Does a BC filled to capacity maintanin a constent amount of lift at different depth's

[GSmith]

I'm not a very religious person, but I can't help looking back on expriences like this and saying thanks!
Great story!
Do you still do the tournaments FredT?

I've got to go find a picture of a Bristtleworm

 

[roakey]

The source of my concern actually comes from a "Lessons For Life" feature in the July '03 Rodale's Scuba Diving Mag.(pg 69, #2). I states, "Extra weight is dangerous on a deep dive. The compression of wetsuits and BC's compounds negative buoyancy. At about 200 feet, negative buoyancy is self-propogating."

We had a long discussion about this statement in some other note, I'll see if I can find it. Basically, the overall conclusion is that this statement was yet another example of how Rodales doesn't have a clue when it comes to anything other than shallow, warm water diving (going down the same road as Skindiver Magazine).

As FredT points out, the greatest delta buoyancy is near the surface, not at depth, and as long as you have enough lift to overcome the squashed wetsuit, you can't get into a situation where your inflator can't keep up with the compression of the BC as you descend.

Basically these cases come down to divers that are narced out of their gourds that panic, screw up, and die. Not because of "self-propogating negative buoyancy."

Roak

 

[FredT]

Do you still do the tournaments FredT?

I've got to go find a picture of a Bristtleworm

Occasionally, but not as a "vocation". We have a few folks that like to hunt for meat together as a team. Most Rodeos/Tournaments are individual competition first, team second.

Bristle worm photos;

http://scubaboard.com/t40246.html

http://fins.actwin.com/species/index.php?t=9&i=311

 

[FredT]

Greg,

If I'm reading your post correctly, Fred doesn't mean that the BC contains 0.4psi - he means 0.4 actual pounds of air. Weight, not pressure.

Ben

nope. That is 0.4 to 1.5 psig in the bag.

 

[GSmith]

Thanks for the links for the pictures Fred, I'll be on the lookout for those.
0.4 to 1.5 psig in the bag? Hmmm, there is something more for me to learn here. I'm sure it winds up being a very simple problem, it just isn't coming together for me yet. I guess that is part of what makes this interesting.

nope. That is 0.4 to 1.5 psig in the bag.

 

[pipedope]

The pressure in the BC will be exactly the depth pressure until it is filled to the maximum and then it is limited by the OPV (over pressure valve) to a low enough pressure that you won't pop the bag.

The depth pressure does compress the air in the BC so at depth the BC will have a slightly lower bouyancy. The amount og change is so small that it is not important in the overall scheme of things. My quick and dirty calculations tell me that a large BC (60 lbs lift at the surface) will still lift more than 59.5 lbs at 120'.

The compression of your suit and the air in the BC is why you need to add a little air to the BC to remain neutral when you go deeper.

 

[Bob3]

I'm in agreement the "self-propagating" thing is pretty off base (polite term).
Depending on what kind of material your wetsuit is made out of, most all compression is going to have taken place by 120', it's not going to squish a whole lot more.
I'm not sure what the OPV on BCs average out at, I'm guessing maybe 3psi, & no need for more than 5psi at all.

 

[paolov]

BTW Has anybody here ever needed to FILL their BC at 130'?

Yes i have FIlled my BC to full capacity at 130 feet. and it wasn't enough to get the lift i needed (as opposed from wanted).

paolo

 

[peterwhennessey]

relates to a theoretical (in my mind) point of no return. A x FSW your pressure relief valve my realease air before effectely increasing your buoyancy.

If you are at 200 FSW, your ATA is approx. 7x14.7=102.9 psi. It seems that your bladder must be capable of containning maybe 150 psi?

Is this correct?

Hmmmm... seems to me that the "burp pressure" on the overpressure release valve is xlbs above ambient- so the relative pressure inside the bladder will be about the same at any absolute pressure- so the bcd will always fill to max at any depth unless ambient is above your interstage pressure, and the bladder is no more likely to burst at depth than at surface ( if you fill to max at depth you are gonna come up way too fast)
Cheers PwH

 

[peterwhennessey]

In this respect - maximum density occuring at a temperature above the freezing point - water is an extraordinary substance.

This is another thing altogether , but I used to melt lead solder for a living, and noticed if you throw a new stick of solder into the melting pot it floats!! So I believe water is not the only substance to get less dense as it cools to solidication (freezing) point.

PwH