Buoyancy Control Tips When Scuba Diving

Neutral buoyancy is what enables you to hover completely motionless, enjoying the sensation of "zero gravity." Use these six scuba diving buoyancy control tips to nail this fundamental (and fun!) dive skill

Weight

The weights you wear in your BCD weight system or on a weight belt are often the biggest problem, so that's the place to start.

When you have more weight than you need, buoyancy control is more difficult: Every extra pound has to be balanced with about a pint of BCD air. Since this volume expands and contracts with depth changes, you have to constantly compensate to keep the volume at one pint. Five extra pounds (which happens) means five un-need pints of volume that grow and shrink every time you change depth.

To check for proper weighting, with a nearly empty tank (500 psi) and an empty BCD at the surface, hold your breath (reg in mouth in case you sink). You should float at eye level and slowly sink when you exhale. If you check with a full cylinder, add 5 pounds to compensate for the weight change as you use up your air. This is because your cylinder gets lighter as you use the air in it. The air in an aluminum 80 weighs almost exactly 6 pounds and at 500 psi, it's about five pounds less. So, you should start the dive about five pounds heavy.

Despite what many divers think, it is not true that this doesn't apply to steel tanks. Steel tanks are typically less buoyant than aluminum in and of themselves, but 80 cubic feet of air weighs the same regardless of the cylinder type. This means the buoyancy gain from using a given amount is the same. A steel tank allows you to take a few pounds of weight from your weight system because tank itself is heavier, but you have to account for air use the same.

There are specialized situations where you want extra weight, but in general, scuba diving buoyancy control is easiest with the right amount – neither too much nor too little.

Exposure Suit

Wetsuits float because neoprene is rubber with gas trapped in thousands of tiny bubbles – that's how it insulates. Their buoyancy (and warmth) varies, but, in general, a new full wetsuit has two to three pounds of buoyancy for every millimeter of thickness. So a thin tropical suit might have less than two pounds of buoyancy at the surface while a thick cold-water suit might have 20 pounds or more. The suit compresses and loses buoyancy as you descend, which is why you use your BCD throughout a dive wearing a wetsuit.

The change is not linear. Buoyancy changes fastest in the first few feet below the surface—that's why it's often hard to get submerged, but once you're down a bit, you sink easily. Based on gas theory (Boyle's Law) your suit has half its surface buoyancy at 33 feet, a third at 66 feet, a quarter at 99 and so. In reality it's not quite this much reduction because the suit resists compression a little – but not much.

Want to perfect your buoyancy? Get started today with PADI's Peak Performance Buoyancy course.

Of course, this buoyancy change is goes in both directions, which is why you put air into your BCD going down and let it out coming up. Nailing this buoyancy skill requires you be alert to buoyancy changes as you move throughout the water column and adjust accordingly your BCD.

Do note that, while the buoyancy of your wetsuit won't change noticeably from one dive to the next, over time it loses buoyancy as the neoprene ages and as the bubbles in it break down with repeated compression and release. It's not only less buoyant, but also less insulation for the same reason.

Trim

Trim is the position your body takes in the water when you're neutral and still. This can affect buoyancy because if your fins want to be lower than your body line, either kicking forward also sends you up, or more likely you spend a lot of effort staying level. The ideaal is that once you are exactly neutral and hold your body absolutely still with your legs stretched out, you're in a level horizontal swimming position. If your legs sink or rise, shift some weight up or down on your body to balance.

Breath Control

Your lungs are a natural buoyancy compensator with as much as 10 pounds of buoyant lift. A normal, resting breath expands your lungs by about one pint, giving you one pound more buoyancy. Breathing in and out, your buoyancy fluctuates within a range of about one pound. So as long as you are nearly neutral with a half-breath, you can make fine-tune buoyancy adjustments just by controlling how deeply you breathe – but never hold your breath, of course.

Putting It Together

Once you get your weight and trim dialed in, you've come a long way toward perfect buoyancy control. Now fine-tune your BCD as you change depth and use breath control to drop gently down to that cleaner shrimp, hover inches above it as long as you want and lift away from it without disturbing anything. These are the basics, but to really get into the zone straightaway, ask your PADI Instructor about the Peak Performance Buoyancy course. Also, everything you learned here applies to diving with standard recreational dive gear, but buoyancy control is quite different using a closed-circuit rebreather (CCR). As about the PADI Rebreather Diver or Tec 40 CCR Diver courses for more about that.