Here ye go...
Fine Tuning Buoyancy
© 2002 G.R. Murchison, CDR USN(Ret); SSI DCSI
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. Lets take a little closer look at neutral buoyancy and why it is so elusive.
Neutral Buoyancy isnt!
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 were 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 (dont!), neutral buoyancy would be fleeting. Lets assume we could actually get perfectly neutral for a moment. So long as we and the water are perfectly still and static were 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, lets 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 doesnt 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. Lets examine the dynamics of a displacement from neutral to see how to use breathing for fine tuning buoyancy control. As a starting point, lets 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 well continue to sink unless we do something to counter it inhale. But this exhale while rising, inhale while sinking doesnt 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. During ascent it may be necessary to vent some gas from the BC to maintain the rate of ascent we want, and to vent a bit more when reaching a new shallower depth to re-establish neutral. Likewise, during descent we may need to add some gas to maintain the desired rate of descent, and add a bit more when we reach our desired depth to re-establish neutral there.
Proper Weighting
Weighting should take into account the gas to be used during the dive. Since we want to be able to make very precisely controlled ascents, and safety and decompression stops in open water without the aid of any down-line or anchor line at the end of the dive, we must carry the weight of the gas were going to use at the beginning of the dive to assure we can achieve neutral buoyancy at the end of the dive. Nitrox or air weighs, on average, about .08 pounds per cubic foot. With an Aluminum 80, for example, 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, and well need to carry that five pounds in extra lead along from the beginning 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 another. 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.
Rick
