Bouyancy Questions/Help

[JVM4.0]

Brief history: My wife has been certified for a couple of years with only a few dives under her weight belt. I have just completed my confined water training, and am looking forward to my OW test, as soon as we can chip enough ice off the local pond.

Yesterday, my wife and I were doing some practice work in a local pool (11' deep) while our instructor was working with other students, and found some interesting things:

1) During my training, the instructor suggested I use about 20# of weight. I complied, but thought that sounded kind of heavy. Yesterday, since we were kind of "on our own" I experimented with weighting. I ended up using 8# and still had to give a couple of short bursts into the BC to be neutral.

2) While playing with the weights, my wife and I would face each other, and see how long we could hover without any kind of input. We could go for a relatively long time before something would cause my wife to virtually shoot up to the surface. Kind of a weird feeling, watching her go. She seemed to be breathing normally, not moving a muscle, trying to outdo each other in the hover, and then......going up!.......for no apparent reason.

Questions are these:

1) How much weight SHOULD an overweight (210#), balding, ex-hippie be using to be neutral? Should I go with a few more pounds, just to be "safe"? I seemed to be better able to control myself with less weight, for obvious reasons. Is that a good thing?

2) Why did my wife just kind of go positive all of a sudden? There was no breath holding, and she denied far**ng in her wetsuit! Are these related to any kind of experience issues we are missing?

I'll look forward to seeing what the experts have to say about all this...........

 

[Walter]

1) How much weight SHOULD an overweight (210#), balding, ex-hippie be using to be neutral?

It's impossible top tell without getting you in the water and working with weights. You should float at eye level with no air in your BC, full lungs and an almost empty tank. You will need to add weight for salt water.

Should I go with a few more pounds, just to be "safe"?

No.

I seemed to be better able to control myself with less weight, for obvious reasons. Is that a good thing?

Yes.

2) Why did my wife just kind of go positive all of a sudden?

She probably breathed more deeply. Buoyancy control is much more difficult in shallow water.

 

[MXGratefulDiver]

Walter provided good answers ... here's my contribution ...

1) How much weight SHOULD an overweight (210#), balding, ex-hippie be using to be neutral? Should I go with a few more pounds, just to be "safe"? I seemed to be better able to control myself with less weight, for obvious reasons. Is that a good thing?

Well, this overweight (250#), balding, ex-hippe uses a four lbs of weight in the pool with a full 3 mm wetsuit. You might need a bit more, but 20 lbs sounds very excessive.

2) Why did my wife just kind of go positive all of a sudden? There was no breath holding, and she denied far**ng in her wetsuit! Are these related to any kind of experience issues we are missing?

As Walter said, her breathing pattern changed. Remember that your lungs are just another BCD. Change the amount of air in them and it changes your buoyancy. Chances are she retained a bit of air in her lungs on the exhale, and it caused her to start rising. Once started, Boyle's Law takes over ... the air starts to expand, and you have to expel even more air to stop your ascent. At that point, simply exhaling might not be enough, you'll probably have to vent from the BCD as well.

... Bob (Grateful Diver)

 

[Jason B]

Hard to add to what has already been stated...
Just remember that once she started going up (probably due to the reason given above) that not only did the air in her BC start to expand, but the wetsuit also starts to expand. All these things combined play part in her "virtually shooting", as you put it, to the surface.
Keep practicing, sounds like you two are getting it.

Jason

 

[scubacurt]

1) How much weight SHOULD an overweight (210#), balding, ex-hippie be using to be neutral? Should I go with a few more pounds, just to be "safe"? I seemed to be better able to control myself with less weight, for obvious reasons. Is that a good thing?

There is no "should" amount of weight that you "should" use. The rough (very rough) rule is 10 percent of your body weight. Unless you are overly obese or wearing bunches of neoprene 10 percent of your body weight will usually leave you overweighted. Most instructors (me included) use this rule so that we don't have to lug bunches of weight around for our students. It is easier to start them out over weighted than it is to keep getting out of the water to add more. If 8 pounds workes for you then go with it. If you are going to be diving in the ocean where there is some current or surge, overweight yourself so that you can "stick" to the bottom when you want to. It would be ideal if we could all dive like the Navy guys did in the Ole days. No BC and a few pounds of weight.



2) Why did my wife just kind of go positive all of a sudden? There was no breath holding, and she denied far**ng in her wetsuit! Are these related to any kind of experience issues we are missing?

