Negative or positive buoancy

[mattiedread]

That is the one right answer to the question asked, and a good one for a newbie like me. I probably would have spent a good portion of my diving career guessing what I needed and jumping in adding /subtracting weights as necessary. Over the long run wasting lots of psi/bars. Those are going into my log book next week. (I have a bunch of wetsuits for surfing whose bouyancy I have not a clue).

Do you assume a human is positive, negative or neutrally boyant? (How much counter weight or lift per pound of body weight?)

 

[haggis]

human bouyancy depends pretty much on the individual, as the different proportions of fat/bone/muscle affect your "density". This is why some people can quite happily sit on the bottom of a swimming pool, and some will float like a cork. In general fat is less dense than muscle, so the fatter you are, the more likely you are to be positvely bouyant. Also of course it depends on how much air you have in your lungs, a full breath will make you more bouyant than a half breath or an empty set of lungs.

HTH

 

[Mario S Caner]

Haggis is right on the money with this one:

The tendancy for substances to float or sink in water is called specific gravity. SG compares the density of a given substance by forming a ratio of the densities. Since pure water has a gravity of 1.0, substances with less than that are less dense and therefore float, while those with values greater than 1.0 will sink.

Fat tissue has a SG of 0.7-0.9
Bone has a SG of 1.9
Muscle has a SG of 1.08

Most human beings have a SG near 1.0 Some higher, some lower. Everyone's is a little different and as you can imagine, can change pretty easily.

 

[Rick Murchison]

Again, the question is "how much lead do I need to allow for my body?" And the way to find out is, once again, in the pool. Some folks (like me when I was a young hardbody) sink, even with a full breath, while most folks (like me now that I've cultivated a respectable table muscle) need a few pounds to sink. Just get in the shallow end and hold weights in your hands until you figure out how much you need to sink you (if any) with a half breath by leaning forward and trying to lie on the bottom. If your feet tend to sink first hold the weights closer to your head, or even in your outstretched arms if necessary. If your feet rise hold the weights lower, or, if they still rise when you have the weights held straight down along your sides then do the check in the sitting position with your feet tucked up under you in a sort of "half lotus" position.
Record your body weight requirement in your log just like it was another piece of equipment.
Rick

 

[mattiedread]

Love the reference to a human body as another piece of equipement (whose buoyancy is not constant).

Mario, don't get me started on physics (I alluded to specific g in my first reply). My wife used to say I had a greater passion for physics than skiing; and I used to reply, skiing is really only one representation of physics...

That is what my degree is in.

 

[Rick Murchison]

Mattiedread sez the body's "buoyancy is not constant" - which is true, but not true enough to be a concern. The extreme day-to-day variance in your body's buoyancy (due primarily to trapped gas in the bowels, but also to a lesser degree to other factors) is no more than a pound or two at most, easily countered with breathing and the BC. (We do need to make occasional adjustments to our body's baseline buoyancy due to long term changes, like middle age spread, but that's a slow and ongoing correction)
As a practical matter, (and as an explanation to us physics nuts) what we're doing here is "measuring with a micrometer, marking with chalk, then cutting with an ax."
We can figure our weighting needs to the third decimal, but we generally buy weights in whole pound increments.
Rick

 

[mattiedread]

And, I know I'm a 'floater' so all I really wanted was a little insight as to whether there was a standard, like 1lb per every 50 of your body weight. The variance in human composition is too high for a real benchmark, I guess.

You're not an engineer by chance? You replied to a post a couple of weeks ago (can't remember which), but I do remember saying to myself, this guy is an engineer.

By the way, I didn't mean to put the 'thumbs down' sign on that post. Hope I didn't offend you. Purely accidental, I've been working in a Unix environment for 16 years. Although I've had PCs for years, 95% of their usage has been dedicated to unix emulation. But the planet looks to be going browser based.

It is a nice planet to visit, but I think this is my last lifetime on earth.

 

[Rick Murchison]

Mattiedread asks if I'm an engineer...
Nope.. I'm just a man with eclectic interests. Biologist originally (general biology - toyed with marine biology but the Vietnam war intervened - minors in chemistry and physics), Carrier (Naval) Aviator for 20 years, Masters in "Management and Human Relations" (poor man's MBA), been diving since the 60's (certified in '71 when my last source of air-without-c-card dried up). These days I own and run a computer store (it's for sale if you want it), am an independent agent for Rexall (nutritionals, preventatives, homeopathics), and teach Scuba for fun. I have a keen interest in the technical side of diving and in "how things work," and so study a lot and have built up a fairly respectable reference library. (and I'm not bad at finding stuff using the internet, if I do say so myself). As a Naval Aviator I studied aerodynamics, of course, and hydraulics, electronics, hypobarics, meteorology, weaponeering etc.. and as a Catapult and Arresting Gear Officer on a Carrier I got a lot of experience with the world's biggest hydraulic and steam machines...
But nope, I can't claim to be an engineer.
Rick
PS - you didn't insult me at all - my only concern was that there may be some who might get confused by our posts which *could* have been interpreted as being in disagreement even though they really were not.

 

[Mario S Caner]

No worries guys, the thumb just did a 180