Basic Buoyancy

So I'm working on becoming a DM and my instructor has given me a homework assignment to explain why being overweighted makes buoyancy difficult. We hear this all the time -- if you wear too much weight, you have a tendancy to pop or have difficultly hovering -- but I don't think I've ever hear a clear, succinct explanation. And frankly, I never really thought that much about it until this assignment. After considering the problem, I decided to post my thoughts here for other to comment on. So here goes.

As we know, buoyancy while diving is the result of two opposing forces, your weight which is the result of the earth pulling you "downward," counterbalanced by the "upward" pressure exerted by the water we displace. In order to become more negative, we use lead, which for its relative weight, displaces very little. It is, therefore, very efficient at creating negative buoyancy.

To become more positively buoyant, we add air, which is at the opposite end of the spectrum -- it weighs very little, but when contained in our bcd bladder, it displaces quite a bit of water.

So, why do we pop? Well, the issue seems to me to come down to the fact that air is compressible (and lead is not). More importantly, for this issue, air does not expand on the way back up in a linear fashion as a function of depth. For example, while ascending from 30 meters to to 10 meters (20 meters total), the volume of air in your BC doubles. It doubles again from 10 meters to the surface. In other words, as you ascend, the rate at which the air expands increases and your buoyancy increases at the same increasing rate. You need to vent faster and faster to prevent the pop. The larger amount of air you have in your bladder to offset the lead (which has constant buoyancy characteristics), the greater the problem. The solution, therefore, is to use a little air as possible (and also therefore to use as little lead as possible).


Gurus, how did I do?

I'm no guru but it makes sense to me. I might rephrase it as "The more air you have ..." rather than "The larger amount you have..." but other than that it seems like you have it understood and can explain it relatively well to others.

Thank you, thank you, thank you! And thank your instructor for giving you this assignment!

That is beautiful and should be put on the wall of every shop where it is thought that planting divers on the bottom is ok!

Just change "The larger amount" to "The greater volume". And I personally would say " The solution therefore is to use as little lead as possible to lessen the amount of air needed in the BC. While still maintaining the ability to stay neutral at the safety stop with an empty cylinder. An empty cylinder being one that contains 300-500 psi."

There is another factor related to what you wrote that you might want to include:

You actually have two kinds of buoyancy compensating devices in play_ your BCD and your lungs. In the best of all situations, you can handle most of your buoyancy adjustments merely through adjustments in breathing, and that is what a skilled diver tries to do. As your buoyancy changes during a dive, those breathing changes can override those other changes so that you don't have to resort to adjusting the BCD.

When a diver is overweighted, the BCD has much more air in it, and the ability of the lungs to overcome its variations is limited. When properly weighted, the lungs are the most important factor; when overweighted, the BCD is much more in control.

I see this regularly. When I do an OW pool session, I demonstrate to the students how I can slowly rise from the floor of a 12 foot deep pool, go to the surface in full control until my head breaks the surface, and then return to the floor of the pool, all using nothing but my breathing to control the ascent and descent. If I go into that same pool to practice technical diving while wearing my backplate and double steel 108s, I am so overweighted that I can only do that withing a range of a few feet.

I wrote a series of articles that go into some detail on buoyancy - including the effects of over-weighting. You might want to check them out:

Deals with the issue of over-weighting and the problems that it causes -
Scuba Buoyancy Masterclass 3of9 - Achieving Great Buoyancy Control -Scuba Tech Philippines

Deals with the impact of weighting on trim/performance -
Scuba Buoyancy Masterclass 5of9 - Trim and Position -Scuba Tech Philippines

t-mac, Yeah. The more weight you wear the more air the BC needs to get you neutral. So there's more in there to expand (or contract). Of course we all know you need about 5 pounds more when starting the dive so you can be neutral at the end--especially if doing a safety stop. This means that except for some undefinable exact point in time, you are never perfectly weighted.

The explanation the PADI Peak Performance Buoyancy course has is a great write up on several different effects that over weighting can cause. Using more air, more air in the BC has a greater effect when changing depths, less streamlined when over weighted, causes more physical effort so your breathing harder, trim issues for a few. Good luck with your assignment.

I second Laplenta's comments, and think t-mac did a nice job answering the question. I also like to do boulderjohn's demonstration, and spend extra time with advanced open water students working on that level of buoyancy control. It all starts with proper weighting.
DivemasterDennis

I like the OP's explanation, because I think it makes the point that it isn't IMPOSSIBLE to control your buoyancy when overweighted, it's just more difficult. Tech divers routinely go into the water as negative as the typical overweighted open water diver, and handle that just fine. But it requires acute awareness of the need to manage your air spaces, and most beginning divers simply don't have that.

Proper weighting is very important to neutral buoyancy, and when I teach an open water class I start mentioning how your best buoyancy device isn't a device at all, it's your lungs, right after I introduce myself and ask why they are taking the class. I teach not only teach the required skills in the pool, we do buoyancy practice, and a lot of divers don't realize that proper kicking habits also help with buoyancy, so right from the beginning of class, students work on flutter kicking properly, not bicycle kicking and adding weight to stay down.