Diving

[mattnj]

Every time the notion of diving under the boat comes up, some diver instructor rips in with the death threats. Without training (that they sell) you probably risk a horrible DEATH!! Yeah yeah. Then they come out with baloney physics about (this time) fishtanks on your chest - unaware of the fact that your chest is withstanding about 10metres water of absolute air pressure whilst you are ashore. So 2metres is a bit more, not near-death, not anything like it. It's not any multiple of 1metre deep fish tanks sitting on your chest, jeez. Even quite a small fishtank on your chest would crush you, and diving doesn't reliably do this.

Yeah, there are lots of numpties, but lets just pretend that someone who can sail and considering diving under the boat is wel-above averagely practical, and possibly a lot more practical and aware of safety issues too.

Even beginner dive training includes zipping down to 5+ metres on the first day, so it ain't that argh dangerous is it? No. We're talking about going less than two metres down, plenty of time to unclip if you get jammed (unlikely but just about possible i spose) and lots more.

Oh and whilst we're about it - diving under your boat is likely MUCH more safe than going on some dive trip and courses - usually on a flimsy RIB in the UK and almost invariably with an illegally-small alpha flag that risks other boats thinking they've found a free rib and approach the boat with divers down - cos the flag isn't the regulation 1 metre high and visible from all angle (ie a proper rigidly-constructed flag).

/rant

Exactly, they start off suggesting you will probably die due to physics, then when challenged simply answer "well you wont understand it" and when challenged again (at boat draft depth) they change onto well you wont have experience clearing a mask and will probably have just jumped off a boat with 50kg on your back, without thinking about what you are doing and the tender wont be ready....in deep water...so will panic....

Listen to what we are saying.....we are saying at boat depth....less than 2m.....

Walk off the beach on a nice calm day at shoulder height, stick your head under and get the hang of it for a tank full, dont be tempted to go deeper, and keep breathing normally the whole time (no holding of breath) then be sensible about going about cleaning your boat, like not in 10knt of tide, 5deg water, in 100m depth, whilst drifing with no one else on board and no way of getting back out of the water....oh and dont padlock the tank to your back either for safety as you need to be aware that you may need to ditch it!

 

[Alcyone]

Exactly, they start off suggesting you will probably die due to physics, then when challenged simply answer "well you wont understand it" and when challenged again (at boat draft depth) they change onto well you wont have experience clearing a mask and will probably have just jumped off a boat with 50kg on your back, without thinking about what you are doing and the tender wont be ready....in deep water...so will panic....

Listen to what we are saying.....we are saying at boat depth....less than 2m.....

Walk off the beach on a nice calm day at shoulder height, stick your head under and get the hang of it for a tank full, dont be tempted to go deeper, and keep breathing normally the whole time (no holding of breath) then be sensible about going about cleaning your boat, like not in 10knt of tide, 5deg water, in 100m depth, whilst drifing with no one else on board and no way of getting back out of the water....oh and dont padlock the tank to your back either for safety as you need to be aware that you may need to ditch it!

What is the pressure of the air you are breathing at 2m? Do you know? Just want to see whether or not you understand it.

 

[agurney]

Exactly, they start off suggesting you will probably die due to physics, then when challenged simply answer "well you wont understand it" and when challenged again (at boat draft depth) they change onto well you wont have experience clearing a mask and will probably have just jumped off a boat with 50kg on your back, without thinking about what you are doing and the tender wont be ready....in deep water...so will panic....

I responded to your "challenged again (at boat draft depth)" request with a couple of simple examples of what you might come across. In none of my responses did I suggest you'd panic, die, jump in with 50kg or anything of that nature. In an earlier post you stated that you'd done some basic training, so you do have an inkling of what's been talked about.

I was a SSAC diving instructor for many years and have witnessed plenty of beginners in the shallow end of a swimming pool, in controlled circumstances, who just can't handle breathing from a regulator if their mask is removed.

My advice was only that you should become familiar with the equipment. [and I'd give similar advice wanted to buy a sailing dinghy and take it to sea without any preparation. After all it's only wind and water, physics won't hurt, we won't be more than 50m from the shore...]


Alisdair

 

[Alcyone]

Well said Agurney. I think there are a few here on a wind up now. Best left to it, probably.

