.

We can make the fishtanks smaller if you like, perhaps just a foot cube - about a tenth the weight - so that being 40metres down is now like having just two large cars on your chest. Hm, i don't think so. No aspect of diving is like having a fishtank on your chest.

However, i can see that you're wedded to the idea of fishtanks on chest to illustrate pressure underwater. I wonder if you could possibly pack it in? It's definitely driving me further and further away from the idea of training - there a risk that the instructor will come out with this sort of rubbish.

I think you should restrict your mathematics to counting the flippers - try get that right before having a go at me...

You can do what you choose, tcm. Keep up with the schoolboy nonsense or actually discuss some maths.

Here it is:

The pressure at the surface is 14.7 pounds per square inch. The pressure at 1m is 10% more (you've already agreed this in post 65, where you say going down 2m increases it by 20%)

14.7 plus 10% is 16.17 lbs per square inch. or 1.47lbs per square inch more.

Your fishtank has an area of 1m X 0.5 m. That's 39 x 19.5 inches. That's 760.5 square inches.

760.5 square inches at 1.47 lbs per square inch is 760.5 x 1.47 = 1118 lbs

That's 508 kg.

You said If I was right it would be 250kg on a 1m x 0.5m x 0.5m tank, i.e. a tank that was 1/2 metre deep. So, for a 1m tank, the depth I am calculating for, it would be 500Kg. My calculations say 508kg.

So, what is it to be, schoolboy stuff, or show me where the maths and physics are incorrect? Of course, you could always just admit you are wrong - I've a lot of time for people who are big enough to do that.

When I first did these calculations, they didn't seem at all intuitive to me either, but they are right. Just think about the pressure, pounds per square inch, and you will see it.

Just off to water my cacti. Have a think and get back to me.

 

[Ariadne]

How much weight = how long is a piece of string sort of question.

If I'm free diving in bathers I use 1Kg, and it works for me. If I'm in my 5mm wet-suit I'll use between 6 & 8Kg to counteract the buoyancy of the suit and me. I'm 6' and about 13/14 stone in weight.

As suggested before they all work, I also use a 150mm wide scraper on a 1m long handle as well. I find that I can clean the boat in about 2 hours if I work at it.

 

[TQA]

I use an 8 inch wide flexible SS scraper attached at an angle of about 15 degrees [ this matters ] to an 8 foot pole.

Using a snorkle mask and flippers with the above I can scrape my 44 footer in about an hour.

I do not need to dive at all using the above, but just keep my head under water to see what I am doing and use the flippers to maintain station. The angle of the scraper matters and I can flip it over as work down the keel.

Now I just need to find something that will let me do the prop withiut diving.

 

[tcm]

diving physics explained

You can do what you choose, tcm. Keep up with the schoolboy nonsense or actually discuss some maths.

Here it is:

The pressure at the surface is 14.7 pounds per square inch. The pressure at 1m is 10% more (you've already agreed this in post 65, where you say going down 2m increases it by 20%)

14.7 plus 10% is 16.17 lbs per square inch. or 1.47lbs per square inch more.

Your fishtank has an area of 1m X 0.5 m. That's 39 x 19.5 inches. That's 760.5 square inches.

760.5 square inches at 1.47 lbs per square inch is 760.5 x 1.47 = 1118 lbs

That's 508 kg.

You said If I was right it would be 250kg on a 1m x 0.5m x 0.5m tank, i.e. a tank that was 1/2 metre deep. So, for a 1m tank, the depth I am calculating for, it would be 500Kg. My calculations say 508kg.

So, what is it to be, schoolboy stuff, or show me where the maths and physics are incorrect? Of course, you could always just admit you are wrong - I've a lot of time for people who are big enough to do that.

When I first did these calculations, they didn't seem at all intuitive to me either, but they are right. Just think about the pressure, pounds per square inch, and you will see it.

Just off to water my cacti. Have a think and get back to me.

Yeah, i though about it for about .3 of a second - tell me where to come round - i've got a car that weighs about 2.5 tonnes = 2500kilos which when i park it on your chest (according to your still-daft analogy) is gonna be just *exactly* the same as you being 5 metres underwater. That's 25 chunky people somehow balanced such their full weight is on your chest. And you're stll only 5metres down. So, with an experienced diver going down to (say?) 40metres - you think that's exactly the same as eight big cars or fully-loaded juggernaut parked with its full weight on your chest. So my car parked right on top of your chest whilst you're lying down will feel like a feather, i suppose? Of course it won't. You know there's summink wrong and hence you went on about some numbers - but if they look wrong they often are wrong.

