Drysuit vs BCD bouyancy control

[Sideband]

The previous poster had said that " it becomes impossible to get positive"
This should never happen. EVER. If You "accidentaly forget" to plug in your suit and you find that you are dropping like stone,( you must be pretty overweight in the first place) you should be able to swim to the surface drop your belt and maintain positive bouyancy. This is worst case mind you.
If you have figured out your weight at the start of your dive there should be a squeeze on the suit and by the end of the dive there should olny be about 3-6lbs of extra air in the suit, which is really not much. Plus it keeps you warmer with more air.

Of course it shouldn't. Divers should NEVER ever ever ever run out of air either, but the fact that they shouldn't doesn't make it not happen.
I know a lot of BP/wing users that don't have a weight belt to drop. The incident I mentioned happened to be such a case.

 

[wcl]

It is probably appropriate and instructive to use only drysuit for training purposes at the beginning, keeping the task loading low, and allowing one to learn the drysuit bouyancy characteristics isolated from other variables.

Our PADI instructors did advise us to add BC use as soon as we were comfortable with that.

That being said, I wolud be happier if the PADI course materials clearly spelled out that evolution in skills and equipment usage. You walk away from the book with only the first (short) part of the learning curve (use drysuit).


In my first post here, I noted that I didn't really have to use my BC after removing suit squeeze and not freezing. Now that it is getting warmer (48 deg F at depth ... warm is relative, I guess), I actually happily did get to use my BC this weekend, and it is just the right thing to do: The BC is something I am familiar with, good at using, designed to add and remove bouyancy efficiently and quickly, with multiple dumps usable in most any attitude, and has a better-controlled bubble placement. Sweet!


Cheers,
Walter

Maybe that is why PADI teaches divers to use the dry suit - it is easier. A lot of things are easier, but not necessarily better.

I believe that if have the ability to use the BC for buoyancy and the dry suit for squeeze, then you can also use only the dry suit for buoyancy if you wish. However, if you only know how to use the drysuit for buoyancy, you don't have the skills to do both. Learn both and then choose.

Using the suit and BC is not a big deal task-loading wise, and that it is easier, should not (IMHO) be the deciding factor.

.02

 

[Canadian_Diver]

Hi,

I did my best to find a similar post with poll and nothing. I personally only use the drysuit for bouyancy. But, I was wondering how many people do?

Thanks

I was just mentioning to someone in another thread my preference on this topic.

Up until two weeks ago, I used to use only my BCD for buoyancy with only a little air in my suit to get the squeeze off. Since then, I had smaller different boots fitted on to my drysuit and 1.5 inches taken off the legs.

I now only use my BCD to float on the surface. My drysuit controls my buoyancy now. I find I'm warmer, more comfortable, and Big Jim n' The Twins aren't crushed. Also, one less piece of equipment to worry about dumping air out of.

 

[lairdb]

This is only sport diving guys!! You don't have mandatory deco, you don't have stages, keep it simple.

Not always true here. Many of the folks here regularly engage in multi-stage, mandatory deco, etc. etc., and most of the discussions are able to keep that in mind and remain polite.

 

[cameron]

Well sunshine if we are talking about multi-stage deco dives then the whole thing changes. If you are using your drysuit for back up, asalot of people do, then you should be able to fly while only using it for obvious reasons. But when most people are saying "I'm a new drysuit diver" I assume that most of them arn't conductiong staged deco and that most are on a single. It such case I feel that it is perfectly reasonable to say that you could use a drysuit exclusivly for bouyancy.

"Of course it shouldn't. Divers should NEVER ever ever ever run out of air either, but the fact that they shouldn't doesn't make it not happen.
I know a lot of BP/wing users that don't have a weight belt to drop. The incident I mentioned happened to be such a case."

Well running out of air is onething, but starting and finishing a dive severly over weight is another. Running out of air could be attributed to freeflow or whatever but startign a dive grossly over weight in just plain stupid. Lets jump in and hope it all works out.

