Why neutral buoyancy isn't

[TSandM]

A quote from Garrobo on another thread:

I was diving with some experienced divers who seemed to never breathe. I've never mastered that skill. Air's cheap and I don't like to punish myself. I am an air hog by the way.

Some of this may be related to the weighting strategy he describes.

 

[D_B]

Great post Rick!

as Warren said, try carrying 6 extra lbs around after you've figured out your proper # lbs to be neutral ... you will be amazed at how touchy your buoyancy becomes, even the slightest change in depth is magnified (found this out after finding some weights on the bottom .. was an eye opener for sure)

 

[Wildcard]

I wish I could write like that....May I use that for PPB classes? Very good job outlining bouyancy along with the proper input form the new diver that knows it all to prove the point.

 

[donacheson]

Like clockwork, certain topics crop up over and over... from some recent threads on the subject, looks like it's time to review "Buoyancy Basics" again...
Enjoy,
Rick
--
... But this “exhale while rising, inhale while sinking” doesn’t keep us stationary does it? To achieve near stability, we must get back to neutral as soon as we stop at a desired depth, so it goes something like this… As we are descending we inhale to stop the descent; as we come to a stop, we must immediately exhale to get neutral, because in order to stop the descent we had to get positive. Continuing to exhale, we will eventually become negative again, and need to inhale before a descent can start, then exhale before an ascent can start, and so forth. With a great deal of practice, we will find that we can breathe slowly and deeply without ever making any noticeable vertical excursions at all! Now we are fish![/SIZE][/FONT][/COLOR]

A minor quibble - I don't believe it's necessary to synchronize one's breathing so as to exhale to stop ascent and inhale to cease descent to achieve "Now we are fish!". A diver breathing normally cycles a couple of liters 5 to 10 times per minute. Assuming the diver to be neutral with a half-breath in his lungs, during the breathing cycle, upward forces average about 1 pound for 6 to 3 seconds and downward forces the the same for the other 6 to 3 seconds. The combination of a small force, inertia of the diver and all equipment, and viscous drag limit the excursions - to a negligible magnitude, IMHO.

That said, the fact that neutrality is unstable should be qualified by noting that the instability is exaggerated by air in the diver's BC and any compressible gear, typically an exposure suit. Air in the BC is miniminized by minimal weighting - just enough lead to permit neutrality with a near-empty tank, empty BC, just below the sirface, and breathing normally.

 

[427Dave]

Great write up Rick. You have a gift. I am going to print it out and take it to our club meeting, if that is OK. A good reminder for us ole coots that do not know everything.

I am going salty water diving for week beginning tomorrow, it is a good reminder on the differences between fresh and salt water.

Thanks.

 

[TSandM]

The combination of a small force, inertia of the diver and all equipment, and viscous drag limit the excursions - to a negligible magnitude, IMHO.

It may depend on your definition of negligible, but I actually enjoy the gentle rise and fall I experience while hovering and "neutral". It's quite perceptible. But one really doesn't have to make a conscious EFFORT to synchronize the breathing -- if your breathing is rhythmic and you have adjusted your buoyancy appropriately, the exhale (arrest rise)/inhale (arrest descent) process proceeds pretty much by itself. But it's fun

 

[Garrobo]

Oh well. I wish that the other gals thought of me as "tall, young and stong" like TS+M. You left out dark and handsome though. Really, I am 67 years old, 5'8" and 190 pounds, half bald with a pot belly and flat feet. A real dork by anyones standards. I am somewhat stronger than the average bear because I worked construction all my life, but that's about it. I just don't think that buoyancy or proper weighting is a problem for me. Maybe it is for others. I seem to have gotten it at the start. I have dove (dived, diven) with as much as 8 pounds too much weight when I was using an AL100 a couple of months ago and didn't notice the difference while getting along under water. Maybe it's because I didn't pay attention. I just want to be sure that when I come up to the 5 meter stop that I can do it without struggling to stay down. Extra weight is my friend.

 

[Wildcard]

Ah yes, I think I have dove/dived/divened with you and your hundred other clones many times. Dont want advice/help/hints/tips yet dont understand why everone else on the boat is giving you the stinkeye becaues you ended everyones dive early.

 

[wedivebc]

Extra weight is my friend.

Hopefully you will come to realize how wrong that statement is

 

[D_B]

I have dove (dived, diven) with as much as 8 pounds too much weight

how did you know it was 8Lbs too much:huh:
... what test(s) have you performed to determine your correct weight?
... and why would you not use that weight if it was the correct amount?

As Dave has pointed out, the correct weight is your friend ... more, or less weight, is not