Diving

[Alcyone]

Ooh, you're trying very hard to wriggle out!

I contested specific pieces of YOUR misinformation - you say that being one foot underwater is the same as having 250kilos on your chest, Will you prove or otherwise withdraw this?

Yeah, I can answer your simplistic question, there's 15psi at sea level = equivalent of about 10m =33feet of water and so a foot underwater there's another 1/33 of that = tenth of a third of an atmosphere which is .5 psi, making 15.5 psi absolute ish.

Now, please return to the anology that being a foot underwater "is like having 250kilos on your chest whilst lying flat on your back" and confirm that that's not true. Or do you really think it is?

Ok, so 1/2 pound per square inch more at 1ft than at the surface.

By your calculation.

So, if a man's chest, is (say) 16" by 25", that's 400 square inches.

Are you ok with that?

 

[tcm]

hm i think that's a huge chest. Halve that figure to allow for a bit of realism and safety

 

[Alcyone]

hm i think that's a huge chest. Halve that figure to allow for a bit of realism and safety

Apologies for that, but I measured my own. Albeit it's not clear where chest ends and stomach begins.

But call it 16 x 12.5 then. 200 square inches?

So, 1/2 lb per square inch x 200 square inches is 100lb. 100lb weight on a small chest just a foot under water.

Yes?

 

[SAWDOC]

The wetsuit makes you more buoyant - during summer you don't really need it. Ditch the wetsuit and don't bother with the weights - use a rope brought under the hull to hold on to and you will manage ok.

 

[tcm]

I think you're gonna forget the gravity, are you? I looked up your site. There's an example there and I worked some figures for you. The example is from that site.

Example: 
Find the pressure on a scuba diver when she is 12 meters below the surface of the ocean. Assume standard atmospheric conditions.
Solution: 
The density of sea water is 1.03 X 10 3 kg/m3 and the atmospheric pressure is 1.01 x 105 N/m2.

Pfluid = r g h = (1.03 x10 3 kg/m3) (9.8 m/s2) (12 m) = 1.21 x 10 to the power 5 Newtons/m2 = 121 kPA
Ptotal = Patmosphere + Pfluid = (1.01 x 10 to the power5) + (1.21 x 10 to the power5 ) Pa = 2.22 x 10 to the power 2 kPa (kilo Pascals) = 222kPascals


Alcyone says this that a foot underwater is like having 250kilos of weight on you, and that a metre is 3 times that, ish, making 750kilos, and 12 metres undewater is therefore like about 10,000kilos, all ish. What’s that like?

Force = mass x acceleration = 10,000 x 10 (g=10ish=accelration due to gravity) = 100kN

Pressure = force per unit area = 100kN/ area of chest (say about 20cm x 30cm ish?) = 100kN/ 600cmsquared = 100kN /0.06
= 100kN divided by 0.06 = 1,666 kPA =

more than eight times the absolute pressure - or four times if chest 2x as big ...and about ten times (ie, he forgot about gravty?) relative pressure and note that this pressure is applied locally so it'l crush just one part of you squishingly out your lungs up your neck.

Conclusion: diving is ten times safer than Alcyone thinks?

 

[Alcyone]

I think you're gonna forget the gravity, are you? I looked up your site. There's an example there and I worked some figures for you. The example is from that site.

Example: 
Find the pressure on a scuba diver when she is 12 meters below the surface of the ocean. Assume standard atmospheric conditions.
Solution: 
The density of sea water is 1.03 X 10 3 kg/m3 and the atmospheric pressure is 1.01 x 105 N/m2.

Pfluid = r g h = (1.03 x10 3 kg/m3) (9.8 m/s2) (12 m) = 1.21 x 105 Newtons/m2
Ptotal = Patmosphere + Pfluid = (1.01 x 10 to the power5) + (1.21 x 10 to the power5 ) Pa = 2.22 x 10 to the power 2 kPa (kilo Pascals) = 222kPascals


Alcyone says this that a foot underwater is like having 250kilos of weight on you, and that a metre is 3 times that, ish, making 750kilos, and 12 metres undewater is therefore like about 10,000kilos, all ish. What’s that like?

Force = mass x acceleration = 10,000 x 10 (g=10ish=accelration due to gravity) = 100kN

Pressure = force per unit area = 100kN/ area of chest (say about 20cm x 30cm ish?) = 100kN/ 600cmsquared = 100kN /0.06
= 100kN divided by 0.06 = 1,666 kPA =

more than eight times the pressure - or four times if chest 2x as big ...and note that this pressure is aplied locally so it'l crush just one part of you squishingly out your lungs up your neck.

No, I did not say that.

I said it was like having a fishtank on your chest.

