Simple bouyancy question

[Kharon]

Sorry, but I have a problem with the initial post. Who is teaching to dump all of the air from a wing or BCD to start a dive? You only need to dump as much as necessary to begin descending. Even if you are weighted perfectly, you will still have some gas in your wing to compensate for your weight and buoyancy differences at the end of the dive. The more tanks you are carrying, the greater the weight difference will be.

Seriously????? Not in my world. When I am weighted properly with a full tank I will have NO air in my BC and will descend when I exhale. In salt water with no neoprene I almost never need to add air to my BC if I'm using 12# lead. If I bump it to 13# I might need to add a tiny bit of air at about 60 feet. That doesn't hold when I'm at home in cold water. I need to futz the air in the BC a bit more but still, properly weighted, no air in the BC to descend. If you need to have air in the BC/Wing you are overweighted.

 

[Diver0001]

...snip...

Theoretically speaking, in terms of maintaining neutral buoyancy most of us are taught to dump all of our air during initial decent and then add small amounts as we near the bottom to establish neutral buoyancy again. Wouldn't it make more sense or would it work at all to just establish neutral buoyancy at the surface and do a tuck roll and just swim down to the bottom.

...snip...

What I teach is to vent the BCD until your head just goes underwater and then to *stop* venting and breathe out. This way the initial part of the descent can be arrested or brought to neutral just under the surface by simply breathing in.

This is important for control to avoid issues like losing your buddy with different rates of descent and the possibility of barotrauma caused by a descent that can't be brought under control (as) easily. With this technique every dive begins with a maximum of control over tempo and a minimum of things happening quickly that could lead to mistakes, like losing ones buddy, crashing to the bottom, kicking up silt, descending further than you wanted to, etc etc.

(As an aside, I use this technique to introduce hovering (although I don't call it that) from the first time the student makes a descent and to drill control during descents to such an extent that going from descent to neutrally buoyant is second nature by the end of the OW course. It forms the foundation upon which instruction in a neutrally buoyant manner can be achieved in OW )

Back on topic; venting the BCD to empty at the beginning of the dive would result in a descent that is generally too quick and could perhaps, especially for rookie divers, create complications. Even if you've never tried descending like I described above, I'd recommend trying it to see what you think.

R..

 

[CT Sean]

Seriously????? Not in my world. When I am weighted properly with a full tank I will have NO air in my BC and will descend when I exhale. In salt water with no neoprene I almost never need to add air to my BC if I'm using 12# lead. If I bump it to 13# I might need to add a tiny bit of air at about 60 feet. That doesn't hold when I'm at home in cold water. I need to futz the air in the BC a bit more but still, properly weighted, no air in the BC to descend. If you need to have air in the BC/Wing you are overweighted.

That is technically true. If you are "perfectly" weighted at the end of a dive with no wetsuit to compress and no air in your BC, they you are "overweighted" at the beginning of the dive by the the weight of the gas that you will breathe during the dive.

You can certainly weight yourself to be "perfect" at the beginning of the dive with no air in your BC, but then you are using your lungs for buoyancy compensation as you "lose weight" over the course of the dive. I did this once by accident, seeing how little weight I can get away with on a shore dive in the tropics, however I wasn't a big fan of it.

 

[herman]

I dive the way you describe most of the time. Proper weighting, breathe control and understanding the physics of the decent or accent are the keys to doing it correctly. The vast majority of divers today are improperly weighted and relay on their BC to compensate for the improper weighting. Fact is, unless you are diving thick exposure protection or large volume tanks, BCs really are not needed, they are after all "buoyancy compensators" and with nothing to compensate for there is no need for them. It is entirely possible to be grossly overweighed and still be neutral at any depth but it requires constant adjustment of your BC. The vast majority of divers are at least somewhat overweighed. You are correct in your assumption that with little to no wetsuit your buoyancy will not change as you descend, that is assuming you have your BC completely empty. If done properly, runaway decents are not a problem....you are neutral all the way from the surface to any depth. Your decent is controlled by your breathing patterns and your understanding of the physics involved, not contant adjustment of your BC. I have no problems at all starting a decent, governing its speed or stopping at any depth I choose and never touching my BC, the same goes for the accent. The idea that safety stops are not possible is pure BS; they are no problem at all to do. My normal decent is to enter the water with a completely empty BC and a full breath. This brings me back to the surface to join up with other divers. As soon as my head gets above water, I begin breathing normally again. When I want to descend, it’s a simple matter of exhaling fully and waiting a few seconds. I will start to slowly sink, at which time I resume normal breathing, momentum along with an occasional deep slow exhale to compensate for water resistance and the decent continues. Once I reach whatever depth I want, I change breathing cadence to stop the decent then resume normal breathing. Accents are done the same way, a long slow inhale starts the accent followed by the occasional deep slow inhale to keep it going. Stopping at any depth is a matter of changing my breathing pattern to stop the accent and return to neutral. There is no need to constantly deflate, inflate and adjust your BC if it's done properly but that is a sure sign you are not correctly weighted.

