Why is Buoyancy So Hard?

Maybe this scene sounds familiar: you start your descent, letting air out of your BC at the surface and breathing out, only to find yourself accelerating faster and faster until you hit the bottom. Once at the bottom you add some air to your BC, relieved to establish neutral buoyancy as your feet and hands come off the bottom as you inhale. Phew! But your relief is cut short, as you soon find yourself floating slowly, then quickly right back to the surface in an uncontrolled ascent to repeat the cycle all over again. 

How can you break the cycle and stop diving like a Yo-Yo? And why can buoyancy feel so unstable in scuba as a new diver?

Let's review the basics of how buoyancy works in water:

1. Objects that sink are "negatively" buoyant. Full tanks and lead weights are in this category.

2. Objects that neither sink nor float are "neutrally" buoyant. Achieving this state should be your aim for all dives. Most human bodies are nearly neutrally buoyant or a bit postive when holding a full breath.

3. Objects that float are "positively" buoyant. A BC full of air, your lungs, a neoprene wetsuit (because it contains tiny bubbles embedded in the neoprene), and a drysuit (because it traps air between you and the water) are all positively buoyant.

4. As divers, to achieve neutral buoyancy, we start by adding enough weight to our kit to counteract the positive buoyancy of our exposure suits, our body (if you have higher fat composition), and sometimes our tanks (aluminum tanks will float when low on gas) - this gets us roughly neutral.

5. Depending on your tank capacity, we start most dives with about 4 or 5 pounds of "extra" weight in the form of compressed gas that is gone (breathed out) by the time the dive is done. 

6. We literally lose 4-5 pounds of weight during each dive (the gas weight lost from inside our tanks to the atmosphere). To compensate for this, we should start each dive 4-5 pounds negative when measured with an empty BC. 

7. We offset that negativity by keeping about half a gallon of air in our BC - enough to displace 4-5 pounds of water - so we can start our dive neutral.

OK. Here's where we start to understand why buoyancy can take effort. It is because the buoyancy of your scuba kit is inherently unstable. 

• As we descend, gas in our BC and exposure suits is compressed by water pressure and shrinks - becoming less positive. This makes us more negative, accelerating our descent, which means more pressure the deeper we go, which means we get more negative. Without taking any action - every descent would become a runaway descent. 

• As we ascend the opposite happens. Gas in our BC and exposure suits expands - if we don't vent that gas - the expansion leads it to take up more volume, displacing more water, making us more positively buoyant. As we become more positively buoyant we ascend faster - reducing the ambient water pressure as we ascend - leading to more gas expansion, even more positive buoyancy and up we go! Without taking any action - every ascdent would become a runaway ascent. 

• Neutral buoyancy is a bit like a delicately balanced ball on top of a hill. If it holds still it is stable, but a nudge in any direction and it will roll faster and faster away. 

How do we fix this?

1. As we descend we add small amounts of air to our BC and (if applicable) drysuit - as well as our ears and sinuses - to maintain the same air volume that will keep us neutral.

2. As we ascend, we need to do the opposite and vent a bit of air. 

3. But the true secret to maintaining neutral buoyancy most of the time on a regular dive (without big depth changes) is that you use your breath. Your breath is your secret weapon! 

4. If you find yourself going up slightly you will, if you take no action, end up going to the surface in a runaway ascent. So don't do that! Breathe out a bit (or a lot) until you arrest the ascent... then resume breathing normally (slightly shallower if you need to descend, slightly fuller breaths if you need to ascend). 

5. If you find yourself going down slightly, you will, if you take no action, end up slowly, then quickly descending into the depths. Don't do that either. Breathe in deep! 9 times out of 10 that will arrest your descent. Add a bit of air to your suit/BC if needed. 

The secret to neutral buoyancy is - use your BC and drysuit to get in roughly the right ballpark - and from there, you have to make constant small adjustments to your breathing volume to bring yourself back to neutral. It is like driving a car. We don't steer in one direction and expect to arrive at our destination. Even when following a "straight road" (e.g. neutral buoyancy at a set depth) we are constantly making tiny, almost imperceptible adjustments to the wheel - a little left, a little right to stay in our lane.

Maintaining your depth and buoyancy when diving is exactly like that. Experienced divers are making small, constant adjustments to their breathing volume - because if they didn't they would either have a runaway ascent or descent. 

Troubleshooting Problems

• PROBLEM: Even when I breathe in or out, the buoyancy changes in my BC quickly overcome the changes in my lung capacity and I still end up yo-yo diving. 

• TRY THIS: You may be overweighted. When you are overweighted you have to carry a larger air bubble in your BC to counteract the extra weight. Bigger air bubbles change volume faster with changes in depth. Try taking off a few pounds and see if you can still stay down at the end of your dive. Alternatively you may not be using the full range of your lung capacity. At times you may need to breathe in VERY deeply to arrest a descent (or start an ascent) or very shallowly (breathe out most of your air) to descend - until you have a chance to adjust your BC/drysuit. Diving overweighted increases your risk and can be dangerous - not least because it can increase your risk of runaway ascents and descents.

• PROBLEM: I don't realize I'm ascending till it is too late! TRY THIS: dive closer to the bottom or another reference point (2-3 feet away) so it is easier to tell when you are drifting away. If you dive far off the bottom, it is very difficult to tell if you are moving up or down.