Stupid bouyancy question

[eggman131]

Ok, I got my c-card this past weekend and things went very, very well and very smoothly. I feel very comfortable in the water but it's always good to ask questions when you're unsure. My question is related to bouyancy and is one that I suppose can be/was overlooked (by me, not my instructor) in the process of learning all that needs to be learned. I should also note that I did my OW checkout dives in a lake; my first dive will be in the ocean (which I know will make me more bouyant).

Assuming an offshore ocean dive, when entering the water how do you address bouyancy? Do you jump in with your BC partially inflated? How do you establish neutral bouyancy on the surface and know that the weight you have is appropriate? I know that if you are properly weighted you will dip below the surface upon exhaling--but again, does this assume a partially inflated BC? If so, HOW inflated should you be? Do you go in over inflated and lose air until you get to a neutrally bouyant point? What is YOUR technique for establishing bouyancy? It can't be that tough!

I suppose that this kind of thing probably becomes automatic once you know your proper weighting . . . but it's a question I felt I needed to ask.

Thanks!

Brian

 

[SubMariner]

Please don't be offended but these are very basic questions that should have been addressed during your Open Water dives. How did your Instructor weight you? How did you enter the water?

Although some of the variables change, the basics remain the same.

I suggest you go back & review the open water portion of your course in your manual & talk with your Instructor BEFORE you do those ocean dives!

BTW, you have not mentioned whether or not the ocean dives are being done from a boat or shore. This makes a difference.

Meanwhile, a review:

Rough weighting formula: for an inexperienced diver wearing a 6.5 mm wetsuit with hood etc, Al 80 tank, in fresh water: 10% of body weight +7lbs and adjust from there. For salt water add 3 lbs.

For a bouyancy check: put on all your exposure protection. With an EMPTY BC and a full lungful of air, you should float @ eye level. When you exhale you should sink. If you do not, start adding weight, usually in 2# increments. Don't forget the extra wt to compensate for an AL tank with less than 1200 psi.

~SubMariner~

 

[mgri]

I'm surprised that they didn't go over this with in your pool work.

When they teach buoyancy check at the surface, they say you are to have no air in your BC and you should float eye level with the surface with your lungs full. When you exhale you should start to slowly sink. This should be in you OW book.

That's the by the book answer but most of the knowledgeable people around here (which I'm not) will tell you that you should shoot for neutral buoyancy with no air in your BC at 15' sfw with about 500 psi left in your tank. The reasoning behind this is ( I think) is it helps with your obligation safety stop.

 

[GlockDiver]

I think what he is asking, is if/how people do buoyancy checks every time they dive. I would guess probably not (as I'm still in OW training, I don't know for sure). Most people probably already know how much weight they need, and adjust that throughout their lifetime as their bodies change.

Any time you change equipment (new BC, wetsuit, etc.), you should probably go down to the pool and adjust your weights. After that, you should know how much weight is proper at your destination, and no buoyancy check should be necessary.

 

[rollins]

Rough weighting formula: for an inexperienced diver wearing a 6.5 mm wetsuit with hood etc, Al 80 tank, in fresh water: 10% of body weight +7lbs and adjust from there. For salt water add 3 lbs.

I am also a recent OW diver. I weigh 195 lbs and have been using 21 lbs of lead in all my 11 dives so far. I dive in 7mm wetsuit with hood, AL 80, 5mm gloves, fresh water.

Using the eye level test at start of dive (full tank, empty BC, full lungs) I seem to be correctly weighted, but I am finding I often dragging along the bottom unless I stay a good 4 feet off the bottom, then I seem OK and can control my buoyancy via lung volume. I also find that if I empty my BC completely at my 15 ft safety stop that I have to fin to stop sinking (but if I keep some air in, I will be stable).

Is this just that my buoyancy control is crap and I need more practice, or could I be overweighted slightly? How does this weighting formula change for the experienced people out there?