As I hope you can see, your daft fishtanks analogy is hopelessly flawed - you are treating the situation as if the water above a diver is like concrete or ice, which of course it is not.

Here's the explanation you need to know so that you don't make this silly analogy to me or anyone else:

The human body is mostly water, and when immersed in a fluid the pressure exterted by all that water above them is exactly counteracted by the incompressibility of their bones and flesh. Otherwise (Newton's Laws, lookem up) there would be a net force downwards and they'd sink like a stone. Or they'd be crushed by the pressure all around them - and indeed at a certain depth the ribcage would cave in. Because there wouldn't be an equal an opposite reaction. The lungs are compressed but not too much - the ribcage acts like the walls of a submarine.

For this very same reason (ie the general incompressibility of animals due their fluid content) you may have noticed that when you go down on your diving expeditions, you'll see that the fish even quite deep down are exactly the same flippin size as when they're on the surface!

And likewise people don't get a bit bigger/smaler when atmospheric conditions are high/low pressure. We're very incompressible, just like fish - in the right conditions BUT NOT if you take us out of the fluid exerting that pressure and lie down with several large cars plonked on us - then we WOULD be crushed because that force is acting only above and below and nothing at the sides so blurgh crushed you very early on in the game of stacked-car-jenga that you seemed to be fine about - seeing as how you seem to be fine about the idea of 500kg as only a metre underwater.

The other thing is that your fishtanks physics ignored gravity (force = mass x acceleration due to gravity = mass in kilos x 9.81 ...and pressure = force divided by area ) and hence those fishtanks you were using are ten times bigger than they should have been. That's why you only got a C in physics earlier up the thread, sorry. I purposely used big fishtanks to see if you would catch this but you didn't.


SO! Please confirm that you won't use the fishtank-on-chest analogy again, and that it's completely wrong and misleading. Like you said, it takes a big man?...

I suppose the interesting/worrying thing is that it's me that's had to explain this to you, and neither you nor any diving instructors. Probably because they think - as you did - that it's much much more dangerous than it actually is.

 

[lw395]

tcm, you are making an arse of yourself

The ribcage is like the wall of a submarine indeed. Not quite exactly.

I feel you have missed the point.

Some of those fish which are the same size down at 40m, are known to die horribly as you bring them to the surface, their swim bladder expands and comes out the mouth I think. And fish are generally held to be better designed for the sub-aqua bit.

 

[Alcyone]

Yeah, i though about it for about .3 of a second - tell me where to come round - i've got a car that weighs about 2.5 tonnes = 2500kilos which when i park it on your chest (according to your still-daft analogy) is gonna be just *exactly* the same as you being 5 metres underwater. That's 25 chunky people somehow balanced such their full weight is on your chest. And you're stll only 5metres down. So, with an experienced diver going down to (say?) 40metres - you think that's exactly the same as eight big cars or fully-loaded juggernaut parked with its full weight on your chest. So my car parked right on top of your chest whilst you're lying down will feel like a feather, i suppose? Of course it won't. You know there's summink wrong and hence you went on about some numbers - but if they look wrong they often are wrong.

As I hope you can see, your daft fishtanks analogy is hopelessly flawed - you are treating the situation as if the water above a diver is like concrete or ice, which of course it is not.

Here's the explanation you need to know so that you don't make this silly analogy to me or anyone else:

The human body is mostly water, and when immersed in a fluid the pressure exterted by all that water above them is exactly counteracted by the incompressibility of their bones and flesh. Otherwise (Newton's Laws, lookem up) there would be a net force downwards and they'd sink like a stone. Or they'd be crushed by the pressure all around them - and indeed at a certain depth the ribcage would cave in. Because there wouldn't be an equal an opposite reaction. The lungs are compressed but not too much - the ribcage acts like the walls of a submarine.

For this very same reason (ie the general incompressibility of animals due their fluid content) you may have noticed that when you go down on your diving expeditions, you'll see that the fish even quite deep down are exactly the same flippin size as when they're on the surface!