 

[RJP]

I dive the way you describe most of the time. Proper weighting, breathe control and understanding the physics of the decent or accent are the keys to doing it correctly. The vast majority of divers today are improperly weighted and relay on their BC to compensate for the improper weighting. Fact is, unless you are diving thick exposure protection or large volume tanks, BCs really are not needed, they are after all "buoyancy compensators" and with nothing to compensate for there is no need for them. It is entirely possible to be grossly overweighed and still be neutral at any depth but it requires constant adjustment of your BC. The vast majority of divers are at least somewhat overweighed. You are correct in your assumption that with little to no wetsuit your buoyancy will not change as you descend, that is assuming you have your BC completely empty. If done properly, runaway decents are not a problem....you are neutral all the way from the surface to any depth. Your decent is controlled by your breathing patterns and your understanding of the physics involved, not contant adjustment of your BC. I have no problems at all starting a decent, governing its speed or stopping at any depth I choose and never touching my BC, the same goes for the accent. The idea that safety stops are not possible is pure BS; they are no problem at all to do. My normal decent is to enter the water with a completely empty BC and a full breath. This brings me back to the surface to join up with other divers. As soon as my head gets above water, I begin breathing normally again. When I want to descend, it’s a simple matter of exhaling fully and waiting a few seconds. I will start to slowly sink, at which time I resume normal breathing, momentum along with an occasional deep slow exhale to compensate for water resistance and the decent continues. Once I reach whatever depth I want, I change breathing cadence to stop the decent then resume normal breathing. Accents are done the same way, a long slow inhale starts the accent followed by the occasional deep slow inhale to keep it going. Stopping at any depth is a matter of changing my breathing pattern to stop the accent and return to neutral. There is no need to constantly deflate, inflate and adjust your BC if it's done properly but that is a sure sign you are not correctly weighted.

What sort of exposure protection are you wearing that you do not need to add air to your BCD (or drysuit)?

 

[herman]

The OP ask about little to no wetsuit which is what I am discussing. In warm water - 80s- I usually wear an 1/8 sleeveless shortie or just a fleece lined skin, mid 80s and its just a rash guard and bathing suit. All of which are basically neutral reguardless of depth... obviously, with thick wet suits or large tanks with a lot of swing then you do need some way to compensate for the bouyancy changes...which I state in my 4th line.
Drysuits are a totally different discussion but IMO there is really no need for a BC with them as well for normal bouyancy purposes- in other words, maintianing and staying neutral but they are a good idea for backup bouyancy in case you have a drysuit failure.

 

[george_austin]

Nowadays - BCD's are kind of a crutch (water wings) Almost everyone is trained nowadays to be moderately to excessively overweighted and told - "hey, no big, that's what your water wings are for. . . " -
Trunking it - you use the BCD you were born with. If you can't do a safety stop because you can't control your buoyancy with breathing - you may be either holding your breath or only breathing off the top of your lungs. Even with an aluminum tank so popular in warm water resort type diving.

Next time you're diving warm water without a wetsuit - check out the most competent individuals in your group - my guess is that you won't see them adding or dumping air if they are wearing a BC.

Water wings only became widespread in the 80's - people enjoyed sport diving for many, many years before that and with fewer accidents as well.