In the above example, even when I halved the chest size at your request, I still get 100lb. That's 300lb at 1m. If you take my chest measurement, that's 200lb, 600lb at 1m, 6000lb at 10m. The weight on a whole body at 1m would be getting on for a tonne, getting on for 10 tonnes at 10m.

What is it that you can't see here?

Edit (you changed your post)

Conclusion: diving is ten times safer than Alcyone thinks?

A typical conclusion arrived at by wooly non-maths that you have displayed throughout the thread.

 

[Alcyone]

I think you're gonna forget the gravity, are you? I looked up your site. There's an example there and I worked some figures for you. The example is from that site.

Example: 
Find the pressure on a scuba diver when she is 12 meters below the surface of the ocean. Assume standard atmospheric conditions.
Solution: 
The density of sea water is 1.03 X 10 3 kg/m3 and the atmospheric pressure is 1.01 x 105 N/m2.

Pfluid = r g h = (1.03 x10 3 kg/m3) (9.8 m/s2) (12 m) = 1.21 x 10 to the power 5 Newtons/m2 = 121 kPA
Ptotal = Patmosphere + Pfluid = (1.01 x 10 to the power5) + (1.21 x 10 to the power5 ) Pa = 2.22 x 10 to the power 2 kPa (kilo Pascals) = 222kPascals


Alcyone says this that a foot underwater is like having 250kilos of weight on you, and that a metre is 3 times that, ish, making 750kilos, and 12 metres undewater is therefore like about 10,000kilos, all ish. What’s that like?

Force = mass x acceleration = 10,000 x 10 (g=10ish=accelration due to gravity) = 100kN

Pressure = force per unit area = 100kN/ area of chest (say about 20cm x 30cm ish?) = 100kN/ 600cmsquared = 100kN /0.06
= 100kN divided by 0.06 = 1,666 kPA =

more than eight times the absolute pressure - or four times if chest 2x as big ...and about ten times (ie, he forgot about gravty?) relative pressure and note that this pressure is applied locally so it'l crush just one part of you squishingly out your lungs up your neck.

Conclusion: diving is ten times safer than Alcyone thinks?

1.21 x10^5 Newtons per square metre.

Do you know what that is TCM?

Can you convert that to kilogrammes per square metre, or tonnes?

Let me help.

It's 121000 Newtons.

There's 9.8 Newtons to a Kilogram

That's 12338kg.

Or 12.3 Tonnes per square metre at 12m.

Are we there yet?

Please, God, let us be there now!

 

[Alcyone]

TCM, I'm done mate. Cheers for the discussion. No disrespect.

 

[tcm]

1.21 x10^5 Newtons per square metre.

Do you know what that is TCM?

Can you convert that to kilogrammes per square metre, or tonnes?

Let me help.

It's 121000 Newtons.

There's 9.8 Newtons to a Kilogram

That's 12338kg.

Or 12.3 Tonnes per square metre at 12m.

Are we there yet?

Please, God, let us be there now!

Yes, we're nearly there - as you say 12 tonnes PER SQUARE METRE at 12 metres

1 tonne PER SQUARE METRE at 1metre

300kilos PER SQUARE METRE at 1 foot depth

How big was that chest?

 

[tcm]

Hah! his chest was summink like .4 x .3 = 0.12 square metres so take that fraction of 300kilos...

means that being a foot under water is less nasty than 36 kilos on a fairly big chest or 25 kilos on a ordinary sized chest - which is TEN TIMES less than advertised by Alcyone AND underwater this pressure is applied nice and comfily around you, not horribly squishing you.


Next!

 

[Wunja]

The fish tank is a bad analogy

Whilst the calculations are quite correct, the analogy of the 750 KG fish tank is not a good one. If you put a 750 KG tank on someone's chest, they will go splat where their unsupported sides will explode leaving a horrible bloody mess everywhere.

OK, so there is 750 kg of water on top of you, there is exactly the same pressure pushing up, and from the sides, top and bottom. The watery bits of the body are incompressible, though they may squidge around a bit they don't get squashed flat. So overall the trained and untrained divers are both fairly comfortable.

However the bits that are full of air - lungs, ears, sinuses, and between the ears are compressible. The lungs are not protected by the rib cage because the lung can get compressed by the water pressure on the diaphragm pushing up under the ribs. These maniac free divers who go down to 100 plus meters, their full lungs will compress to the size of a golf ball. Ears via the flexible ear drum or eustation(sp?) tube; sinuses through leaking membranes in the nose etc.

So back to the secondary discussion - should you take scuba training before pootling around at 2 meters under you boat?

I would say yes, on the grounds that it covers equipment familiarity, allowing you to sort out the common issues - mask flooding (it is really difficult to keep breathing when you get a sudden shock of cold water on your face), loosing your regulator and recovering it. Balancing pressures in your ears and mask - mask squeeze is a good way of getting a couple of black eyes!

