Buoyancy control is the most important skill a scuba diver can master. Many divers believe their buoyancy control is good, but when put to the test it fails to come up to scratch. To understand how to attain good buoyancy control, we need to first define what buoyancy means.
When we place an object in water, it will either be positively buoyant (floating on the surface), neutrally buoyant (neither floating nor sinking) or negatively buoyant (resting on the bottom). To understand what will happen to a particular object, we need to know that when an object is placed in water, it pushes some of that water out of the way; this is called displacement. The weight of the object is a force pushing the object down (Force A). The amount of water the object displaces (pushes out of the way) is a force pushing the object up (Force B).
If the object weighs more than the weight of the water it displaces, then Force A is more than Force B and the object will be forced down; it will be negatively buoyant.
If the water the object displaces weighs more than the weight of the object, then Force B is more than Force A and the object will be forced up; it will be positively buoyant.
If the weight of the object is equal to the weight of the water it displaces, then Force A and Force B are the same and the object will hang mid-water; it will be neutrally buoyant.
When we scuba dive there are two states of buoyancy we want to achieve; positive buoyancy on the surface at the start and end of a dive, and neutral buoyancy when we are underwater on the dive. Many divers fail to properly achieve this state of neutral buoyancy; they actually are negatively buoyant and are using their fin (and often arm) movement to propel themselves off of the bottom. This is easily demonstrated when a diver stops swimming and they have kick or scull with their hands to maintain their position in the water. If you have achieved neutral buoyancy this is not necessary, you will be able to ‘hang’ or hover motionless.
As a diver you will know that we wear weights when we dive, either on a weight belt or integrated into the BCD (Buoyancy Control Device). This is because most divers will be positively buoyant on the surface, even when wearing a scuba unit with a fully deflated BCD. The water we displace weighs more than we do, and it pushes us up. To compensate for that we increase our weight (our downwards force), by wearing weights. If we weight ourselves correctly, we will wear just enough weights to increase our weight (our downwards force) to the point where it is the same as the amount of water we displace (our upwards force). Sounds easy, but we have to take into consideration that divers are not inanimate objects. Divers have their own inbuilt buoyancy control devices, our lungs. When we breathe in and add air to our lungs, we increase our displacement (our upwards force). When we breathe out and reduce the amount air in our lungs, we decrease our displacement. It is by wearing the correct amount of weight and by learning to control our breathing, that we can master buoyancy control underwater. Obviously it is important to remember the most important rule in scuba diving, to never hold our breath.
When we first start to learn to scuba dive, we are often a little excited and anxious about being out of a natural terrestrial environment. When we are excited and anxious we tend to breathe faster, but not exhale properly. We are increasing our displacement and start to rise in the water, leading us to believe that we need to wear more lead weight. What tends to happen at this point is that we add the extra weight and feel more comfortable about staying down, and we keep this amount of weight on and neglect to master breath control properly. In fact, if we are wearing too much weight it is very difficult to master breath control. If you are over-weighted, the change in displacement caused by breathing will have little to no effect, as it is not enough to overcome the downwards force created by the extra weight you are wearing.
Now you only have one of two options, neither of which is effective. Option one is to keep moving, either by swimming, or kicking up with your feet, or sculling up with your hands. This makes it difficult to stop and look at anything of interest, and uses a lot of energy and your air supply. Option B is increase your displacement (your upwards force) by adding more air to your BCD. This is ineffective because it creates more drag in the water, which again causes you to use more energy and air. Moreover any slight changes in depth will cause the air you have put into your BCD to change volume. If you go up even slightly the volume will get larger, further increasing your displacement. This is why over-weighted divers will still find themselves rising too quickly in the water when they make minor depth changes. At most you should have only a couple of short blasts of air in your BCD. If you are adding more, you definitely need to adjust the amount of weight you are wearing.
A diver who is correctly weighted and has mastered breath control will use fin motion only to propel themselves forward in the water and not to stay off of the bottom. If there is no current present in the water, the diver may appear to barely move their fins at all. They will only use their hands for tasks. The diver who is not correctly weighted and has not mastered breath control, will always keep moving their fins (and hands), even when they stop to look at something.
You should start by doing a buoyancy check at the surface to ensure you are wearing the correct amount of lead weight. To do this, fully deflate your BCD and hold a normal breath; you should float at around eye level and then as you exhale you should begin to sink slowly. If you are doing the check with a full cylinder of air, you will need to add approximately 1-2 KG (2-4 pounds) to compensate for the fact that your cylinder will become lighter as you use the air. If you change equipment or diving environment or have not dived for a while you should re-do the buoyancy check. Remember that you are much more buoyant in salt water than in fresh water; you will be much less buoyant wearing a steel cylinder than an aluminum cylinder; and the exposure protection you are wearing greatly effects your displacement.
To descend at the beginning of the dive, you should be in a vertical position (to reduce your displacement) and you should be exhaling as you leave the surface (again to reduce your displacement). If you are correctly weighted and are struggling to descend, it is your breath control that is at fault. If you are really struggling, use a descent line to help you to begin with, but do not come to rely on it.
For the majority of the dive we should aim to maintain what we call ‘ideal breathing’. ‘Ideal breathing’ is most simply described as slowly and deeply, but there is more to it than that. Seventy percent of gas exchange occurs in the lower third of the lungs. Therefore we want to breathe in a way that allows as much air as possible to reach this area of the lungs, and spend the time where it needs to be for oxygen to be absorbed. We want to fill our lungs by drawing gas in beginning from the bottom of the lungs and then letting it out from the top. Fill from the bottom and empty from the top. The release of gas should take longer than the inhalation.
It is ok to deviate from this ideal breathing pattern when it is by choice. For example if we want to stop and look at something, and we try to maintain ideal breathing, we will find ourselves rising and falling. When we want to maintain a hovering position we need to shorten the breathing cycle (remember never hold your breath), and take smaller breaths in and out. Many divers struggle to maintain their position on a safety stop, either because they are not controlling their breathing or they have not fully vented the air from their BCD on ascent, not because they are under-weighted. Practice hovering whilst closely monitoring your depth gauge. Change your breathing pattern and notice how it affects your position in the water.
Perfecting your buoyancy control using breath control takes concentration and practice. You would not expect to be able to master driving a manual car without a lot of concentration and practice. Neither can you expect to fine-tune your buoyancy using your breath control when scuba diving, without taking the time to master it. After a while it becomes second nature, just like driving a car.
The benefits of taking the time to properly master buoyancy control are invaluable. You will feel (and look) comfortable in the water, you will use less energy and your air consumption will greatly improve. You will easily be able to stop and look at and under things, allowing you to observe aquatic life that may have previously eluded you. You will be able to avoid damaging the aquatic environment, your equipment and yourself.
It can be a good idea to enroll in a Peak Performance Class with your instructor. Ask at your local dive centre for more details. Happy diving!