 

[tcm]

Boyle's Law for diving numpties

1. I don't sell training, and neither do the majority of BSAC clubs, one of which, I am a member.

2. The physics is not baloney, you clearly just don't understand it.

3. The fish tank analogy is correct. If you don't believe it, make yourself a 3ft snorkel and try and breath through it.

4. Do you actually have any concept of quite why it is that your lungs are not compressed by the weight of water? Do you know how a demend valve works? Does the concept of breathing compressed air actually register with you?

5. It's not really 'some dive instructor' now, is it. at a rough guess it seems to be around four experienced divers here saying the sme thing. That's all of us. All of us with baloney physics trying to sell you something?

Lots of people read these fora. Of course there are numpties who know better when something is being explained to them, but the majority will have a read and have a think, not shoot their mouth off about something they do not understand.

I'm happy to debate the physics with you, and provide references if you feel you do understand and would wish to.

Cheers.

Hum, your earlier post said "An easy way to do think about it is to imagine lying on your back with a fishtank full of water on your chest. That's the pressure 1ft below the surface. Now go down 3 feet. That's three fishtanks full on your chest. That's the pressure you are under, at just 1 metre."

...and that shows that *you* don't understand the physics. And no, just because there are four other people saying the same thing it doesn't make it correct. After all, twelve million people voted for TBlair and they were all stupid as well, see?

To others DO NOT lie on your back with a fishtank (especially not three fishtanks) full of water on your chest. The weight of a smallish (say) 1m x .5m x .5 m fishtank is 250kg excluding the empty tank, three of them over 3/4 of a tonne. So that's like having a hefty motorbike on you, and it'll be applied all at once, erk.

By contrast, to see what it's like 2metres underwater, you could erm, go to the local swimming pool where the incredibly dangerous and life-threateneing conditions are somehow provided free by the council with few if any safety restrictions other than the usual ones of no running, and no petting, remember? Or in the sea, where if you descend to the incredibly dangerous depths of your own feet, you'll experience those very same conditions.

Boyle's Law (and not fishtanks) explains the inverse proportionality between absolute pressure and volume. There's no "debating" needed here as you suggest, since it's not actually a matter of opinion but of scientific fact.

The absolute pressure on surface is approx 15psi or whatever units you want to use - height of water will do and (and i expect this is where the dimly-remembered fishtank example originates...) it's about the same as 10metres height of water - on the surface. So going down 2 metres is another 20percent. So yeah, it's a bit of pressure, your ears pop, but NOT at all the same as having three fishtanks or a whole motorbike on your chest. Yeah, yeah, so the air you breathe in will expand as you come to the surface, but there again, only by 20% and hardly anyone can actually take in full 100% capacity, and most tend to blow out as they begin to ascend. Completely different from diving to greater depths, and even then you (obviously) don't need to know Boyle's Law.

Oh, and i saw you used the word "fora" so you may need some help with use of Latin words whilst speaking/writing English (as we are doing here) as well. Perhaps these lessons escaped you as well and anyway, lots of teachers are a bit hopeless these days, ahem! The general rule you need to know is "if in doubt - anglicise!". So, for example although if you were speaking Latin you would use "fora", when you're using that word in English, you say/write "forums". Likewise, we refer to the relevant small flowers as "crocuses" and not "croci", cactuses and not cacti, octopuses and not octopi, and of course walruses and not walri. Walrus isn't a latin word anyway.

I think that mostly covers the physics, and the latin too. To conclude, it's not super-dangerous to dive to 2metres with or without a dive tank. The most dangerous thing is likely dropping the tank on your foot. Possibly much MORE dangerous is to leap into a ybw forum armed only with a "B" in English GCSE, a "C" in Physics, a leisure-based diving qualification - and then attempt a latin declension, hm?

 

[mitchc]

Diving at shallow depths and the dangers

To all the experienced divers that have jumped in on this thread to support my posts: Thanks, the more good info we can get out there the more people will consider the risks and keep safe.

To all those non divers/inexperienced divers who think it's fine to just jump in:
Please read my post to mattnj on page 5
"Can you just be patient and wait for my PM later in the week and perhaps then you'll understand why I get so worked up about this subject."