It covers good buoyancy control, making the job in hand much easier to accomplish without bobbing up to the surface. And here is where the max depth argument is an issue, the depth you should be able to cope with is the depth of the bottom, not the draught of your boat, because it is the bottom where you end up if you get your buoyancy wrong.

It teaches good practices - diving with a buddy, (who may be on the surface, but is still looking out for you), checking your air supply/bottle contents on a regular basis.

What I don't like about formal training, is the necessity of being able to spout the physics in a test at the lower levels of qualification. One of the most gifted dive trainees we had in the pool, never progressed beyond that as he could get no grasp on the concepts of pressure. This may have changed as it is 10 years since I stopped diving.

The BSAC incident report used to be a combination of entertaining (the daft ways some people get themselves into trouble) and the sombre - the cases where it has gone horribly wrong. In a lot of cases, the cause was put down to lack of appropriate training, for the dive attempted.

 

[Alcyone]

Hah! his chest was summink like .4 x .3 = 0.12 square metres so take that fraction of 300kilos...

means that being a foot under water is less nasty than 36 kilos on a fairly big chest or 25 kilos on a ordinary sized chest - which is TEN TIMES less than advertised by Alcyone AND underwater this pressure is applied nice and comfily around you, not horribly squishing you.


Next!

Here ya go mate:

http://cgi.ebay.co.uk/Small-Aquariu...UK_Pet_Supplies_Fish&var=&hash=item9bff2e1ddb

1 33l fish tank. Just the size to fit YOUR own calculations.

Just as I said. Like having a fish tank on your chest, even just one foot underwater.

Thanks for proving it to anyone else who couldn't get it.

 

[tcm]

hah. Is it stackable dya think?

 

[misterg]

As I put in my first post I haven't got the time right now to put chapter and verse on the dangers of breathing compressed air on here, but I'll try to explain it later in the week when I've more time....

I'll PM you later in the week with the full explanation, including all calculations and supporting evidence... bet you can't wait

I would be interested in what you have to say, so please post rather than PMing.

Please make sure you've resolved the confusion between force and hydrostatic pressure though....

There are a whole raft of potential problems that seem trivial, but if you don't have some basic training you could get yourself into a pickle. Here are a few for starters:

1. Your mask leaks. Not a lot, but an irritating trickle that affects your visibility. How do you clear it?
2. You are scrubbing away and you knock your mask off. Can you breathe underwater without a mask covering your nose? Can you replace the mask and clear it and still get the job done?
3. You jam a strap on something (prop, anode, whatever). Are you sure you would have the presence of mind to sort the problem (remove fin/remove pack/whatever) without panicking?
5. Pick any of the above, factor in the poor visibilty from the crud you're scraping off, maybe even some current or a jellyfish sting, and add a measure of equipment failure (your weight belt slips, sticky demand valve, whatever).

Alisdair

Surely at 2m depth these are trivial (save entanglement which is a hazard regardless of whether you have scuba gear or not) - just blow out and surface - it's not like you need a controlled ascent from 2m.

So far, the rules seem to be "breathe out as you surface", "make sure you can't descend further than you intend to" and "don't panic".

Any more?

Andy

 

[agurney]

So far, the rules seem to be "breathe out as you surface", "make sure you can't descend further than you intend to" and "don't panic".

Any more?

Andy

That's where you need familiarty. Here's the scene: You are under the hull scrubbing away, the water's already poor visibilty because of the plankton, and you've added it to with all the crud you've stirred up and rubbed off. Then, something happens - maybe you're buzzed by a mobo, or feel like sneezing. You remember the mantra above and breathe out .. but you were neutrally buoyant and start sinking .. but you don't have any references for what is up or down so become disorientated.

Lots of things can happen, though they probably won't. Analogies were made in earlier posts to driving .. experienced drivers can 'read the road', they can anticipate certain things happening, and can react to the unexpected (youngster running out in front of them, patch of ice, whatever) whereas someone who jumps in a car for the first time may still be able to drive at 90 .. but the number of police car chases on TV gives an idea of the skill levels involved and the likely outcome.


It's a pity the original post developed into this. I have a navy dry suit, full scuba gear, but my preferred method of cleaning the hull is to use a scrubber of some sort from a dinghy, while wearing a neoprene hood, snorkel and mask .. then to snorkel with a paint scraper for the barnacles and mussels that I can't reach from the surface. All the talk about pressure and volume is getting silly .. it can only get worse, Boyle's is the only gas law that I 've seen mentioned, Charles's and Dalton's still to come



Alisdair

 

[tcm]

Very very good points there, "lots of things might happen but probably won't". Hum.

