Misconceptions and Fallacies

[Rick Murchison]

Water in a BCD doesn't affect buoyancy one way or the other.

What water in the BC does is decrease the available lift of the BC by the weight of the water in it. That is, a BC with 22 pounds of lift that has a quart (about two pounds) of water in it will have the potential of only 20 pounds lift, because the two pounds of water inside the BC displaces (surprise!) two pounds of water outside the BC.
So, in addition to being sloppy, poor technique, a whole bunch of water in your BC isn't a good idea.
Rick

 

[Midnight Star]

...getting narked out of their gourd...

Giving your regulator to a passing by fish at 320' IS NOT being kind.

-----

Mike.

 

[Midnight Star]

What water in the BC does is decrease the available lift of the BC by the weight of the water in it. That is, a BC with 22 pounds of lift that has a quart (about two pounds) of water in it will have the potential of only 20 pounds lift, because the two pounds of water inside the BC displaces (surprise!) two pounds of water outside the BC.
So, in addition to being sloppy, poor technique, a whole bunch of water in your BC isn't a good idea.
Rick

It also decreases by volume the amount of air the BC can hold - decreasing lift; it'd have to be alot of water, just a little wouldn't hurt that much. I wonder, if it would be possible to purge the excess water by going vertical (allowing the water to drain to the bottom of the BC), then holding open the bottom dump valve while adding air to the BC (of course you'd have to be anchored to something, and would run a great risk of an sub-ICBM ascent) - but, would it work?

-----

Mike.

 

[rjack321]

Using the butt dump is a good way to work around a slug of water in your wing. Which can accumulate, despite good technique, after a couple of days of diving.

It also decreases by volume the amount of air the BC can hold - decreasing lift; it'd have to be alot of water, just a little wouldn't hurt that much. I wonder, if it would be possible to purge the excess water by going vertical (allowing the water to drain to the bottom of the BC), then holding open the bottom dump valve while adding air to the BC (of course you'd have to be anchored to something, and would run a great risk of an sub-ICBM ascent) - but, would it work?

 

[Temple of Doom]

Misconception:
"Being overweighted automatically increases air consumption."

The fact is that if your buoyancy control is good, being overweighted has a zero or negligible impact.

I don't know for sure, but here's where my common sense is taking me...

Having more weight requires more air to remain buoyant. Granted a small amount of additional air. However, at depth it would require even more. Add to that a dive profile that has you going up and down a certain amount, and you're magnifying that effect.

Add to that the effect of the additional volume of air in your BC, which can significantly reduce streamlining and make movement more dificult. Even when you're not doing sprints u/w, it will still have an effect. The additional effort required to swim (especially if you've got a surface swim, where you inflate your BC even more) will certainly increase your breathing and exertion.

And in that vein, if you're shore diving there's significant extra exertion packing around additional weight.

On multiple-dive days the additional exertion for swimming and packing your gear around will have an even greater impact (two dives will likely have more than twice the effect on your exertion).

It seems to me that, while under some circumstances being overweighted will have a small effect, it will always (and automatically) have an effect on consumption. I'd even go so far as to suggest that it quite often has a significant effect on air consumption.

Craig

 

[mikerault]

"Using the butt dump is a good way to work around a slug of water in your wing. Which can accumulate, despite good technique, after a couple of days of diving."

Huh?

I always invert the BCD after a dive and dump any (if any) water that accumulated after each dive. The more moisture in the BCD bladder the better chance of issues such as fungus growth, excessive wear and other nasties...

Mike

 

[jepuskar]

He said butt dump...LOLOL

 

[dumpsterDiver]

How about:
Loosing a 10 lb weight belt will surely result in the diver rocketing to the surface in an uncontrolled manner, however any diver can swim up (and do a safety stop) with a rig that is 10 lbs too heavy due to a complete BC failure.

 

[Midnight Star]

He said butt dump...

 

[matt_unique]

My favorites:

1.) Ankle weights are a crutch for people who don't know how to achieve proper trim.

Ankle weights are used for lots of things, some tasks having nothing to do with trim but comfort of lead placement as one example. Also apparently more commonly used in the Northeast than other regions.

2.) You should have just enough lead to sink at the start of your dive by venting all air and exhaling.

You need to be able to sink straight down with your tank at 500 psi or less. If you can just barely sink with a full tank, you are underweighted.

3.) Excess lead or gear results in a measureable increase in air consumption.

Short of some extreme change in weight, there will be no measurable difference in air consumed or effort while diving. No measureable change in air consumption taking a deco bottle, two reels, ankle weights, etc. Adding a few more blasts of air in your wing or BC will make you neutral if you are overweighted. Dive #1 with X lbs and dive #2 with X+10 will not yield measureable results. Other factors affect air consumed much more notably including stress, tasks (hunting vs. cruising), current, etc.

4.) The amount of lead you carry is a measure of skill. "X diver only uses 10lbs of lead - wow he's good!"

You need to know the whole story. A neoprene drysuit diver with thick thermals and an aluminum tank will require more lead than a trilam diver using steel tanks for example.

To be continued...

--Matt