Why neutral buoyancy isn't

[Garrobo]

OK, got a response from my buddy saying that you need to add enough weight to keep you down or neutral with an empty tank. I agree. So jump in the water with an empty tank and add enough weight so you don't pop to the surface like a cork. That's even easier.

 

[simbrooks]

Using the Buoyancy Compensator

Aside from flotation on the surface, the BC should be used only to compensate for the changes in buoyancy experienced due to exposure suit compression with depth change and gas consumption during the dive. From our discussion of buoyancy above, adding air to a BC to initiate an ascent or removing air to initiate a descent when we are already neutral is totally unnecessary, as any displacement upwards or downwards will continue unless corrected.

My comment also kind of links in well with the proper weighting paragraph under this one. This article works well for most divers with "light" tanks like AL80's, but once you have heavier tanks and are slightly overweighted compared to the volume that you and your exposure protection can displace, you might need a little extra volume in the BC to compensate for that. Some tanks are neutrally bouyant or thereabouts when empty or full, but others are always negative and need some air to offset them on top of the weight of the air used from them during the dive.

 

[simbrooks]

OK, got a response from my buddy saying that you need to add enough weight to keep you down or neutral with an empty tank. I agree. So jump in the water with an empty tank and add enough weight so you don't pop to the surface like a cork. That's even easier.

This is coarse weighting and bouyancy, generally being heavy enough to get down from the surface and not too light to cork up, but not too heavy that you waste air on the BC keeping yourself from sinking even with a near empty tank. It works to a degree of accuracy and comfort.

What is mentioned in the article about breathing is fine tuning the bouyancy. People can get by with constantly adjusting their BC depending on their depth without concentrating much on the breath cycle and its effect on bouyancy. They might sink a bit, or float up and then adjust their BC to compensate as they werent concentrating on their breathing. Constantly adjusting the BC is a pain unless you are doing a multi-level dive when it becomes necessary.

I read somewhere the your lungs can displace the equivalent of 6-9# of water during the swing between fully inhaled and exhaled, so you can make up for a couple of pounds of under or over weighting that way, but also it gives you a powerful tool to use to adjust your depth as needed. Its one of those nice things to know that you can go up and down just by breathing - particularly if you are in a tighter spot or an area you dont want to disturb with kicks or sculling of your hands, you can just rise out of there or drop in to see something.

 

[LIVES4SHARKS]

THANKS RICK! It's amazing when NEUTRAL BUOYANCY is achieved. Now trying to maintain that...sheesh...it's very very hard. I work in the pool about every 2 weeks just trying not to lose control, stay in trim, and task load. ALL WHILE BEING HORIZONTAL not semi vertical. But it took working with a mentor for me to understand the concept and the "feel" of it! Gee now I can't wait to learn it all over again when I get a drysuit!

Remember, it's easy to be neutral when you are moving forward, it's another thing when you are holding still, or holding a safety stop with no line in shallower water! Who knew breathing (correctly) could be so difficult, but when done right, can take away weight and allow more freedom from continuous BC air adjustments!

GOOD LUCK AND HAPPY DIVING!
Carolyn :sharks:

 

[mstevens]

Personally, I can't see where buoyancy is such a big deal. You figure about 10% of your actual weight, give or take a few pounds, depending on your muscular content and how thick a wetsuit you are wearing, if you are using one. If you sink after letting the air out of your BCD you should be OK. When you get to the level you want to stay at, pump enough in your BCD to stay at that level. What's so complicated about that?

10% of my actual body weight is double the amount of weight I need. That's why this is a big deal. If I were to dive with the guesstimated weight, I'd need excessive air in my BC, which means more drag. It also means a proportionately larger change in buoyancy as I ascend, which means more fiddling with the BC. The only really good way to tell how much weight one needs is to jump in the water geared up. This stuff becomes important because one can't always jump into the right type of water - I live near a freshwater pond but fly to the Caribbean. It's good to be able to adjust weight rationally based on changes in tank size, moving from one exposure suit to another, or changes in water density.