I'll do a full write up (including the physics, medical back up & examples of when it's gone wrong a 1 mtrs depth) and post it on here so all understand the dangers. But you will have to wait a few days, as this week is a tad busy for me. Lets just hope the anticipation doesn't hurt you before the breath holding scuba diving does!

Shall we all agree to leave it for now! Hopefully most people will take some training and will be safe....but I guess they'll always be the few that won't and may end up in the paper's as a statistic.

Please keep safe onthe sea whether you are on it or under it!

 

[tcm]

"including the physic's.... in the paper's"

Yeah, we'll all be sure to read that really carefully. I can't wait. Someone who can't spell "physics" is gonna explain it to us. Jeez.

 

[mitchc]

Yeah, we'll all be sure to read that really carefully. I can't wait. Someone who can't spell "physics" is gonna explain it to us. Jeez.

Sorry TCM,
I'll get it right next time, thanks for pointing that out...
I've just sent you PM, can you give me a few days?


Regards,

 

[rogerstown]

There's a few things in the PADI Open Water course that you need a certain level of fitness to do, particularly 10 minute tread water and 200m swim (not timed). Being grossly overweight, having a heart condition or asthma might mean it's inadvisable but otherwise go for it! You might choose one to one instruction or at least a small group (no more tha 4 students to 1 instructor, say).

Most diving is (and should be) fairly relaxed and relaxing, not some kind of macho action man activity. Enjoy...

Thank you for advice djbreeze. As a sailor I have great respect for the sea and the more training done the better.
This of course does not include Latin lessons that some on the thread are suggesting.

 

[Alcyone]

Hum, your earlier post said "An easy way to do think about it is to imagine lying on your back with a fishtank full of water on your chest. That's the pressure 1ft below the surface. Now go down 3 feet. That's three fishtanks full on your chest. That's the pressure you are under, at just 1 metre."

...and that shows that *you* don't understand the physics. And no, just because there are four other people saying the same thing it doesn't make it correct. After all, twelve million people voted for TBlair and they were all stupid as well, see?

To others DO NOT lie on your back with a fishtank (especially not three fishtanks) full of water on your chest. The weight of a smallish (say) 1m x .5m x .5 m fishtank is 250kg excluding the empty tank, three of them over 3/4 of a tonne. So that's like having a hefty motorbike on you, and it'll be applied all at once, erk.

By contrast, to see what it's like 2metres underwater, you could erm, go to the local swimming pool where the incredibly dangerous and life-threateneing conditions are somehow provided free by the council with few if any safety restrictions other than the usual ones of no running, and no petting, remember? Or in the sea, where if you descend to the incredibly dangerous depths of your own feet, you'll experience those very same conditions.

Boyle's Law (and not fishtanks) explains the inverse proportionality between absolute pressure and volume. There's no "debating" needed here as you suggest, since it's not actually a matter of opinion but of scientific fact.

The absolute pressure on surface is approx 15psi or whatever units you want to use - height of water will do and (and i expect this is where the dimly-remembered fishtank example originates...) it's about the same as 10metres height of water - on the surface. So going down 2 metres is another 20percent. So yeah, it's a bit of pressure, your ears pop, but NOT at all the same as having three fishtanks or a whole motorbike on your chest. Yeah, yeah, so the air you breathe in will expand as you come to the surface, but there again, only by 20% and hardly anyone can actually take in full 100% capacity, and most tend to blow out as they begin to ascend. Completely different from diving to greater depths, and even then you (obviously) don't need to know Boyle's Law.

Oh, and i saw you used the word "fora" so you may need some help with use of Latin words whilst speaking/writing English (as we are doing here) as well. Perhaps these lessons escaped you as well and anyway, lots of teachers are a bit hopeless these days, ahem! The general rule you need to know is "if in doubt - anglicise!". So, for example although if you were speaking Latin you would use "fora", when you're using that word in English, you say/write "forums". Likewise, we refer to the relevant small flowers as "crocuses" and not "croci", cactuses and not cacti, octopuses and not octopi, and of course walruses and not walri. Walrus isn't a latin word anyway.

I think that mostly covers the physics, and the latin too. To conclude, it's not super-dangerous to dive to 2metres with or without a dive tank. The most dangerous thing is likely dropping the tank on your foot. Possibly much MORE dangerous is to leap into a ybw forum armed only with a "B" in English GCSE, a "C" in Physics, a leisure-based diving qualification - and then attempt a latin declension, hm?