And likewise people don't get a bit bigger/smaler when atmospheric conditions are high/low pressure. We're very incompressible, just like fish - in the right conditions BUT NOT if you take us out of the fluid exerting that pressure and lie down with several large cars plonked on us - then we WOULD be crushed because that force is acting only above and below and nothing at the sides so blurgh crushed you very early on in the game of stacked-car-jenga that you seemed to be fine about - seeing as how you seem to be fine about the idea of 500kg as only a metre underwater.

The other thing is that your fishtanks physics ignored gravity (force = mass x acceleration due to gravity = mass in kilos x 9.81 ...and pressure = force divided by area ) and hence those fishtanks you were using are ten times bigger than they should have been. That's why you only got a C in physics earlier up the thread, sorry. I purposely used big fishtanks to see if you would catch this but you didn't.


SO! Please confirm that you won't use the fishtank-on-chest analogy again, and that it's completely wrong and misleading. Like you said, it takes a big man?...

I suppose the interesting/worrying thing is that it's me that's had to explain this to you, and neither you nor any diving instructors. Probably because they think - as you did - that it's much much more dangerous than it actually is.

And your 2.5 tonnes of car is held up by air in your tyres at 30psi? About 2 bar? The same pressure as being 10m under water?

So air at 2 bar, 30psi will hold up 2.5 tonnes, yes? That's what a demand valve does. That's how a demand valve works. That's teh pressure of the air in your lungs breathing off scuba at 10m.

You can huff and puff and shout and scream all you like.

You just don't get it do you?

LW395 is right.

Hey, tell you what. Let's try a different tack!

You are 100% correct. Goodbye.

 

[tcm]

hm, poor show..

I do think you oughta agree that with even quite a small fishtank on your chest you'd be in a lot more difficulty than being a metre underwater. That's where the argument began. At least you've backed down although with rather bad grace. Heyho.

Your last incorrect analogy - tyres: Some snow tyres have about 10psi but the cars weigh the same or more. Or the tyres might be 100psi but the car won't float away, and nor will a bicycle with a mass of 15 pounds and tyres at 100psi. And those big lorries don't need tyre pressures of eight times that of a car (240 psi?) even though they weigh eight times as much. Again, the construction of the tyre (or ribcage, body tissue or submarine walls) prevent the disastrous results you and others suggest.

 

[mikemonty]

And your 2.5 tonnes of car is held up by air in your tyres at 30psi? About 2 bar? The same pressure as being 10m under water?

So air at 2 bar, 30psi will hold up 2.5 tonnes, yes? That's what a demand valve does. That's how a demand valve works. That's teh pressure of the air in your lungs breathing off scuba at 10m.

You can huff and puff and shout and scream all you like.

You just don't get it do you?

LW395 is right.

Hey, tell you what. Let's try a different tack!

You are 100% correct. Goodbye.

Air at 2 bar will hold up 2000 tonnes or 2000000 tonnes given a big enough area to operate over!
The 2.5 tonnes of car is (assuming a total tyre footprint of lets say 1600 cm2) generating a pressure on the ground of 1.56 kg/cm2
as long as the pressure in the tyre is above this plus atmospheric (ie approx 2.5 bar Abs) it will lift the rims off the ground (and before anyone says it - the 2 bar pressure in your tyres is GAUGE - ie not including the pressure of the atmosphere, it is really 3 bar ABSOLUTE).

The analogy really is worthless.

The demand valve works like a forecourt airline - allowing you to inflate your lungs to say, 1 bar gauge 2 bar abs at 10 metres or 3 bar abs at 20 metres. The only difference is that the valve is operated by depressing the pressure on the low pressure side - clever trick given that the water pressure must be balanced whatever the depth - and delivering a CONTROLLED flow of air reduced from tank pressure to somewhere about water pressure.

The difficulty for the diver is that coming from 10 metres to surface means that you either gotta lose a bar of pressure in your lungs or else expand your lungs to twice their normal volume - no real competition there!

However I'm not a diver (though I've been through the basic training) I'm an engineer, but I picked the following up from http://www.deep-six.com/page63.htm

"If diver takes a breath and then heads toward the surface without exhaling the lungs will overinflate. Each alveolus overinflates. Air breaks the thin walls of the alveoli and is forced directly into the blood capillaries. The air bubbles in the blood stream will travel out of the lungs and back to the heart. The bubbles may grow larger as the divers ascends. They may join together as well. The bubbles than move out of the heart to the body and some to the brain. In the brain the bubbles travel until they can go no further because the blood vessels get too small. The bubbles are trapped. Clots may form around them. The blood may stop flowing at that point and brain cells get starved for oxygen. If the brain cells start dying certain functions are lost. The diver has caused a stroke! This can happen in as little as 4' of water! "

So although there is a lot of mince being talked here - and as usual - diving topics always attract the doomsayers and breath-sucker-inners, there remains one abiding good point.
DONT HOLD YOUR BREATH.

