Buoyancy issues

[Rick Inman]

FatCat once bubbled...

First: contracted muscles take up less space than decontracted muscles. Mass doesn't change, volume (density) does. Usually this is compensated in the body by other muscles decontracting which doesn't change overall body volume. In the case of the abdominal muscles, no other muscles decontract (if the exercise is done right). Body volume will decrease a little, though admittedly not all that much. Body mass will of course stay the same.
the Cat

I've not heard of this before. Can anyone else besides FatCat comment on this idea? Doc?
(Maybe clenching the abdominal muscles causes air to to be expelled somewhere else! )

 

[Warren_L]

I'm no doctor, but as far as I understand, density can only change if your volume changes given the mass stays the same. And since your muscles are in large part fluid, I would expect any volume change to be minimal, perhaps negligible. When you contract your bicep for example, the muscle shortens but widens out.

 

[FatCat]

warren_l once bubbled...
I'm no doctor, but as far as I understand, density can only change if your volume changes given the mass stays the same. And since your muscles are in large part fluid, I would expect any volume change to be minimal, perhaps negligible. When you contract your bicep for example, the muscle shortens but widens out.

Let's get this straight shall we? First, it's not really about the volume change, it's about body position and trim. But! Body tissues are largely fluid (about 60-65 % in the average adult body) but the tissues also contain absorbed gases. Gases compress. In the abdomen, the muscles compress and contract inwards. Try sucking in your gut without holding your breath or puffing up your chest, you'll see what I mean.

]donacheson once bubbled...
Add 5 pounds after getting neutral just beneath surface with an empty BC and near-empty tank! You've missed the point, Fat Cat!

I stand corrected.

donacheson once bubbled...
As for added equipment, one should be weighting oneself with all gear carried on a dive. Aside from some of the large battery packs carried by cave divers, dive lights aren't big factors in buoyancy; wet/dry suits are the biggest variable. For example, my full 1/4-inch wet suit requires about an additional 15 pounds of lead more than skins. On the other hand, my Polartec shortie requires none.

Now this time, my friend, it's you who's missing the point entirely. Let's take one of the frequently used dive lights, shall we? Let's say a Mares Phos. This dive light weighs in at 1.6 kg at the surface. It displaces 0.7 l of water. Net result 0.9kg of downward pull. Negligable? No. Can you compensate for it? Yes. Should you take it into account? Certainly.

Let's take this further, why don't we? I just weighed one of my regulators (just the first stage). Well what do you know: 916 grams (some 2 lbs). So if come winter time I switch from one to two regulators and take a dive light along, I've added almost 2 kg to my configuration.

Since I tend to take two dive lights along - I don't hold with those backup toys - that's 3 kg.

We've experimented a little with tanks here, and we took 6 12 l steel tanks and submerged them in a basin to measure water displacement. The average difference in water displacement was 0.2 l (= 0.2 kg). There was however a difference of 2.3 kg in weight between the tanks due to difference in wall thickness. And before you ask, yes the tanks were filled to the same pressure, ie. 200 bar.

Add all this up will you.

light 1 kg
another light 1 kg
reg 1 kg
tank 2 kg
assorted small gear (extra knife, carabiners,...) 0.5 kg
_____
difference 5.5 kg
(some 11 lbs)

Small difference, I think not.

Since almost no-one I know tends to take this into account - mostly because no-one has ever done the math, or has even stopped to think about it - I see and hear divers all over Europe whine about lack of buoyancy control.

Most divers dive with fixed configurations: this suit and those accessories for summer, another suit and some equipment changes for spring and autumn, and yet another suit and a different config for winter. Get to know your equipment and know which amount of lead to take along for each config.

So my advice is: learn some physics, know your equipment and take a buoyancy class !

 

[donacheson]

FatCat once bubbled...


....

Now this time, my friend, it's you who's missing the point entirely.
...
So my advice is: learn some physics, know your equipment and take a buoyancy class !

What I said was "As for added equipment, one should be weighting oneself with all gear carried on a dive."
I could care less whether or not that's done empirically for each gear configuration or the way you suggested - adding/subtracting weights for each piece of gear.

Oh, BTW, my apologies for not knowing you carried two Mares "Phos".

 

[FatCat]

So why the c**p about dive lights et al not having that much influence on buoyancy?

