Pinpoint accurate buoyancy control is a fundamental scuba skill. Precise control of your buoyancy is what enables you to hover completely motionless, then back out of an area without using your hands at all.
At first glance, buoyancy control looks like a simple matter of balancing the downward force of your ballast weights against the upward force of your BCD inflation. When the two cancel out, you’re neutral and can hover in the water. Since the weight on your belt doesn’t change after you enter the water, it seems as though you have only one variable to contend with: the upward thrust of your BCD. It sounds easy, so why isn’t it?
In fact, pinpoint buoyancy control requires getting at least six things right. There’s good news, though: Once you get all six variables dialed in, you’ll find it much easier to control your buoyancy.
The six factors that affect your buoyancy are your ballast weight and your BCD inflation, of course, and also your trim, your exposure suit buoyancy, your depth and your breath control. Your ballast weight and your trim are the only two factors that, once you’ve selected them, stay put. All the others are variables, changing during the dive along with time or depth or both. Some you can control, some you can’t. Buoyancy control isn’t as easy as it looks.
1. Ballast Weight
The ballast weight on your weight belt or in your integrated pockets don’t change during a dive, but it’s often the biggest problem. Many (if not most) divers are over weighted, carrying more lead than they need. That makes buoyancy control more difficult because every extra kilo of lead has to be balanced with additional air volume for buoyancy. To displace a litre of water and balance the kilo of lead requires an air bubble in the BCD of about one pint in volume.
But because an air bubble expands and contracts with depth changes, you have to be constantly adding or subtracting air from the bubble to keep its volume at one pint. Five extra kilos, which is not uncommon, means a five-pint bubble that grows and shrinks five times as much with depth changes and needs five times as much adjustment in order for you to maintain neutral buoyancy. So extra lead means extra thrust up or down when you change depth, and requires extra fiddling with your BCD valve controls. Sometimes it means nearly constant fiddling.
The first step is to just do it – take off two kilos before your next dive. Can’t get below the surface? Before you reach for the lead again, make sure you really need it. Getting below the surface, especially on the first dive of the day, can be surprisingly difficult and can trick you into carrying more lead than you really need. Here are a few tips:
- Be patient. The plush lining of a dry wetsuit can trap a surprising amount of air, and therefore buoyancy, in its fibers, and it takes a minute or so to get fully wet.
- Reach up. Hold the inflator hose over your head and stretch it upward a little so its attachment point to your BCD is highest. At the same time dip your right shoulder and squeeze the BCD against your chest with your right arm. This maneuver encourages the last few bubbles to find the exit.
- Rock backward a little. Many BCDs trap a bubble of air just behind your head. Rocking backward as if you are in a La-Z-Boy recliner moves the exhaust hose over the bubble and lets it escape.
- Relax. Many of us move our hands and feet more than we realise, especially at the beginning of the dive. It’s nerves: Without realising it, your body is trying to climb out of the water. That generates upward thrust, making you seem lighter than you are. To counteract that, hold your right arm still at your side (your left is holding up your exhaust hose), extend your legs and point your fins straight down so they have the least resistance to sinking.
- Exhale. Another manifestation of nervousness is a tendency to hold your breath, and a lungful of air adds as much as 4.5 litres of buoyancy. Exhale and hold it until you start sinking, then take shallow inhales until you get below 2 meters.
- Force it. Another option is to use your body weight to generate some downward momentum by lifting part of it out of the water, then letting it fall back. Lying on your face, jackknife your upper body downward, then lift one leg, then another, out of the water. The weight of your legs will drive you downward, and once your fins are in the water you can kick down.
But how do you figure out what’s the ideal amount of weight?
With a nearly empty tank, say 50 bar, with lungs half full and with no air in your BCD, you should be close to neutral at the surface – floating with the water at eye level, for example – and only slightly negative at your 5 meter safety stop. Some divers will be even lighter than that, so they’re neutral at 5 meters. That makes them slightly positive as they ascend to the surface, but they can counter that by holding less air in their lungs and taking only shallow breaths.
Once you get close to the right amount of lead, you can fine-tune it at your safety stop when your tank is nearly empty and you don’t have much else to do for three minutes anyway. Here’s one way:
Carry your smallest weight, one or two kilos, loose in a pocket or clipped to a D-ring so you can take it off easily. When you reach your safety stop with 70 bar left, hand it to your buddy temporarily or put it on the bottom if the water is shallow. Now, try to get neutral again. Remember to keep your hands and fins as still as possible. Do the test next to the mooring rope for security if you want, but remember you can always overcome a kilo or so of positive buoyancy by exhaling and kicking downward. When you’re making adjustments so small, there’s no reason to fear an uncontrolled ascent. If you can stay neutral at 5 meters without that small weight you took off, you don’t need it, and your next dive will be easier without it.
Now retrieve your weight from your buddy and do the same favor for them.
The next variable to worry about is your trim – the position your body takes in the water when you’re neutral and still. This matters for buoyancy because if your fins are lower than your body, kicking to go forward will also make you go up. It will seem that you’ve suddenly become buoyant, so you’ll vent air from your BCD. Then, when you stop kicking, you’ll be too heavy and you’ll sink.
