Why neutral buoyancy isn't

[Rick Murchison]

Like clockwork, certain topics crop up over and over... from some recent threads on the subject, looks like it's time to review "Buoyancy Basics" again...
Enjoy,
Rick
--
Buoyancy Basics
© 2002 G.R. Murchison, CDR USN(Ret)

You recall from your 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 changing buoyancy of breathing 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 stationary 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, 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. Since we want to be able to make very precisely controlled decompression 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 lead at the beginning of the dive to assure we can achieve neutral buoyancy at the end of the dive. In other words, if you are properly weighted (able to sink by exhaling or float at eye level with full lungs and an empty BC) with a full AL80, you need to add 5 pounds to be properly weighted with 500 psi.
Remember that salt water weighs about 102.5% what fresh water does, so we displace about 102.5% 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 at 12 pounds in salt water, and I weigh 240 pounds with all my gear on, I would need to remove about 6 pounds for proper weighting in fresh water.
--

 

[SoCalMike]

Reminds me of another controversary, is infinity really infinity? Beacuse infinity is the measure of nothing in reality, can it really be something?

 

[sharkattack]

Reminds me of another controversary, is infinity really infinity? Beacuse infinity is the measure of nothing in reality, can it really be something?

Where does space end and what is on the other side? Can go nuts thinking about this stuff.

Buoyancy? Useless in upwellings or downwellings

 

[tedwhiteva]

Like clockwork, certain topics crop up over and over... from some recent threads on the subject, looks like it's time to review "Buoyancy Basics" again...
Enjoy,
Rick
--
Buoyancy Basics
© 2002 G.R. Murchison, CDR USN(Ret)

You recall from your 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 changing buoyancy of breathing 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 stationary 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, 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. Since we want to be able to make very precisely controlled decompression 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 lead at the beginning of the dive to assure we can achieve neutral buoyancy at the end of the dive. In other words, if you are properly weighted (able to sink by exhaling or float at eye level with full lungs and an empty BC) with a full AL80, you need to add 5 pounds to be properly weighted with 500 psi.
Remember that salt water weighs about 102.5% what fresh water does, so we displace about 102.5% 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 at 12 pounds in salt water, and I weigh 240 pounds with all my gear on, I would need to remove about 6 pounds for proper weighting in fresh water.
--

What I realized recently - this doesn't apply to rebreathers. When you exhale on a rebreather, you don't loose the volume, it just goes to another cavity that is still attached to you.

So you rebreather guys/gals - how do you fine tune your buoyancy like us OC guys do by controlling breathing?

 

[Nudgeroni]

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.

Thanks Rick. As a beginner, this little gem is good to hear. It's easy to get sloppy and use the bc as a crutch for assent/descent. If practiced, bad habits are hard to lose. Being my first year of diving, I will be working a lot on buoyancy, and I'll keep this in mind.

 

[DennisW]

Nice treatise.

 

[Spratman]

Best Reader's Digest version I've seen! Thanks, Rick.

 

[Warren_L]

What I realized recently - this doesn't apply to rebreathers. When you exhale on a rebreather, you don't loose the volume, it just goes to another cavity that is still attached to you.

So you rebreather guys/gals - how do you fine tune your buoyancy like us OC guys do by controlling breathing?

This is not enirely true. Depending on where your counterlungs are mounted and the relative position of your lungs to the counterlungs, your breathing can affect your buoyancy slightly.

If your counterlungs are above your lungs, an exhale will result in an overall increase in the volume of loop gas, albeit slightly. The converse is true if your counterlungs are below your lungs.

Fine tuning buoyancy can be somewhat different. Often times, I will fine tune by either swimming up or down a little, depending on if I am to positive or too negative. You can always add or subtract gas in your wing, but if your wing inflation is plumbed to your diluent, you want to do that as little as possible. As well, in cold water diving, I sometimes add a bit of gas to my drysuit and that can act to adjust your buoyancy as well.

Remember that on CCRs during ascents, air spaces will expand and have to be vented, which means not only the wing and drysuit, but also the loop, lest you get the chipmunk cheeks.

 

[wedivebc]

What I realized recently - this doesn't apply to rebreathers. When you exhale on a rebreather, you don't loose the volume, it just goes to another cavity that is still attached to you.

So you rebreather guys/gals - how do you fine tune your buoyancy like us OC guys do by controlling breathing?

Nice article Rick,
Yes RBs have their own challenges. I miss being able to fine tune my bouyancy with breathing. You learn different techniques though. One big issue is gas volume in counterlung must also be safe to breath so adding oxygen is sometimes not appropriate and adding diluent will require the addition of O2 to get PPO2 back in check. You learn to anticipate depth changes and vent or add gas prior to the change. Proper weighting and minimal loop volume in a RB help alot.
One point to add to Rick's article about RB diving is the static nature of neutral bouyancy in a rebreather is also fleeting to a smalller degree because oxygen is used up[ by the body and that causes a reduction in loop volume, hence bouyancy change.

 

[Garrobo]

Personally, I can't see where buoyancy is such a big deal. You figure about 10% of your actual weight, give or take a few pounds, depending on your muscular content and how thick a wetsuit you are wearing, if you are using one. If you sink after letting the air out of your BCD you should be OK. When you get to the level you want to stay at, pump enough in your BCD to stay at that level. What's so complicated about that?