Buoyancy Question

I was reading the buoyancy charts in the "BC's and Weight Systems" group (this is the sticky that was made) and it has you adjust your weights so your neutrally buoyant at 10' deep at the end of the dive. I was wondering why it seems to be common practice to aim for neutral at 10 feet of depth. It would seem to me that if I were in an OOA situation than I would want it to be as easy as possible to return to the surface (lets assume a CESA for instance). Is there logic behind this that I am not seeing?

The guideline is neutrally buoyant at 10' with 500 psi.

So you are not OOA.

This is to safely and comfortably hold the last stop and then be able to perform a slow ascent to the surface.

If you are neutrally buoyant with an essentially empty tank and an empty BC at 10', you will probably be positive at the surface. This is certainly true for wetsuit divers.

What they're really getting at is that you don't want to be positive at 10' and unable to hold your safety stop. Of course, some folks think 15' is the magic numbe for the stop. But if you're neutral at 10', you will be slightly negative at 15' and that's probably ok.

OOA situations are forbidden. They should NEVER happen.

That sticky spreadsheet is interesting. At the start of a deep dive with a compressible wetsuit (maybe loses 20#) and a -10.5# tank, you are pinned down by 30# should your wing fail. Even if you drop your weights (a really BAD idea), you are still pinned by more than 10#. And if the majority of your weight can't be ditched, you're in even worse shape.

Think about a lift bag or surface marker buoy. Or, dive a drysuit and have redundant buoyancy.

Richard

Ok that makes sense to me now however has raised another question
If you are neutral at 10 feet wouldn't that mean that if you were at depth lets say 70 feet with 1000 psig that you would start to be positively buoyant because you simply don't have the weight?

Note: I had a feeling that I would get comments on the OOA. I just like to plan on the worst case scenario.


Thanks for the clarification so far, It is much appreciated!

Think about what happens to air at depth. The air bubbles in your wetsuit, booties, whatnot are compressed more at 70' than 10'. The air in your tank actually does weigh something. So at 70' with 1000 PSI your wetsuit, etc. are going to displace less water (be less buoyant) and your tank is going to weigh more while displacing the same amount of water (be less buoyant.)

I recommend you check out this link, it is written in a reasonably non-scientific tone:

http://home.flash.net/~table/gasses/boyle1.htm.

@Slonda828: Your link is broken. I can see how the OP would appreciate a non-scientific explanation. That's a good idea.

pjones:

If you are neutral at 10 feet wouldn't that mean that if you were at depth lets say 70 feet with 1000 psig that you would start to be positively buoyant because you simply don't have the weight?

Click to expand...

@pjones: First off, there's a difference between "weight" and buoyancy. Since we're talking about diving, let's just concern ourselves with buoyancy.

What happens to your wetsuit during the dive?
As you travel deeper in the water column, there will be greater ambient pressure. Neoprene wetsuits will tend to become more compressed at greater ambient pressure. Wetsuit compression results in a loss of positive buoyancy. Keep in mind that, when you're talking about a wetsuit, you can only lose as much positive buoyancy as you start out with. For example, a neoprene wetsuit that's 15 lbs. positively buoyant at the surface can only lose a maximum of 15 lbs. of positive buoyancy.
The reverse happens upon ascent. As you ascend in the water column, there will be a lower ambient pressure and, theoretically, this means that the wetsuit should expand back to it's normal thickness and eventually recapture its original positive buoyancy by the time you reach the surface.

What happens to your tank during the dive?
You use up air inside your tank throughout the dive. The air inside the tank contributes to the buoyancy of the tank. As you use up air inside your tank, the tank becomes more positively buoyant. Keep in mind that if you don't change the amount of air inside your tank, then the buoyancy of the tank remains the same. For instance, a full tank that's -10 lbs. buoyant at a depth of 10 feet will still be -10 lbs. buoyant at a depth of 100 feet.

(If anyone else wants to add more info that the OP might find helpful, please feel free to join in.)

OK. Now take a stab at it. Try answering your own question and explain your reasoning. We'll walk you through it.

pjones:

I was reading the buoyancy charts in the "BC's and Weight Systems" group (this is the sticky that was made) and it has you adjust your weights so your neutrally buoyant at 10' deep at the end of the dive. I was wondering why it seems to be common practice to aim for neutral at 10 feet of depth. It would seem to me that if I were in an OOA situation than I would want it to be as easy as possible to return to the surface (lets assume a CESA for instance). Is there logic behind this that I am not seeing?

Click to expand...

The objective of having enough weight is to have just enough weight to always be in control. that means having enough weight to remain neutral when you are in your lightest or least buoyant state. This will be at the end of the dive when you have dropped something like 5 pounds of air weight and are returning to the surface at something like 10 feet. You do not want to get to your safety stop depth and find that you are fighting to stay down. From 10 feet to the surface you should be able to control your ascent with lung volume and body form. Some divers will make that neutral depth a little shallower if they will end the dive spending a lot of time in the shallows or if surge that may be uplifting is expected. Again the point is to have just enough to never be forced to the surface.

This means that in the beginning of the dive and when deep you will be negatively buoyant, that is why you have a BC. It will compensate for this negative buoyancy. If that should fail you should be able to swim your rig up. If you are deep, with a heavy wetsuit and a full tank of air you may need some lift from a buddy or safety sausage/lift bag to get going or you may need to drop a little weight. Having dropped weight you will eventually become positive and that's when you flare your body in a prone position to slow your ascent.

If you were to be OOA at depth then you would want to drop enough weight to be immediately positive and use the same flare position as you vent your lungs on the rise. It may be fast ascent but a case of DCS beats the heck out of drowning. Again this is a fallback to for a breakdown in buddy protocol or solo redundancy.

Buoyancy is an ever changing proposition.

Pete

My above link is fixed, thanks for letting me know.

Bubbletrubble:

OK. Now take a stab at it. Try answering your own question and explain your reasoning. We'll walk you through it.

Click to expand...

So if I were neutrally buoyant at 10' with 500 psi in the tank than at depth the air inside the suit will be compressed therefore making me negatively buoyant and allowing me to remain at the bottom... with more air in the tank it would only be heavier therefor keeping me even more negative...That makes sense to me now, Thanks for walking through it with me. Is there anything that I missed?