Firstly let's deal with fora. Fora, like stadia is a quite acceptable plural. The law you speak of, "general rule you need to know" is it a universal rule, such as Boyle's law? Can you give references to prove that you are correct?

Thanks.

Now. The fish tank scenario. Please explain to me what weight of water would be directly above a man's chest if he was lying in 1.5 foot of water. You seem to think it would not be 250kg? And if this weight is incorrect, what, precisely, is it that causes the compression you speak of when you quote Boyle's law?

The pressure at the surface is 1 bar, approx 14.7 pounds per suare inch. OK with that?

The pressure at 1m under water is 1.1 bar, approx 15.95 pounds per square inch. Ok with that?

The difference is 1.2 pounds per square inch.
No multiply that extra weight per square inch by the area of the bottom of your fishtank and tell me the analogy is way off.........

Thanks.

 

[mikemonty]

The difference is 1.2 pounds per square inch.
No multiply that extra weight per square inch by the area of the bottom of your fishtank and tell me the analogy is way off.........

Thanks.

The analogy is way off...
It depends on the perception that the diver is being squeezed like the burger in a bun, from top and bottom only.
Fortunately for anyone who has ever immersed themselves in water, the water also squeezes from the sides thus holding the entire bag of guts and organs together.
The said bag of guts and organs, being largely incompressible water, is quite happy about this (largely) uniform pressure and simply presses back.
The obvious exceptions are the lungs and the ears, the former which - as you say feels no sensation and does not give any feedback to the lump of (incompressible) fat inside ones head which is also happily accepting the same pressure. The same of course cannot be said about the ears which often protest at changes in pressure (swallow - don't spit)

The solubility of gases at pressure is a separate issue and hardly impinges at 1 metre.

 

[Alcyone]

The analogy is way off...
It depends on the perception that the diver is being squeezed like the burger in a bun, from top and bottom only.
Fortunately for anyone who has ever immersed themselves in water, the water also squeezes from the sides thus holding the entire bag of guts and organs together.
The said bag of guts and organs, being largely incompressible water, is quite happy about this (largely) uniform pressure and simply presses back.
The obvious exceptions are the lungs and the ears, the former which - as you say feels no sensation and does not give any feedback to the lump of (incompressible) fat inside ones head which is also happily accepting the same pressure. The same of course cannot be said about the ears which often protest at changes in pressure (swallow - don't spit)

The solubility of gases at pressure is a separate issue and hardly impinges at 1 metre.

Solubility has nothing to do with this. You are right to say that the human body, being largely made of water, is largely unaffected by the pressure. But we are talking about air spaces here, in particular, the lungs. And if you finish the calculation I started above, you will see that the figures agree EXACTLY with TCMs 250kg.

So, way off?

Not really, no.

Anyway, when diving, make sure you enjoy the florums and faunums, and the cactuses when you come out.

 

[lw395]

No, it's not super dangerous, but there are mistakes that novices make that can make it dangerous.
I would not want the responsibility of saying to a novice, 'yeah, have a go, not much to it really'.
When my nephew wanted a go, I was happy to wheel him off to Andark for the evening, let someone start him off on the right track. It only cost the same as a couple of air fills.

If you have a chat with the pro instructors they may tell you that now and then, beginners get it badly wrong and panic a bit. No big deal when you've got an instructor to put you right. They've seen all the mistakes before and will sort it before it becomes a drama.

It is possible to do yourself damage at modest depths. People have proved that the hard way.

 

[Alcyone]

No, it's not super dangerous, but there are mistakes that novices make that can make it dangerous.
I would not want the responsibility of saying to a novice, 'yeah, have a go, not much to it really'.
When my nephew wanted a go, I was happy to wheel him off to Andark for the evening, let someone start him off on the right track. It only cost the same as a couple of air fills.

If you have a chat with the pro instructors they may tell you that now and then, beginners get it badly wrong and panic a bit. No big deal when you've got an instructor to put you right. They've seen all the mistakes before and will sort it before it becomes a drama.

It is possible to do yourself damage at modest depths. People have proved that the hard way.

That's the second time I've thought you were spot on in about 2 minutes!