 

[lw395]

All I can add is that the training is, IMHO, good value and very enjoyable.
Possibly less so if TCM is in your class. I don't think the instructors deserve that.

Diving to modest depths is pretty safe if you follow reasonable precautions, some of which were not entirely intuitive to me, and I have a degree in physics. (OK degree level physics is not focussed on useful things like keeping your boat clean).
It's also very good fun and going on a course or joining a club is sociable too. If you are going to buy some kit, you'll get better value for money if you invest a little in learning to use it. Bit like sailing perhaps?

The numerical stuff about pressure is all very well, but it might be more useful to think that a painfully loud ear damaging noise is noise is not many psi...

130dBA is comparable to about 66pA, or just under 7mm of water!
Your ears are quite fragile then.

Take care and have fun.

 

[Rum_Pirate]

For those that intend to dive and not get training and/or ignore good diving practice, I bid you permanent farewell (may you demise be quick and not drawn out and painful) to you forthcoming trip over the river Styx and please accept my sincere condolences and sympathy to your friends and family in advance.


PS There are bold divers and old divers, but there are few - if any - old bold divers.


PPS A trinidadian expression "May you be guided accordingly!".

 

[Alcyone]

Air at 2 bar will hold up 2000 tonnes or 2000000 tonnes given a big enough area to operate over!
The 2.5 tonnes of car is (assuming a total tyre footprint of lets say 1600 cm2) generating a pressure on the ground of 1.56 kg/cm2
as long as the pressure in the tyre is above this plus atmospheric (ie approx 2.5 bar Abs) it will lift the rims off the ground (and before anyone says it - the 2 bar pressure in your tyres is GAUGE - ie not including the pressure of the atmosphere, it is really 3 bar ABSOLUTE).

The analogy really is worthless.

The demand valve works like a forecourt airline - allowing you to inflate your lungs to say, 1 bar gauge 2 bar abs at 10 metres or 3 bar abs at 20 metres. The only difference is that the valve is operated by depressing the pressure on the low pressure side - clever trick given that the water pressure must be balanced whatever the depth - and delivering a CONTROLLED flow of air reduced from tank pressure to somewhere about water pressure.

The difficulty for the diver is that coming from 10 metres to surface means that you either gotta lose a bar of pressure in your lungs or else expand your lungs to twice their normal volume - no real competition there!

However I'm not a diver (though I've been through the basic training) I'm an engineer, but I picked the following up from http://www.deep-six.com/page63.htm

"If diver takes a breath and then heads toward the surface without exhaling the lungs will overinflate. Each alveolus overinflates. Air breaks the thin walls of the alveoli and is forced directly into the blood capillaries. The air bubbles in the blood stream will travel out of the lungs and back to the heart. The bubbles may grow larger as the divers ascends. They may join together as well. The bubbles than move out of the heart to the body and some to the brain. In the brain the bubbles travel until they can go no further because the blood vessels get too small. The bubbles are trapped. Clots may form around them. The blood may stop flowing at that point and brain cells get starved for oxygen. If the brain cells start dying certain functions are lost. The diver has caused a stroke! This can happen in as little as 4' of water! "

So although there is a lot of mince being talked here - and as usual - diving topics always attract the doomsayers and breath-sucker-inners, there remains one abiding good point.
DONT HOLD YOUR BREATH.

The point being that the demand valve is able to supply air to support and inflate the lungs. The pressure in the second stage is 10-14 bar, typically, so supplying air at 6 bar, typical for the BSAC 50m suggested limit is no problem.

What is important, as you allude to, is the pound per square inch or kg per square cm if you prefer. And that pressure, weight if you like, is far higher, at even relatuively shallow depths than the car on it's tyres.

But some simply cannot grasp that, whichever way it is explained to them.

The rest of your post relates to burst lung, air embolism, pneumothorax and interstitial emphysema, and all of the other injuries I referred to right at the start of this. Spot on.