Sometimes after a few days of diving even warm water, I will start wearing a 3 mil suit so I might wear a couple pounds if it's a real shallow dive - surgy, maybe a dive with a lot of current and I want to get to the bottom quick and stay there easily, crawling over rocks instead of swimming hard and blowing thru air.

Still though, I hate myself if I ever need to put air in my BC unless I'm on the surface flirting with some nice lady ;-)

 

[kwinter]

Seriously????? Not in my world. When I am weighted properly with a full tank I will have NO air in my BC and will descend when I exhale. In salt water with no neoprene I almost never need to add air to my BC if I'm using 12# lead. If I bump it to 13# I might need to add a tiny bit of air at about 60 feet. That doesn't hold when I'm at home in cold water. I need to futz the air in the BC a bit more but still, properly weighted, no air in the BC to descend. If you need to have air in the BC/Wing you are overweighted.

Wow! I wish I had your superhuman abilities. Let's do an experiment. Try this next time and get someone to do a video.

Weight yourself "perfectly" so that you are exactly neutral at the surface with a full tank and no air in your BC. Make one of your weights a 5 pound drop weight. Now exhale to start your descent and drop down to 60 or 80 feet or so, adding air to your BC only as needed to become neutral. Now remove that 5 pound weight and drop it there, letting air out of your BC to compensate, and head to the surface. Have your buddy video you as you try to hold your safety stop at 15 feet with 5 pounds less than you started and no compensating compression of air in thermal protection. That 5 pounds is about the amount of air you would use on the dive.

Get back to me after you post the video. I need a good laugh.

 

[fjpatrum]

Sorry, but I have a problem with the initial post. Who is teaching to dump all of the air from a wing or BCD to start a dive? You only need to dump as much as necessary to begin descending. Even if you are weighted perfectly, you will still have some gas in your wing to compensate for your weight and buoyancy differences at the end of the dive. The more tanks you are carrying, the greater the weight difference will be.

And as for adding gas as you near the bottom, the rate you do that all depends on how deep you go and how fast you want your descent. I usually want a quick drop and don't add much until I'm nearing bottom, then blast it full force and come to a gentle stop. Yes, I have crashed into a few wrecks when the vis is less than 1 foot, but that's part of the fun.

I have yet to require air in my BC down to about 60 feet, period. I regulate buoyancy with my breathing. Why would I add air if it's unnecessary?

Why would I have extra air in my bc to account for less air in my tank at the end of a dive? If I am weighted properly, I am essentially 4 pounds heavy at the start of a dive, right? (Assuming an AL80 tank with 3000 psi). Any wetsuit I wear will compress enough to compensate for that and certainly my drysuit, when diving dry, will accommodate as well. Why on earth would I ever need to keep air in my BC while descending from the surface?

I think you and I also have a different concept of fun. Crashing into anything on the bottom doesn't make my list.

 

[tursiops]

I have yet to require air in my BC down to about 60 feet, period. I regulate buoyancy with my breathing. Why would I add air if it's unnecessary?

Why would I have extra air in my bc to account for less air in my tank at the end of a dive? If I am weighted properly, I am essentially 4 pounds heavy at the start of a dive, right? (Assuming an AL80 tank with 3000 psi). Any wetsuit I wear will compress enough to compensate for that and certainly my drysuit, when diving dry, will accommodate as well. Why on earth would I ever need to keep air in my BC while descending from the surface?

I think you and I also have a different concept of fun. Crashing into anything on the bottom doesn't make my list.

Your post makes no sense....it reads like you've got it all backwards, or there are serious typos.
If you are 4 (actually 5) pounds heavy at the beginning, at the surface, and your wetsuit compresses as you go down, you will be even MORE negative at depth, maybe another 5-15 pounds, depending on your wetsuit. So you need air going into the BC, to compensate for being 10-20 pounds heavy. Then as the dive goes along, you use 5 pounds of breathing gas, but you are still 5-15 pounds heavy because of the suit compression. Then you ascend, the suit expands again, you are no longer heavy. You reach the surface neutral.

The OP postulates NO wetsuit, so the thread is about losing weight during the dive only because of the gas in the tank being used. So in his case, he is 5 pounds negative at the surface, and also 5 pounds negative at the bottom, at the beginning, and zero at the bottom at the end, and also back at the surface.

Right?