 

[Lightning Fish]

How can it take up extra air? It doesn't matter whether the air is in the DS or BCD, the same amount is used, otherwise the buoyancy would not be the same.

Is this true? I can't be bothered to delve into the books, so... feel free to correct any mistakes.

Bouyancy is a function of volume and weight. If one cubic foot of air is added to my BCD, what is the volume increase? Now, instead of adding air to my BCD, if I were to add one cubic foor of air to my drysuit, what is the volume increase? I would say the weight increase is negligible (unless you add your whole tank .) I'm not sure the volume change would be equal due to different materials, different shape, different internals that can 'trap' the air, (i.e., fleece perhaps?).

Comments?

Cheers,
Bill.

 

[JeffG]

Comments?

Cheers,
Bill.

1 ft3 of gas is 1ft3 of gas. It displaces the same amount of water and occupies the same space regardless of the shape of the container.

 

[Lightning Fish]

1 ft3 of gas is 1ft3 of gas. It displaces the same amount of water and occupies the same space regardless of the shape of the container.

Get a refund???

So a rigid tank containing 1 ft3 of gas will displace the same amount of water as a BC containing 1 ft3 of gas?

Let me rephrase what I said, replace 1 ft3 of gas with the equivalent of 1 ft3 of gas at 1 atm. Or, in other words, a diver is at some depth, say 100 fsw, with an aluminum 80 containing 2000 psi of air and their inflator for their BC and drysuit provides the same flowrate. The diver can either press the inflate button for either the BC or drysuit for the same amount of time, let's say 5 seconds. I'm suggesting the volume change is not neccesarily equal and thus the bouyancy change is not neccesarily equal.

Why? The relationship between pressure and volume change is K, the bulk modulus:
K = pressure/volume change
or
K = sigma/(delta_V/Vo) where sigma is the stress or hydrostatic pressure, delta_V is the change in volume, and Vo is the original volume.

Another way to look at it is Hooke's law:
Stress = Y x Strain, where Y is Young's Modulus. This is written as 1-D, but I think it can be worked into a volumetric strain.

The bulk modulus is related to Young's Modulus. For steel Y ~= 30,000,000 psi (200 GPa), while the BC and drysuit has a much lower value. So when a tank is filled to its standard operating pressure, its deformation is negligible while the BC and drysuit will visibly change in volume. Now, since the BC and drysuit are made of different materials, I would expect them to have different values for Young's modulus or the bulk modulus. Thus, the volume change would be different.

Cheers,
Bill.

edit: steel Y ~= 200 GPa, rubber Y ~= 0.01-0.1 GPa, nylon Y ~= 2-4 GPa, from
http://www.nationmaster.com/encyclopedia/Young's-modulus

 

[RIDIVER501]

BCD on the surface only.
drysuit U/w for Bouyancy.
Way have to worry about managing 2 air spaces?

 

[JeffG]

Get a refund???

I was having a bad hair day. :eyecrazy:

So a rigid tank containing 1 ft3 of gas will displace the same amount of water as a BC containing 1 ft3 of gas?

Wasn't what you said. With a pressurized vessel (ie tank) will always displace the same amount of water regardless of how much gas is in it. Its buoyancy will change becuase the weight of the tank becomes less as you consume the gas durning the dive.

Let me rephrase what I said, replace 1 ft3 of gas with the equivalent of 1 ft3 of gas at 1 atm. Or, in other words, a diver is at some depth, say 100 fsw, with an aluminum 80 containing 2000 psi of air and their inflator for their BC and drysuit provides the same flowrate. The diver can either press the inflate button for either the BC or drysuit for the same amount of time, let's say 5 seconds. I'm suggesting the volume change is not neccesarily equal and thus the bouyancy change is not neccesarily equal.

Then you would be wrong. The gas in both the drysuit and the BCD are under the same ambient pressure. The only way there could be a difference is if you create a situation where your BC or Drysuit becomes a pressurized vessel (ie filling it beyond ambient pressure.)