Another issue is that weights are heavy (and therefore have mass that affects inertia). I have to drag them to where I've diving them walk around with them, climb in and out of boats with them, and change my velocity with them. Somebody might have to rescue me in them. Much better to dive with the proper weight instead of excessive weight "balanced" by excessive BC air.

 

[crpntr133]

Proper weighting is a big key factor in this whole equation. I started diving doubles and drop 2lbs off my original set up, which helped my trim. I could tell a difference on the next dive. Not only was it easier to move through the water but my buoyancy was a bit better as well.
If you think have your buoyancy down try photograhy or video. You might be surprised!

 

[Garrobo]

So you're worried about a little extra weight? What's the big deal? Every breath you take is going to pull air out of your tannk and change your buoyancy to some degree. You're only going to be down for an hour or so and the trip up the ladder shouldn't take you more than 15 seconds. If you are in any kind of decent physical condition that shouldn't be a problem.

 

[rahuck]

So you're worried about a little extra weight? What's the big deal? Every breath you take is going to pull air out of your tannk and change your buoyancy to some degree. You're only going to be down for an hour or so and the trip up the ladder shouldn't take you more than 15 seconds. If you are in any kind of decent physical condition that shouldn't be a problem.

Let's see...here are just a couple of interesting quotes:

"[FONT=Verdana, Arial, Helvetica, sans-serif]An over-weighted or under-weighted diver is at increased risk in a number of ways. In both, the diver has to work harder to maintain control and thus increases risk."
[/FONT]

-- September/October 2006 Alert Diver
​

"Controlling your buoyancy is one of the most important skills you'll master. Buoyancy control improves your safety, reduces fatigue and enhances the enjoyment of diving. It also enables you to avoid destroying delicate portions of the underwater environment.



Diving incident reports often cite overweighting and/or poor buoyancy control as a contributory factor to, or a factor associated with, accidents or near accidents. For example, in a study of 100 diving fatalities that occurred in Australia and New Zealand between 1980 to 1987, 45 percent of the victims were believed to have been overweighted, with 40 percent being more than 4.5 pounds/2 kilograms negatively buoyant on the surface."

-- The Ups and Downs of Buoyancy Control By John Lippmann
​

It looks to me like being properly weighted and having good buoyancy control makes you a safer diver.

 

[TSandM]

So you're worried about a little extra weight? What's the big deal? Every breath you take is going to pull air out of your tannk and change your buoyancy to some degree. You're only going to be down for an hour or so and the trip up the ladder shouldn't take you more than 15 seconds. If you are in any kind of decent physical condition that shouldn't be a problem.

Garrobo, from reading that, I have an image of you as being tall, young and strong. I'm none of those things, and carrying extra weight up a ladder, or up a long slope out of the water (especially in my doubles) is a major concern. In addition, although buoyancy compensation negates weight , it does nothing about mass, and extra weight taken into the water has to be moved around by muscle power, and that uses gas. If you don't believe me, strap on a big set of doubles and find out how much more work it is to swim them around!

One CAN most certainly dive significantly overweighted, and instructors often do, as they carry extra weight in case students need it. But it is inelegant and inefficient and not nearly as much fun as being properly weighted and doing almost everything with breath control. One of the things I love about diving in the tropics is that I can enter the water so close to neutral that I almost don't have to adjust the air in the BC at all, except for a vent every few minutes to make up for the gas I've used. Everything else is breathing. It makes you feel much more a part of the water.

 

[Warren_L]

Personally, I can't see where buoyancy is such a big deal. You figure about 10% of your actual weight, give or take a few pounds, depending on your muscular content and how thick a wetsuit you are wearing, if you are using one. If you sink after letting the air out of your BCD you should be OK. When you get to the level you want to stay at, pump enough in your BCD to stay at that level. What's so complicated about that?

One additional thing to note about carrying around extra weight that is not required - as you say, you add more air to your BC to stay neutral. That's fine, but consider that the more air that is required to keep you neutral, the more of a buoyancy change there's going to be as you ascend or descend. This means you're going to need even more air as you go down and you need to release more gas as you ascend - this is inefficient and can contribute to quicker changes in buoyancy as your depth changes versus having no excess weight.