My guess is that as your wife kept concentrating she was holding more and more air in her lungs. As we concentrate on a task we have a tendency to do this. That is why most students float to the surface while performing tasks such as mask clearing or BC removal and replacement underwater. Practice this. Breathe normally now dont exhale the last 10 percent of your air and take another breath. Before you know it you arent exhaling the last 25 percent of your air. Also when we take deeper breaths we have a tendency to occupy the space in our upper lung lobes which are beneath the ribs. It is not so easy to see how expanded someones lungs are while it occurs beneath the ribs. They appear to be breathing normally but are retaining air and maintaining a larger surface area with their lung expansion. Next let's look at the physics of shallow water buoyancy. The most drastic pressure change occurs in your first 33 feet of water (1 full atmosphere, doubling your pressure) which means that in the first 11 feet of a column of water leaves you more sensitive to pressure changes. The first 11 feet of water is equal to about 1/3 of an atmosphere change. Which means by the time your wife goes up 1 - 2 feet she is already on her way to an uncontrolled ascent. I think that you will find that if she recognizes early and exhales completely she will be able to "catch" her buoyancy. Yet one more factor. If she is weighted "lightly" or with just barely enough weight to keep her down with a normal lungfull of air, she has less mass. In the water the less mass you have, the faster your body can react to buoyancy changes. Imagine that you have a 5000 pound block supported by a lift bag in the water. Everything is neutrally buoyant. If you put one breath of air into the lift bag it will become positively buoyant but will not react as quickly because all of that mass has to gain momentum. We are no different than the block. The more we weigh the longer it takes us to gain or lose momentum.

I hope this answered your questions.
-curt-

 

[JVM4.0]

Thanks for the input, guys. We didn't have a chance to follow up with our Instructor yesterday, and I keep getting these nagging thoughts about what was going on.

I think in reality it's the difference between what I "know" and what I "realize".

All good answers, but I still think she was far**ng in her suit!

 

[Charlie99]

Should I go with a few more pounds, just to be "safe"? I seemed to be better able to control myself with less weight,...

Just adding a bit of detail on "add a few more pounds, just to be safe" -- you've already gotten lots of good advice.

Neutral buoyancy is an unstable equilibrium. Your wetsuit and any air you have in your BC will expand as you rise, increasing your buoyancy and tending to make you rise even faster, which further expands the wetsuit and the air in your BC which increases your buouancy, which make you ........

Carrying excess weight means that you have to add more air in your BC to be neutral making this equilibrium even more unstable. The roughly 5 pound change in buoyancy as you breathe in and out can be used to fine tune your buoyancy and to keep a constant depth.

Someone who has had a runaway ascent incident would be tempted to try and cure the problem by adding even more weight, but in reality it only makes a future runaway ascent even more likely.

 

[jbd]

Neutral buoyancy is an unstable equilibrium. Your wetsuit and any air you have in your BC will expand as you rise, increasing your buoyancy and tending to make you rise even faster, which further expands the wetsuit and the air in your BC which increases your buouancy, which make you .......

This is a very important concept to understand and it also works in the other direction i.e. descending. All new divers and any students in an OW course should read and understand Charlie99's above comments thoroughly.

JVM4.0 the instructor should have started you out with proper weighting for you, as opposed to a suggested weight of 20 lbs. Also farting in the wetsuit just moves gas from one place to another so no effect on buoyancy

 

[theskull]

. . . Also farting in the wetsuit just moves gas from one place to another so no effect on buoyancy

Have to disagree with this one. The gas is compressed when it is in your body and expands as it is released into the suit.

Similarly, we just move gas from the tank into the BC, but it does indeed increase your buoyancy.

Of course, this portion of the discussion is just silly, but it is fun.

he he he, he said, "fart".

theskull

 

[donacheson]

Brief history: My wife has been certified for a couple of years with only a few dives under her weight belt. I have just completed my confined water training, and am looking forward to my OW test, as soon as we can chip enough ice off the local pond.

Yesterday, my wife and I were doing some practice work in a local pool (11' deep) while our instructor was working with other students, and found some interesting things:

1) During my training, the instructor suggested I use about 20# of weight. I complied, but thought that sounded kind of heavy. Yesterday, since we were kind of "on our own" I experimented with weighting. I ended up using 8# and still had to give a couple of short bursts into the BC to be neutral.

2) While playing with the weights, my wife and I would face each other, and see how long we could hover without any kind of input. We could go for a relatively long time before something would cause my wife to virtually shoot up to the surface. Kind of a weird feeling, watching her go. She seemed to be breathing normally, not moving a muscle, trying to outdo each other in the hover, and then......going up!.......for no apparent reason.

Questions are these:

1) How much weight SHOULD an overweight (210#), balding, ex-hippie be using to be neutral? Should I go with a few more pounds, just to be "safe"? I seemed to be better able to control myself with less weight, for obvious reasons. Is that a good thing?

2) Why did my wife just kind of go positive all of a sudden? There was no breath holding, and she denied far**ng in her wetsuit! Are these related to any kind of experience issues we are missing?

I'll look forward to seeing what the experts have to say about all this...........

You should try to carry minimal lead weight. Minimal is that needed to remain neutral with an empty BC, near empty tank, and breathing normally just below the surface. It is at the end of your dive when you are most buoyant and when you need to comfortably execute a safety stop and maintain control to ascend slowly to the surface.

More wetsuit means more lead. More lead is needed in salt than fresh water, about 2.5% of your total - you and your equipment - more.