neutral without the aid of a BC

[nimoh]

In a recent discussion on scubaboard, a comment was made that a diver may use a BC only for surface flotation, never adding air at depth. Rather than completely hijack that thread, I thought I would start a new one.

At the start of my dive I am negative to begin the descent, then add short bursts to keep my descent slow and controlled, winding up at the target depth neutral. I am pretty sure if I didn't add any air to my BC, I would be laying at the bottom.

So, my question is, for divers that don't add air to their BC, how do you get neutral at depth?

I remember that Mike Nelson never wore a BC, and always seemed to be neutral, but how?

 

[chrpai]

http://www.scubaboard.com/forums/ne...ending-what-best-practices-4.html#post6382772

I have no problem with this approach other then he was trying to talk all smart in a "best practices" thread while saying that he never developed the "safety stop habit".

My buddy sometimes dives wetsuit sans-BC and I usually have him hand me his weightbelt and then give it back to him at the safety stop.

 

[Dhboner]

For most people using exposure protection (wet suit or drysuit) it is almost impossible to be neutral at the start of a dive with full tanks and not have to add air to the wing or BC. Wetsuits compress at depth causing them to lose buoyancy, aluminum tanks become positively buoyant as they empty. All these things generally require the diver to be negative at the beginning of the dive, use wing lift to compensate then end up neutral at the end of the dive so they can hold a safety or deco stop.

 

[nimoh]

when I started this thread, I didn't realize that the process commonly involved dumping some weight to get neutral, and then finding some weight to begin the ascent. So, I have probably heard enough on this topic.

thanks, chrpai for providing the reference to the other thread.

 

[fdog]

It's mostly in the lungs. ....And not much for exposure protection.

For example, something we do a lot of in the tropics is "Monkey Diving", which is using a simple scooter harness & a stage bottle - no wing or BC.

As you can see, this aluminum 80 will suffer from weight swing over the dive. And, as you descend, the wet suit will crush, losing buoyancy, too.

The answer is careful weighting. I am picky about my weighting to the extent I will adjust by a half a pound if I need to. I adjust my weight so that I'm neutral, at the end of the dive, at 1/3 lung volume at 10' (insert asterisk).

The rest is simply lung compensation, which is like riding a bicycle. It works well enough that you can easily hold buoyancy and trim at 40':

This is all with skimpy (3 mil) exposure protection, which we can easily dive in this fashion down to 90'. If I'm wearing a 5 mil, I can do it, but need to have a max depth of about 45', or else I'll need a wing of some kind deeper. And of course, a monkey diving with a dry suit is diving with a BC of sorts:

So, it's mostly in the lungs. And being smart with your weight(s) to begin with.



All the best, James


(Asterisk) this is not my normal weighting procedure for when wearing a wing, where I weight to be neutral at the surface with an empty tank.

 

[chrpai]

fdog- agreed. Except the diver previously alluding to this was saying that he did it in a 5mil.

 

[nimoh]

thanks for posting that James, it seems that you have it perfectly planned out. This is exactly what I was looking for. Not that I plan on trying it, just curious

 

[-hh]

For most people using exposure protection (wet suit or drysuit) it is almost impossible to be neutral at the start of a dive with full tanks and not have to add air to the wing or BC. Wetsuits compress at depth causing them to lose buoyancy, aluminum tanks become positively buoyant as they empty. All these things generally require the diver to be negative at the beginning of the dive, use wing lift to compensate then end up neutral at the end of the dive so they can hold a safety or deco stop.

Agreed, although one one clarification and one caveat.


The clarification...

... is that exposure protection is often a big part of the buoyancy variation, but it isn't the only part. As you do go on to state, one's air supply tank (not just aluminum ones) also change buoyancy during a dive as one consumes their contents.

And while the buoyancy shift is more noticable with an AL80 than some other tanks (because the tank goes from being negative to positive), all tanks shift, because it is the mass of air inside the tanks that gets exhausted...as a simple rule of thumb, figure 1lb change for every ~13ft^3 of air consumed...ie, an AL80 going from 3000psi to 500psi results in a 5lb difference.


The caveat...

...is that what constitutes being "sufficiently" neutrally buoyant is a YMMV. For example, must one be within an ounce of perfect in order to be truely "neutral"? Or is a half pound good enough? Or maybe a kilogram?

The answer is that it depends in no small part on how demanding we're being and also how much control is "easy" for us to accomplish. We should also not forget that because of our breathing cycle, we're a dynamic system, and breath control techniques can include a purposeful shift in one's average tidal volume to add or subtract from the system without ever lifting a finger towards one's purge button. It will depend on one's lung capacity and comfort level for how much of the weighting imbalance is offset. Of course, when starting out heavy and ending light, one will pass through one's point of "perfectly neutral" at some point.

As such, one dive without a BC and be "sufficiently" neutral throughout the dive...if one isn't too demanding for what is "sufficient". And of course, with less thermal protection, the shift is smaller, which makes the task less challenging.


...PLUS, of course, other ways to 'Skin the Cat', such as by using drop weights.



-hh

 

[fire_diver]

Yes, a lot can be done with just lung volume but I prefer to bale to breath normaly. I prefer to use a BC and adjust it's air volume during a dive instead of my lungs.

Changes in exposure protection change weighting needs dramatically, even for the same thickness. It all dpends on manufacturer.

 

[fdog]

fdog- agreed. Except the diver previously alluding to this was saying that he did it in a 5mil.

Didn't catch that thread... it sounds like some interesting reading.


All the best, James