Why will a diver become less buoyant as he swims deeper?

[RiverRat]

Right On, Captain!!--I think it's a conspirancy by the BCD manufacturers!

ROFLMAO......Oh it is for sure! Don't forget to wrap your computer in tinfoil so the cosmic rays don't ruin it on your SI. :lol:

 

[markfm]

Part of the when-men-were-men series. OK, under modern standards, people are taught to take reasonably full breaths, including reasonably full exhales. Minor adjustments at depth are absolutely done with my lungs, but I sure don't use them for gross control. Some folks may choose to hold an extra 172 cubic inches in their lungs, on average, to counteract the weight of the air in the full tank, great, more power to them.

Me, I usually carry a few lb spare, purposefully, to hand off, if I'm with newer divers.

It's great to shave pounds off, there's usually a pretty rapid curve on the first couple dozen dives, but I don't sweat a few pounds.

 

[Walter]

Part of the when-men-were-men series.

It has nothing to do with gender or the size of your ego, it's an easy skill to learn. Good divers of all genders, dive with as little effort as possible, that includes proper weighting.

 

[The Horn]

So, whats the final word?

Bouyancy depends on displacment and the amount of the force acting on a diver will be dependant on the amount of water mass the diver displaces compared to his own mass.

So a nikked diver as H2 says decends into the depths. Pressure acts on his body and will try to compress him. The only thing that will compress is air space so if he crushes slightly due to air in his lungs he will displace less water than he did at the surface and thus loose bouyancy.

A fully suited up diver will also loose bouyancy given the fact that he will displace less water as the pressure acting on him will compress any air entrained in his suit etc.

The only way not to change bouyancy would be to be nikked with surface supply so your lungs would stay full with equal pressure to the water pressure acting on you, however, expanding and contracting your chest while you breath will change your displacment and you will gain and loose bouyancy accordingly.

I like the word nikked now.........

 

[spschneer]

So, whats the final word?

Bouyancy depends on displacment and the amount of the force acting on a diver will be dependant on the amount of water mass the diver displaces compared to his own mass.

So a nikked diver as H2 says decends into the depths. Pressure acts on his body and will try to compress him. The only thing that will compress is air space so if he crushes slightly due to air in his lungs he will displace less water than he did at the surface and thus loose bouyancy.

A fully suited up diver will also loose bouyancy given the fact that he will displace less water as the pressure acting on him will compress any air entrained in his suit etc.

The only way not to change bouyancy would be to be nikked with surface supply so your lungs would stay full with equal pressure to the water pressure acting on you, however, expanding and contracting your chest while you breath will change your displacment and you will gain and loose bouyancy accordingly.

I like the word nikked now.........

Okay, now I'm thoroughly confused...

I thought H2 said (correctly, I think) only when freediving will the air in your lungs compress....

If you're breathing off surface supplied or Scuba tank (with normal functional regulator, that adapts to the ambient pressure), I thought that meant the regulator supplied air at the surrounding (water) pressure at the depth you are diving...in other words, more than 14.7psi (surface) pressure...therefor, the air in your lungs won't compress unless you stop breathing (momentarily....equivalent to free-diving from that point downward) and decend. That would compress that air, but it would re-equalize as soon as you resumed breathing, and the next breath would be equal to the surrounding (new & greater) pressure. Likewise, when you ascend, the pressure you're breathing is based on the surrounding pressure (and lower) thus your lungs are not really going to get involved in the equation at all, if you're on a continuous supply(regardless of whether it is surface supplied or not) and you are continuing to breathe...

Or do I need to go back to my OW books and relearn this???

 

[markfm]

H2 was correct, and so were you.

 

[The Horn]

It all has to do with your displacment. Bouyant force is equal to the difference between the mass of water you displace times acceleration due to gravity and your mass times the acceleration due to gravity:

your mass is 100 kg
gravity = 9.81 m/s^2
Force = m x a = 100 kg x 9.81 m/s^2= 981 Newtons

You displace 103 kg of fresh water

Force = 103 x 9.81= 1010.43 Newtons

Difference = 29.43 newtons of positive bouyancy to obtain neutral bouyancy you would need to add 3 kg (roughly 6.6 lbs)

The deeper you go more water pressure causes anything that is compressible to reduce in size and thus displace less water thereby lowering the difference between the mass of water you displace and your mass. If you still have an extra 6.6 lbs to obtain neutral bouyancy at the surface you will then be negative and sink.

Think of a submarine. The only way they can dive is to take on ballast water to reduce thier bouyancy: the mass of the submarine and the water inside becomes greater than the mass of the water the submarime displaces.

When you obtain neutral bouyancy notice how you slowly rise and fall as you breath, breath out you will sink breath in and you slowly rise. Add air to your BC and you increase the mass of the water you displace, however your mass stays the same so you rise.

 

[The Kraken]

Just cause . . .

the K

 

[Michael Schlink]

Nekkid, not nikked (pronounced Neck-Kid)
To the OP..... all answers above are equally correct, pick the one you like the best and then tell everyone you know where you heard it

 

[The Horn]

"e" dang.......no wonder my wife told ne to put my cloths back on.....