 

[Wendy]

GlockDiver once bubbled...
I think what he is asking, is if/how people do buoyancy checks every time they dive. I would guess probably not (as I'm still in OW training, I don't know for sure). Most people probably already know how much weight they need, and adjust that throughout their lifetime as their bodies change.

Any time you change equipment (new BC, wetsuit, etc.), you should probably go down to the pool and adjust your weights. After that, you should know how much weight is proper at your destination, and no buoyancy check should be necessary.

I think the other 2 folks answered this question. I don't think most people know how much weight they need, and I also think that a lot of time students are overweighted in ow classes.

Going down to the pool won't really help here cuz Eggman131 will be doing an ocean dive and he would need to wear more weight than he does in a pool. Going to the pool to see how much weight he would need in fresh water would be a good idea, so that way he would have a starting number for his ocean dives.

 

[Wendy]

rollins once bubbled...
Using the eye level test at start of dive (full tank, empty BC, full lungs)

Try doing this at the end of your dive when you have @ 500 psi in your tank.

 

[herman]

No doubt a good bouancy check is the best way to go but there are times that it is not possible. So let me see if I can answer some of your questions. First, leave the boat/shore with a good amount of air in your BC, how much depends on how close your weighting is to optimal. Too much is not problem, too little can be a big problem if you are way overweighted. Sinking like a rock is never a good thing. A word of caution, back inflate BCs will tend to plant you face forward at the surface with excessive air in them, but IMHO that's still better than an uncontrolled decent. When you are ready to decend, start releasing air slowly, a little at a time until you are neutral. How much and how fast will come with experience but too slow and too little is never a bad thing. Then you can use breath control to start the decent. Remember, your wetsuit will compress as you decend so you will start to become negative and you will have to adjust to air in your BC to compensate, again, slow is good, sinking like a rock is not. When I am neutral at the surface, I perfer to have no air in my BC at the beginng of the dive. This helps in 2 ways. An uncontrolled decent is almost impossible as I am neutral with no air in my BC and at depth I need only a small amount of air in my BC to adjust for wetsuit compression and loss of air in my tank. Lots of air in your BC at depth makes neutral bouyancy difficult to control and is a sure sign you are overweighted. At the end of each dive, try to do a quick bouyancy check. Keep notes in your log as to how much weight you had on, what equipment you had on and how close to correctly weighted you were. After a while, you can go back in your logs, look for a similar dive and have a good guess as to the correct amount of weight you need.
If you are boat diving, I would suggest that you stick close to the anchor/decent line for at least the first few dives. Try and do a free decent/accent next to it if currents allow. That way, you can practice your bouyancy control and decent/accent control but if things get out of hand you can grab on to it and stop what could be a nasty ride.

 

[eggman131]

Thanks for your answers! Guess I should have been more clear: I do know my weight--I was 15 on the lake for my OW, so I am estimating 20 for my ocean dive (colder water, thicker suit). And we DID cover this in our class; again: "My question is related to bouyancy and is one that I suppose can be/was overlooked (by me, not my instructor) in the process of learning all that needs to be learned." And given all the answers, it seems that there are conflicting ways in which to achieve neutral at the surface.

I think I'm straight. Thanks.

 

[saf_25]

eggman131 once bubbled...
Guess I should have been more clear: I do know my weight--I was 15 on the lake for my OW, so I am estimating 20 for my ocean dive (colder water, thicker suit).

I suggest taking extra weights along with you, to have in case you need them. I know that, with all gear/exposure protection staying the same, I use 6#'s more in the ocean than I do in the pool. So, your 5#'s more sounds close, assuming the same gear, but if you're going to be using a thicker suit as well, you might find yourself needing a couple extra pounds to get down. Try it with the 20 and see how it works - worse case, you'll need to go in to shore and add a few extra pounds (hopefully there won't be a long surface swim) but at least you'll have them with you and you'll be able to continue your dive.