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I want it all....
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Discussion Starter #1
If I've got this right, a litre of water weighs 1kg, so an item with a volume of 1000cc and weighing 1kg will be neutrally bouyant.
In seawater, you need to increase the weight by about 27g to maintain neutral bouyancy .

If the volume of the item remains unchanged, then it's bouyancy is unchanged whether at the surface or 20m below, is this right ?
 

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I want it all....
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Discussion Starter #3
Thanks, no, not a trick question. Just needed to confirm before starting work on a project
 

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Nil carborundum illegitamae
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Problem is that humans and neoprene compress at depth and therefore change the volume.
 

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I want it all....
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Discussion Starter #5
Actually though, can anyone tell me how much positive bouyancy a diver at the surface has, with his little head peeking above the water ?
 

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Keyboard Worrier
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Actually though, can anyone tell me how much positive bouyancy a diver at the surface has, with his little head peeking above the water ?
Well, it depends on the amount he is willing to use his lungs to provide that bouyancy.

In principle if you accept that a diver is correctly weighted with empty bcd and full lungs and floating at eye level and then breaths out to descend, then the bouyancy is equal to the air breathed out - 1/2 l will equal 1/2 kg and begin descent - my lungs are approx 4l so breathing out provides negative of -4kg approx.


I just noticed 'little head' - you don't mean 'that' do you? lol
 

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I want it all....
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Discussion Starter #8
"little head" !, Oh goodness me no.

Sorry, I meant a diver on the surface with wing/bcd inflated, how much +ve bouyancy does he have ?
 

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Keyboard Worrier
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"little head" !, Oh goodness me no.

Sorry, I meant a diver on the surface with wing/bcd inflated, how much +ve bouyancy does he have ?
How long is a piece of string....
One of my bcds has a lift capacity of 30kg - so in principle fully inflated - 30kg...
 

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Nigel Hewitt
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7,142 Posts
Archimedes did it all years ago.
Displacement is everything. If you push 1Litre of water (1Kg fresh, 1.027(ish)Kg salt) out of the way you get that amount of up-thrust.
BCDs/Wings tend to be marked with their volume in litres or something. If it's quoted in lbs or Kilos assume fresh water to be safe.

If things compress more than water naturally they displace less water at depth. Water might as well not compress as I think it's about 2% down by volume at the bottom of the deepest ocean trenches.

Divers don't compress even a tiny bit as they are mostly made of water, cylinders not enough to measure, wet suits and neo drysuits far to much.
The air in your nice membrane suit compresses but you top it up so it stays constant and comfortable.
For the record I did a total flood in the pool of my Otter Britannic with 300gm undersuit and Weezle booties and lost 7Kgs of buoyancy so, I assume, when I opened the zip 7L of air came out.

I estimate changes by dangling things in my rain water butt on a fisherman's scale.
For example my nice winter hood and gloves give 1.4Kgs of up-thrust.

I'm not quite sure what the question was...
 

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I think that humans do compress. At least, by my empirical data. Wearing a t- shirt and swimsuit I have to cinch in my weightbelt at 20m or so.
 

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The worlds slowest sailor.
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I think that humans do compress. At least, by my empirical data. Wearing a t- shirt and swimsuit I have to cinch in my weightbelt at 20m or so.
its probably the gas in your little tummy.
 

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Nigel Hewitt
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Do you mean 7l of water came out of your flooded drysuit, not air?
Well I progressively packed my pockets and the belt I used until I was neutral in the shallow end.
The suit was getting a bit tight with all the moving about and the inflate wasn't hooked up but I felt that was good for the test.
Then I unzipped it all the way and started unloading the weights onto the side until I was neutral again.
The pile added up to 7Kgms

To me this means I reduced my displacement by 7L and, since 7Kgs of lead only displaces about half a litre I guess 6.5L of that must have been in air that wasn't in the suit any more.

As to humans compressing...
Freediving I compress hugely. At 30 meters my big full lungs are collapsed to nothing so all the works in my tummy are moving up into my rib cage.
I have a rubber weightbelt to take up the slack or it would go loose.

Scuba diving I get some movement because gravity ceases to be a problem and things move up a bit too.
I don't think it is compression just redistribution.
 
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Scuba diving I get some movement because gravity ceases to be a problem and things move up a bit too.
I don't thing it is compression just redistribution.
Hmmm - good point. I shall have to see if my boobs get bigger ;)
 

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Take some before and after pics, I am sure a few of us would be happy to give an opinion of any perceived increase.
 
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