Buoyancy control

As an inexperienced diver... I can get neutrally buoyant lying prone however as soon as I turn vertical ie. head towards the surface & fins down, I start to descend. Perhaps I'm not as level as I thought I was (no mirrors down there), so I'll try asking a buddy to comment on my trim.

Taken the reverse, if I get neutral in a standing or sitting position to look at an object etc. then I start finning away, my fins will have to come way over my head in order to go horozontal to the wall or reef.

So far it seems everytime I change position in the water it requires an adjustment to my BCD. Any thoughts?

Next time try and be conscious of how much air in your lungs in the different orientations. You might unconsciously be changing your breathing pattern based upon orientation, and that will have a large effect on buoyancy.

While theoretically your orientation can slightly change your buoyancy by changing the depth of your wetsuit, that effect is small even in a pool, and is pretty much non-existant at any reasonable depth.

As to having to put your fins way above your head, you should go ahead with your plan to get a buddy to look at you. Many people think they are horizontal when their feet are really pointed down quite bit. True horizontal position can feel nose-down at first.

I'm having about the same problem. While diving @ about 50fsw the other day, the release on my fin came undone. Thinking that I was neutral I stopped and tucked to fix the problem. I found myself sinking. I had to inflate to re-establish neutral bouyancy.

fmw625, the problem you describe is due to "angle of attack". You were not truly horizontal and like an airplane wing angled slightly up you were generating lift as you moved forward through the water. When you stopped moving forward that lift ceased and you discovered the truth... you were not neutral at all but negative to counter the lift!

To become neutral you need to be motionless... and it won't matter what your orientation is: horizontal (supine or prone) or vertical. As Charlie stated folks often change their breathing pattern when they are in different orientations so to eliminate that just get neutral in a position of comfort without any finning... hands still and clasped and feet motionless.

Once you have achieved neutrality then begin swimming forward. If you start to ascend you are are swimming head up... so aim more downward. If you start to descend your are swimming head down... so aim more upward.

Don't try to figure out trim (horizontal swimming position) until you can achieve neutral buoyancy or you will just be chasing a mirage.

Charlie99:

Next time try and be conscious of how much air in your lungs in the different orientations. You might unconsciously be changing your breathing pattern based upon orientation, and that will have a large effect on buoyancy.

While theoretically your orientation can slightly change your buoyancy by changing the depth of your wetsuit, that effect is small even in a pool, and is pretty much non-existant at any reasonable depth.

As to having to put your fins way above your head, you should go ahead with your plan to get a buddy to look at you. Many people think they are horizontal when their feet are really pointed down quite bit. True horizontal position can feel nose-down at first.

Click to expand...

Very good answer... nuff said.

Yep- cheers! I'm gonna take my hallway mirror down with me to 20m to check my form next time.

Here's an article of mine on the subject (controlling buoyancy on open circuit Scuba)...
Love,
Uncle Ricky
----------
Fine Tuning Buoyancy

You recall from open water training that buoyancy is determined by the relationship of the weight of an object to the weight of the water it displaces when it is submerged. If more water is displaced than the object weighs, it will be positively buoyant and will rise or float; if less water is displaced than the object weighs the object will sink. As Scuba Divers we generally seek to achieve that balanced state where we displace the same weight of water as our own weight – neutral buoyancy. This is easy to understand, but not so easy to achieve. Let’s take a little closer look at neutral buoyancy and why it is so elusive.

Neutral Buoyancy isn’t!

Theoretically, all we need do to achieve neutral buoyancy is to get our weight equal to the water we displace – simple… and impossible!

(1) We cannot get our weight exactly right because the amount of water we displace is always changing, because we’re breathing. As we inhale we increase our volume, displacing more water. Since our weight remains the same we become more buoyant; likewise we become less buoyant when we exhale.
(2) Neutral buoyancy is an unstable state. Even if we were to stop breathing (don’t!), neutral buoyancy would be fleeting. Let’s assume we could actually get perfectly neutral for a moment. So long as we and the water are perfectly still and static we’re fine, but any displacement will upset the balance. If we are displaced upwards, the air in our BC and in our body cavities will expand, and we will become positively buoyant, thus tending to continue upwards. And as we continue upwards those gases will continue to expand and we will get ever more positively buoyant, until we reach the surface or something ruptures. Likewise if we are displaced downward the gasses in the BC and body will be compressed; we will become less buoyant and we will sink at an ever increasing rate until we reach the bottom.