In order for your kicking not to disrupt your buoyancy, your body needs to be trimmed so your legs are nearly horizontal and your fins push you only forward. Here’s how to check your trim:
Once you are exactly neutral, hold your body absolutely still with your legs stretched out behind you. If your legs sink, you should move a little weight from your waist to a point higher on your body.
3. Tank Weight
If using an aluminium cylinder, your tank gets lighter as you dive and use up the air in it. The 80 cubic feet of air pumped into your full tank weighs approx 2.7kg, and when you breathe it down to 50 bar, you’ve used up 2.2kg of that air, so the tank weighs 2.2kg less. That’s a buoyancy shift that has to be countered by venting 2.2kg of buoyancy from your BCD. And that explains why you have to start the dive 2kg+ heavy – so you have 2kg+ of buoyancy in your BC to lose and be neutral at the safety stop.
Fortunately, this weight loss and buoyancy gain is gradual. If a tank can last you 60 minutes, it gains only 500 grams in 10 minutes and you hardly notice it. Also, the tank’s buoyancy gain is affected by depth only in the sense that you use up air faster when you are deeper. Because the tank is rigid, its buoyancy does not change immediately just by going 5m deeper or shallower.
So you will have to adjust for the tank’s buoyancy change, but it won’t take you by surprise. You probably won’t notice any change until nearly halfway through the dive.
Incidentally, it’s not true, as many divers believe, that you can escape this buoyancy gain by using a steel tank. Steel tanks are typically less buoyant than aluminum to begin with so they may end the dive slightly negative while an aluminum tank is positive. But 80 cubic feet of air weighs just as much in either tank, and the buoyancy gain when you use it up is just as much. Using a steel tank allows you to take a few kilos of weight off your belt, but you have to carry some or all of it in the tank itself, which is typically heavier.
4. Exposure Suit
Wetsuits float. There’s no escaping the fact, because the same thing that makes neoprene warm makes it buoyant: the gas trapped in thousands of tiny bubbles. Their buoyancy (and warmth) varies, but, in general, a new men’s wetsuit has 1 to 1.5 litres of buoyancy for every millimeter of thickness. So a thin tropical suit might have around 1 litre of buoyancy at the surface while a thick cold-water suit might have 9 litres or more.
It’s tempting to minimize the neoprene to make buoyancy control easier. Some tropical divers wear no neoprene at all. But that might be a bad bet, because getting cold is fatiguing and increases your risk of decompression sickness.
The buoyancy of your wetsuit won’t change noticeably from one dive to the next, but over time it does lose buoyancy because the thousands of tiny bubbles in the neoprene lose their resiliency and collapse or fill with water. At that point, the wetsuit has less buoyancy and less insulation than when new.
The good news here is that if you don’t change depth, your wetsuit’s buoyancy doesn’t change either. Once you have your buoyancy dialed in for a given depth, you can forget it. More good news: The very thin wetsuit you’d wear in the tropics has so little buoyancy to begin with that you can pretty much ignore any changes with depth.
Whatever the surface buoyancy of your wetsuit, it will change dramatically with depth. Because pressure flattens those thousands of gas bubbles, your wetsuit gets thinner and displaces less water. In effect, it gets heavier. The change is not linear. You lose half of your surface buoyancy in the first 10 meters of your descent and a third in the next 10 meters. Below 20 meters, there’s only one-sixth of the original buoyancy left to lose no matter how deep you go. The single larger bubble in your BCD behaves the same way.
Buoyancy changes fastest in the first few meters below the surface–three times as fast at 1m as at 20m. That’s why it’s often hard to get submerged, but once you’re down 2m or so, you seem to get heavier and sink easily.
Unlike the buoyancy change in your tank, this buoyancy shift is immediate and goes in both directions. When you ascend, you get back the buoyancy of your wetsuit and your BCD instantly. So you have to be alert to buoyancy changes whenever you change depth, and especially when you ascend.
6. Breath Control
Your lungs are a natural buoyancy compensator with about 4.5 litres of buoyant lift. A normal, resting breath expands your lungs by about 0.5l, givingyou 0.5l more buoyancy. Breathing in and out, your buoyancy fluctuates within a range of about 0.5l to 1l. But you can place that one-litre fluctuation almost anywhere in the total 4.5l range. You can breathe from nearly full lungs and cycle between 3.5 and 4l of buoyancy, for example, or you can breathe with nearly empty lungs and cycle between 1l and1.5l. So as long as you are nearly neutral with a half-breath, you can rise or fall at will just by controlling your lungs.
Putting it all together….
Once you get your ballast weight and trim dialed in, you’ve come a long way toward perfect buoyancy control. Now you can fine-tune your BCDinflation to compensate for the very predictable changes due to breathing down your tank and changing depth and use only breath control to drop gently down to that nudibranch, hover inches above it as long as you want to get that perfect photo and lift away from it harmlessly.