Bouyancy

[StephenT]

i always thought buoyancy was the easy part, and trim was the hard part. You can self judge if you have decent buoyancy, it's alot harder to self judge trim...

[Twinset & Wing]

Green on way Howard

[sleaford_diver]

Thank you Howard and everyone that has replied.

It has been very usefull. I have been mulling over for a while whether to ask the question and kept putting it off however I am off to Sharm at the beginning of March and i wanted to have it right in my mind before I went . So thanks for that

Green for you as well howard

Andy

[Skiver]

As someone who still has a lot to learn I was vey interested to read this thread, a lot of what has been written here was very useful, particular thanks to Howard who explained the different factors involved clearly and concisely.

Thanks again
John

[StephenT]

hmm, people seem to find howards post very useful, why not make it a stick and stick it at the top of the forum or something? :>

[Spacehopper]

Howie? My money's on Garf writing that by remote control to raise Juniors rep a few notches .

Well it works for me, nice one Howie or whoever you are. Incoming

Gareth

[uwila]

Originally Posted by sleaford_diver

My OW instructor said I did have good bouyance control for someone who was new to diving, at the moment I have a crisis of confidence because this experienced dive has said that i should always have air in my bcd.

I may be able to explain why you were told this. Bear in mind what Howard explained and think of coming up to a 5m stop. In the worst case, your cylinder may be close to empty (say only 30 bar left and you started with 230 bar), so you have breathed the biggest weight of air possible (short of actually being out of air) and you still need to be neutral so that you can hold the stop. Assume that you have zero air in your BCD. This means that you have absolutely the minimum amount of lead to dive safely. Perfect, but now consider 2 minutes before the stop when you were at the bottom just about to ascend. On the bottom, your wesuit will be compressed more than it is at the safety stop and therefore less bouyant, so you will need a little air in your BCD to get neutral.

When you first got to the bottom, your cylinder would have been almost 3Kg heavier and so you would have needed another 3 litres of air in your BCD to be neutral. (200 bar of air, in a 12L cylinder, assuming the International Standard Atmosphere weighs 2.94 Kg)

This is why some people say you always need some air in your BCD - you let it out as the dive progresses to compensate for the air you've breathed and you still need a little at the end of the dive to compensate for wetsuit expansion as you ascend.

Things are a little different with a dry-suit as you've got another source of bouyancy to consider, but all the same principles apply.

Howard points out that you may have 6 litres of lung volume to play with (I'm getting old, so mine is now only a shade over 5 l) but you don't really want to be neutral on your stop with empty lungs

Anyhow, you seem to be a natural, so just get on with it and enjoy the diving.

Cheers, Chris

[Miss Roxy Chablis]

Its a fair point Chris that if you have to do a weighting check before you dive, chances are you'll be doing it with a full tank and you'll need a bit of extra weight to cope with the loss of 2-3 kilos of gas - hence the suggestion to do it with a nearly empty tank.

The other major culprit with good weighting is wetsuits. A twin layer 5/7mm wetsuit looses on average 8kg of it's buoyancy in going from 0-20m (as the little air bubbles inside the neoprene are compressed by the increase in pressure) More simply put - a diver that is correctly weighted at the surface is around 8kg overweight at 20m. Bearing in mind wetsuits give you no redundant buoyancy and most BCD's are a single buoyancy cell - its a fairly compelling arguement to be diving an either an incompressible drysuit with underlayers if it's cold enough to need a full wetsuit, or just a thin shortie or rash vest in tropical waters.

The huge weightbelts that a thick wetsuit necessitates make it so hard to get good trim in the water as well: Think of your body as an underwater see saw and then attach a big weight to one side and a big balloon to the other and it doesn't take much imagination to work out that the combination of a BCD full of air around your chest and a weightbelt round your waist will constantly try to tip you legs down in the water.

Some of the more modern BCDs have built in ditchable weight and trim weight pockets which help because they are putting the weight and the buoyancy in broadly the same place so you don't get the same pivotting effect.

I appreciate this is all fairly obvious if you've been diving for a while - but I thought it might be helpful info for people starting out who read the original bit of the thread.

[B]

Originally Posted by Howard Payne

Don't change a thing mate (except perhaps the council of the "experienced" diver )

Yep.. me too... experienced does not mean best, go with what you feel comfortable with....

B xx

[thehappychappy]

Good thread, nice little info bits and discussion.

YD & the assimilated on-line collaborative community wins yet again.