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

[FredT]

Depends on the strength of the spring used in the dump valve. The spring keeps the valve closed so, for the BC to burp off pressure, it must overcome the force of the spring. It's going to differ for each BC as I'll willing to bet that no two springs are exactly alike. You could probably calculate how much pressure it would take if you had some special measurement tools, but it seems like a waste of time.

Halycon got the "wrong" spring in the vents they used on a batch of closed lift bags. They replaced a bunch of bags when they split on ascent. It wasn't a problem with the bag, just the wrong spring in the OPV. Burp pressure is often listed in inches of water on the more sensive units. These have the spring "molded in" as a preset on the valve diaphram. 1.5 to 2.5 psi is common on several of the drysuit and BC OPVs I've checked. This makes sense as the wing/BC/DS should not burp from the static head pressure of the water at the bottom of the bag if the bag is vertical with the vent at the top. (roughly .5# per foot for head pressure)

 

[Snowbear]

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."

Haven't read the article, but what I believe they're saying is as the diver is in an uncontrolled descent, the wetsuit and BC are continuing to compress. As the diver plumets into the abyss, his low pressure inflator cannot keep up with the compression forces of the water on the air in his BC.

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?

I don't think so. Buoyancy in this case is related to the VOLUME of air that displaces the water. Trouble with uncontrolled descent - as the diver is descending, the air already in the bladder is compressing due to the external pressure of the water and again - the inflator can't keep up with the compression. Does this make any sense at all? It's clear in my head - but I'm not always so good at 'splaining stuff

I can pretty much guarantee your BC bladder is NOT capable of containing 150psi without rupturing. Think about it - a standard car tire holds about 32-35psi, a HD pickup tire holds 80psi, a semi truck tire holds up to 120 psi.

 

[FredT]

Haven't read the article, but what I believe they're saying is as the diver is in an uncontrolled descent, the wetsuit and BC are continuing to compress. As the diver plumets into the abyss, his low pressure inflator cannot keep up with the compression forces of the water on the air in his BC.

Provided of course you have even a halfway decent first stage and inflator valve. The rate of change the divers buoyancy is decreasing as he drops. The wetsuit has already lost it's buoyancy, and the largest rate of change is in the top 30'. IF the diver has properly sized his BC he should be able to get positive with no problem. I've filled an 80 pound lift BC in well under 10 seconds at 160' with a MkV first stage and SP inflator valve. If you want the details search this site for "bristle worm" in text and the post should show up.

 

[Scubaroo]

I can pretty much guarantee your BC bladder is NOT capable of containing 150psi without rupturing. Think about it - a standard car tire holds about 32-35psi, a HD pickup tire holds 80psi, a semi truck tire holds up to 120 psi.

At the surface, no, but at depth? Riddle me this - what is the ambient pressure of water at 300'? What is the ambient pressure of the air in your BC at the same depth?

It's the pressure differential that matters - on the surface - 150psi in your BC is about 135psi MORE than the surrounding air. At 300', ambient water pressure is about 148psi - your BC cranked to 150psi is only 2psi more than the ambient water.

Now what would the pressure in your BC be if you emptied half of the air out and stayed at 300'?

Packet of gummy bears to the first correct answer.

 

[Snowbear]

At the surface, no, but at depth?

I was talking about the relative pressure I guess... Your BC will not tolerate a pressure of 150psi more than ambient. Better?

 

[Snowbear]

This doesn't make much sense...

we read the stories of the overweighted divers who suddenly realize they're at 200' and hold their inflators and cannot stop descending? I agree that the rate of change is decreasing and the amount a wetsuit can reasonably compress is finite as the diver descends, but I think the poster said something about the article talking about the "point of no return" at 200' (sorry - I don't subscribe to Rodales, so cannot read it myself)? Isn't this referring to that already overweighted diver who's inflator can't catch up to the comression rate of his descent? Like I said - I'm thinking it, but not good at saying it (and I forgot to mention I'm stupid), so please correct me if I'm thinking (and/or saying) it wrong?

p.s. - Yes, I've read your bristle worm story a couple of times and thought you were lucky to have survived!! At least you had the sense to let go of the speargun to stop your descent, eh

 

[Scubaroo]

More likely the diver is overweighted, and their BC just doesn't provide enough lift even when full once their wetsuit has compressed to keep them neutral. eg some BCs in small and medium sizes are only 22lb of lift (for example), and there's plenty of divers who feel they can't dive with less than 30lb on their weightbelt...

 

[Otter]

As for the original question about the amount of lift of a fully inflated bcd remaining constant regardless of depth, I believe technically the answer is NO -- but probably not perceptible ( I didn't do the math). Since we are talking about the weight of the water displaced by the air in the bladder, BOTH the air and the salt water are denser at depth. Now I don't know the exact number for salt water, but its one of the reasons (the other being the minerals) that salt water doesn't freeze at depth at 0 C.

 

[donacheson]

As for the original question about the amount of lift of a fully inflated bcd remaining constant regardless of depth, I believe technically the answer is NO -- but probably not perceptible ( I didn't do the math). Since we are talking about the weight of the water displaced by the air in the bladder, BOTH the air and the salt water are denser at depth. Now I don't know the exact number for salt water, but its one of the reasons (the other being the minerals) that salt water doesn't freeze at depth at 0 C.

You had it right - up until the last sentence, then you lost it. <G>

For a fixed volume of air (presumably an always-full BC holds a fixed volume of air), cooler temperatures do increase the air density and hence increase weight, while cooler water temperatures increase water density and hence increase buoyancy. In other words, the're offsetting effrects - until one approaches water temps of about 39F.

Near this temperature the density of water DECREASES as the temperature drops, so 32F water is less dense (lighter) than 39F water. Additionally, ice is less dense than water; it floats. That's why even moderately deep ponds and lakes and the ocean never freeze to the bottom. The consequences on lake and marine life would be dramatic if those bodies of water did freeze to the bottom.

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

 

[FredT]

Isn't this referring to that already overweighted diver who's inflator can't catch up to the comression rate of his descent? Like I said - I'm thinking it, but not good at saying it (and I forgot to mention I'm stupid), so please correct me if I'm thinking (and/or saying) it wrong?

Basically the LP inflator will fill the bladder faster than compression can shrink the air. The problem is that too small a lift bladder won't help the overweighted diver once the OPV valve starts venting.

FT