 

[misterg]

Alisdair: Thankyou.

Andy.

 

[William_H]

We can't let good post die

When I did SCUBA training one of the exercises was to go to 10 metres remove regulator from mouth and surface. The exercise was to get a diver to breathe out. This is done by saying Ah and blowing bubbles.
What is amazing is that i can say Ah for some seconds then run out of breath. In this exercise however I could say Ah for twice as long. The breath just kept coming.
What a load of waffle we have seen regarding pressure. the analogy of filling a balloon with water then taking down 10 metres with no change in size is correct. Most of the human body does not have a problem with the pressure at depth. this is because it is water and solids so does not compress but happily supports all that pressure. It is the air (and gases) in the body that have the problem. The lungs have to be inflated with an air pressure that is twice as much as on surface to support the surrounding body and ultimately the sea. Likewise the air in the inner ear has to be increased in pressure to balance the added pressure on the outside of the ear drum. This additional pressure comes from the scuba tank.
When you free dive (holding breath) from the surface down to 10 metres the lungs actually collapse to half size so that the air increases in pressure to balance the outside forces. No problems. The air in the inner ears is not so easy but is balanced by swallowing etc via the eustation tubes.
The real problem is this concept of taking air into the lungs at 10 metres then coming to the surface without exhaling so lungs must expand to twice their normal size.
Interestingly contrary to training I have drained scuba tanks of air while diving. Sounds horrible doesn't it. Actually it is no problem at all. The first sensation is that you seem to be sucking a bit harder than usual to get your air. it can take quite a few breaths to realize what is happening. So you slowly head for the surface. As you rise you need less absolute pressure of air from the tank so although you continue to breath you don't actually run out. It is all a very graceful process running ouyt of scuba air. The instructors will be appalled at this comment. saying never use the last 10% etc.
Anyway as I said before I would neveer use a tank to clean a boat. just snorkel.
Now another mad story from grumpy olewill.
Yesterday I was working on the boat on swing mooring and I put down my screwdriver on the deck. Sure enough soon there was a clatter and I realised the screw driver had gone to Davy Jones Tool Locker. It was just an old 8 inch long plastic handled screw driver.
Grumpy old bugger that olewill is he went home got into a wetsuit and with mask and snorkel and weight belt went back to get that screwdriver. I found it easily enough in about 4 metres of water. I also cleaned some krill off the bottom of the hull. There is not much A/f left but weed and animals are not growing much at the moment. It is mid winter. Anyway all good exercise but I am always dismayed at how unfit I seem to be when diving. Oh well Old age pension next year so that is some compensation. olewill

 

[Alcyone]

That's where you need familiarty. Here's the scene: You are under the hull scrubbing away, the water's already poor visibilty because of the plankton, and you've added it to with all the crud you've stirred up and rubbed off. Then, something happens - maybe you're buzzed by a mobo, or feel like sneezing. You remember the mantra above and breathe out .. but you were neutrally buoyant and start sinking .. but you don't have any references for what is up or down so become disorientated.

Lots of things can happen, though they probably won't. Analogies were made in earlier posts to driving .. experienced drivers can 'read the road', they can anticipate certain things happening, and can react to the unexpected (youngster running out in front of them, patch of ice, whatever) whereas someone who jumps in a car for the first time may still be able to drive at 90 .. but the number of police car chases on TV gives an idea of the skill levels involved and the likely outcome.


It's a pity the original post developed into this. I have a navy dry suit, full scuba gear, but my preferred method of cleaning the hull is to use a scrubber of some sort from a dinghy, while wearing a neoprene hood, snorkel and mask .. then to snorkel with a paint scraper for the barnacles and mussels that I can't reach from the surface. All the talk about pressure and volume is getting silly .. it can only get worse, Boyle's is the only gas law that I 've seen mentioned, Charles's and Dalton's still to come



Alisdair

Realistically, I can't see where Dalton and Charles come into this at all. At a stretch you might have a problem with Charles on a particuarly bad fill where CO levels were dangerously high to begin with, pushing them to lethal toxicity at 10m, but that's pushing things a bit far really.

In the context of the discussion, I'd say Boyle was the awkward bugger here.

 

[mattnj]

cant belive you are all still banging on about this

all it has done for me is confirmed to me why i gave up diving about 5years ago...(never did more than about 50 dives anyway at "open water" level)

there was so much banging on about how dangerous it was and how i was going to die due to physics it just got boring, and made it no "fun"...i was only really interested in a few meters depth to check out the sealife and mess about....

....i think all the "dive masters" get so caught up in there 80m+ trimix pitch black, freezing cold super scary (and stupid if you ask me) diving that it messes up their heads, and they go into autopilot banging on about how incredibly dangerous it is at any depth....

BORING!!!!