Think before you speak. And then speak clearly.

Oh yeah, read what's written as well, before shooting your mouth off.

So excuse me for taking my task as an instructor seriously. I'll never again take the time and effort to illustrate a point (as if).

Hell, what am I wasting my time for? I've got to pack for my dive trip.

 

[The Iceni]

Rick Inman once bubbled...
I've not heard of this before. Can anyone else besides FatCat comment on this idea? Doc?
(Maybe clenching the abdominal muscles causes air to to be expelled somewhere else! )

When a muscle "contracts" it does not actually contract, the muscle fibres simply slide over each other making it shorter and fatter. The volume remains the same! Just tense your biceps to see this effect.

I am no instructor but may I comment on this case?

The more gas and equipment you carry, the more you weigh in the water. This must be compensated for by some form of buoyancy, ideally a BC or wing, to attain that nirvana of neutral buoyancy.

As the open-circuit dive progresses the gas in the cylinders is gradually lost so the diver gets more and more buoyant (A single 15 litre tank contains about 3 Kg of useable gas, so if the diver is neutrally buoyant at the start he will be VERY buoyant at the end of every dive.)

Thus the amount of weight needed for buoyancy control must be established with near-empty cylinders and it will take a few dives to get this spot on and has very little to do with muscle mass or finess and has everything to do with your body's density and the submerged weight of the equipment carried.

The idea of a BC is to start the dive with enough gas VOLUME in it to compensate for the weight of gas carried in the tanks;- and gradually vent off during the dive as needed. (Of course, at the start of any dive the diver will be negatively buoyant without it.)

I emphasise VOLUME because as a diver descends Boyles Law will compress any gas in their BC (and suit) reducing its volume. At the bottom of the descent an injection of gas into the BC (and suit) is needed to restore neutral buoyancy.

This is a skill that cannot be learned from a book or easily taught. It is like riding a bike - learned only by trail and error and it is always better to start off overweight and trim down once you are comfortably neutral.

Once he has his weight and BC sorted (by experience not by any calculation), an experienced diver can fine tune his buoyancy using the variable VOLUME of the air in his lungs. Inhaling a litre of gas results in 1 Kg of lift, while an exhale adds 1Kg of weight.

In the UK scene we usually dive with dry suits and heavy doubles, HID torches and the like, so have a lot of additional variable buoyancy to contend with solely because of Boyle's Law. Most of us only dive with our own equipment for this reason. (It is such a headache setting up to dive with someone else's kit.)

Guys! Watch your manners!

I hope that helps,

 

[love2godeep]

Thanks, Paul, your reply made a lot of sense.

I've learned quite a bit, following the threads of this forum. Didn't know it would set off a row, though!

I'll definitely do some more experimenting with the buoyancy, and also realize that it can take time for some things (such as the breathing) to become intuitive.

We're in for a snowstorm tonight, here in Washington State. I like the snow, but can't wait to get somewhere tropical!

L2

 

[donacheson]

love2godeep once bubbled...
....
I've learned quite a bit, following the threads of this forum. Didn't know it would set off a row, though!

...
L2

Glad that you did glean a bit from the discussion. Fat Cat and I are really on the same page, just disagreeing a bit on how we got there. <GG>

 

[Warren_L]

FatCat once bubbled...


Let's get this straight shall we? First, it's not really about the volume change, it's about body position and trim. But! Body tissues are largely fluid (about 60-65 % in the average adult body) but the tissues also contain absorbed gases. Gases compress. In the abdomen, the muscles compress and contract inwards. Try sucking in your gut without holding your breath or puffing up your chest, you'll see what I mean.

When it comes to buoyancy, it actually is about volume, given no change in mass. I really doubt that muscles change their volume too much during contraction. Perhaps someone in the medical field can comment on this? But from a physical standpoint, yes, volume will affect buoyancy, so if there is little or no change in volume, buoyancy will remain relatively unaffected.

 

[Warren_L]

The Iceni once bubbled...


When a muscle "contracts" it does not actually contract, the muscle fibres simply slide over each other making it shorter and fatter. The volume remains the same! Just tense your biceps to see this effect.

Oops, thanks Iceni.... that's what I thought. Didn't get to you post before my last post.

W