Controlling instability

Much like balancing a stick vertically on a finger, maintaining neutral buoyancy requires constant adjustments.
First, let’s consider the options of the closed circuit rebreather (CCR) diver. Since the CCR diver inhales and exhales into a counterlung, there is no net change in volume – and so no change in buoyancy – due to simply breathing. Therefore the CCR diver must counter any vertical displacement with fin action rather than breathing. This is both a blessing and a curse, for while breathing doesn’t start the CCR diver on an ascent or descent, neither can it be used to start or stop one when desired.
In open circuit (OC) Scuba we have the additional change in buoyancy as we breathe to deal with as well as the natural instability of neutral buoyancy itself. However, the fact that breathing changes our buoyancy can work to our advantage, because if we pay attention to the timing of our breathing it can actually provide most if not all the corrective force to keep us at constant depth in the water. Let’s examine the dynamics of a displacement from neutral to see how to use breathing for fine tuning buoyancy control. As a starting point, let’s assume we have achieved perfect neutral buoyancy with half a breath in our lungs. If we are displaced upwards, we will rise until we provide a counterforce to stop the rise. So, we begin to exhale as we start upward, and continue to exhale until our upward movement has stopped. Remembering that an object in motion will remain in motion until a force in the opposite direction is applied, we must have exhaled enough to have applied a downward force – we have become negatively buoyant to get our ascent to stop, and so if we do nothing at this point we will begin to sink. And as we sink gasses in our body and BC will compress and we’ll continue to sink unless we do something to counter it – inhale. 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!

Using the Buoyancy Compensator

Aside from flotation on the surface, the BC should be used only to compensate for the changes in buoyancy experienced due to exposure suit compression with depth change and gas consumption during the dive. From our discussion of buoyancy above, adding air to a BC to initiate an ascent or removing air to initiate a descent when we are already neutral is totally unnecessary, as any displacement upwards or downwards will continue unless corrected.

Proper Weighting

Weighting should take into account the gas to be used during the dive. Nitrox or air weighs, on average, about .08 pounds per cubic foot. With an Aluminum 80, starting a dive at 3000 psi and ending it at 500 psi, we use 64.5 CF, or about 5 pounds of gas during the dive. Since we want to be able to make very precisely controlled decompression or safety stops in open water without the aid of any down-line or anchor line at the end of the dive, we must carry that five pounds in extra weight at the beginning of the dive to assure we can achieve neutral buoyancy at the end of the dive.
Remember that salt water weighs about 102.5% what fresh water does, so we displace about 2.5% more water by weight in salt water what we displace in fresh water. We must therefore compensate at the rate of about 2½ pounds per 100 pounds total weight (our body and all our gear) when we move from one to the other. For example, if I am correctly weighted carrying 14 pounds of lead in salt water, and I weigh 240 pounds with all my gear on, for fresh water I would need to remove about 6 pounds, and my proper weighting would be carrying 8 pounds of lead.
--- (c) 2002 G.R. Murchison ---

One thing you may want to try when you think you are neutrally buoyant. Orient yourself in a vertical position and then cross your arms and your legs. Do you stay at the same level or start to sink like a rock or bob like a cork?
Sounds to me like a simple weighting issue. The key to great buoyancy is having the proper amount of lead. When somewhat overweighted you will find you need to add and release a lot more air from your BCD when changing depths up and down.
Try one of the old tests from Open Water class the next time you are out, at the end of your dive when your tank is mostly empty do the eye level bouyancy check at the surface and let all the air out of your BC (remember to keep the reg in mouth)
The key to this test is again to cross arms and legs. You will probably find you are a little overweighted and start to sink. You will be amazed that as you gain experience in diving that you actually need less weight than you did when you first started even with the same gear set up. THis is simply due to having more underwater confidence and not breathing as much, as deep, or as quickly as you used to do.
This is actually a great test to do from time to time as your experience grows and will help you maximize your trim and buoyancy control
M

supergaijin:

So far it seems everytime I change position in the water it requires an adjustment to my BCD. Any thoughts?

Click to expand...

as a new diver myself (logged #10 on saturday ), i was becoming very frustrated with my buoyancy. diving in minnesota usually requires me to use a 2 piece farmer john (6.5 mil), gloves, boots & hood. the frustrating thing is, it takes a lot of weight to sink all that neoprene (i was using 30 lbs, which i know i too much). when i thought it was right on the surface i was very much heavy below 10'.

this past weekend the water was warm enuff for me to dive with my lycra suit and the farmer john part of my wet suit. i decided i was going to dump a ton of weight and ended up only having to use 15 lbs, it was perfect!! at 10' my BC was completely empty of air and i could hang motionless rising and falling with only my breath. i felt feet high/head low, but looking back at previous dives, i always seemd to be finning to maintain my position (which i'm sure adds stress and makes you use more air).

next dive, plan on working on your buoyancy control? spend the whole dive working on weighting and trim. position of weight changes things, where the tank sits on your BC changes things, equipment you carry changes things. when it's right (or close to it), the dive is much more pleasurable.

good luck