 

[mikemonty]

Solubility has nothing to do with this. You are right to say that the human body, being largely made of water, is largely unaffected by the pressure. But we are talking about air spaces here, in particular, the lungs. And if you finish the calculation I started above, you will see that the figures agree EXACTLY with TCMs 250kg.

So, way off?

Not really, no.

Anyway, when diving, make sure you enjoy the florums and faunums, and the cactuses when you come out.

Well again the analogy breaks down depending on whether you are breathing air under pressure from a cylinder regulated by a valve or at atmospheric - through a snorkel.

I watched a HUGELY entertaining Mythbusters recently where a pig (dead) in a diving suit at tens of metres depth was allowed to suddenly depressurise to atmospheric - pork sausages in an instant.
However with pressurised air you are holding the air in your lungs at the same pressure as the water round about and the nett effect is the same as if your lungs were operating at around atmospheric pressure (setting aside that nitrogen solubility issue again).

Stick you free diver with a lungful of air down at tens of metres and then give him a clear pipe to the surface and he can watch his own lungs and heart shoot up a tube.

So your analogy only works if the guy under the fishtanks is having his lungs inflated artificially, sufficient to support the weight of the fishtank, a bit like a inner tube lifting a weight - ie nett result zero squish.
This is again ignoring the hamburger element to your analogy where the poor unfortunate gets extruded out from under the fishtank.

 

[Alcyone]

Well again the analogy breaks down depending on whether you are breathing air under pressure from a cylinder regulated by a valve or at atmospheric - through a snorkel.

I watched a HUGELY entertaining Mythbusters recently where a pig (dead) in a diving suit at tens of metres depth was allowed to suddenly depressurise to atmospheric - pork sausages in an instant.
However with pressurised air you are holding the air in your lungs at the same pressure as the water round about and the nett effect is the same as if your lungs were operating at around atmospheric pressure (setting aside that nitrogen solubility issue again).

Stick you free diver with a lungful of air down at tens of metres and then give him a clear pipe to the surface and he can watch his own lungs and heart shoot up a tube.

So your analogy only works if the guy under the fishtanks is having his lungs inflated artificially, sufficient to support the weight of the fishtank, a bit like a inner tube lifting a weight - ie nett result zero squish.
This is again ignoring the hamburger element to your analogy where the poor unfortunate gets extruded out from under the fishtank.

That is exactly correct. See my post above about understanding the difference between snorkelling and scuba. I think that is the bit that most people don't get. A scuba diver is, in effect, getting his/her lungs inflated artificially. Well, almost, the air supplied is ambient, you still use your diaphragm and rib muscles to inflate the lungs, but you are essentially correct.

Anyway, it's labouring the point. I think most here show a sensible attitude.

Cheers.

 

[Deleted member 36384]

What depth do you have to be before you cant breath through a tube to surface then? Walking off the beach with my 3 meter pipe, could my chest muscles work my lungs if they are 2m below the water?

 

[Alcyone]

What depth do you have to be before you cant breath through a tube to surface then? Walking off the beach with my 3 meter pipe, could my chest muscles work my lungs if they are 2m below the water?

Most people are very surprised to find that you wolud not breathe through a snorkel 18" long!

Edit:

The optimum design length of the snorkel tube is at most 40 centimetres (about 16 inches). A longer tube would not allow breathing when snorkelling deeper, since it would place the lungs in deeper water where the surrounding water pressure is higher. The lungs would then be unable to inflate when the snorkeller inhales, because the muscles that expand the lungs are not strong enough to operate against the higher pressure.[1]

http://en.wikipedia.org/wiki/Snorkeling

 

[Deleted member 36384]

Ok thanks. I didn't think it was as small as that. Having the wife lower me over the side on the bosuns chair, anchor slung from the bottom of the chair, a water hose in the mouth, scrubbing brush in one hand, probably would not be the smartest way to clean my hull.

Not much removed from messing up at 2 m with a regulator due to ignorance.

 

[Alcyone]

Ok thanks. I didn't think it was as small as that. Having the wife lower me over the side on the bosuns chair, anchor slung from the bottom of the chair, a water hose in the mouth, scrubbing brush in one hand, probably would not be the smartest way to clean my hull.

Not much removed from messing up at 2 m with a regulator due to ignorance.

Having a snorkel shorter than you imagined is no big deal. Just explain to the missis that you are more than capable with it, and get down there.......