Buoyancy Control

buoyancy-control

The most important skill you will ever learn as a diver is buoyancy control. Your ability to control your buoyancy will dictate where you are in the water. It is what allows you to relax on the surface when not diving, it allows you to sink slowly at a comfortable pace and it allows you to glide elegantly above a coral reef.

The Science:

If an object displaces more water than its own weight then the object will float, however if the object displaces less water than its own weight then it will sink. This explains how a huge ocean liner can float, however a tiny marble will sink. A lot of it is to do with the shape of the object, if the ocean liner was crushed into a cube it would sink as well. When an object floats we say it is ‘positively buoyant’, when it sinks we say it is ‘negatively buoyant’ and it if does neither we say it has ‘neutral buoyancy’.

Buoyancy is effected directly by the weight of the water volume displaced, and as salt water weighs more than fresh water you will find you are much more buoyant in the sea than you are in a fresh water lake. Some seas in the world, such as the Red Sea or the Mediterranean Sea have more dissolved salts in the water than other seas or oceans. Because of this if you were diving in these places you would probably need more weight. If you are diving in fresh water with only a 3 mm wet suit on then it is very likely you would need very few weights, if any. This is why it is important before going for a dive in a new area you should do a quick buoyancy check.

Lung Control:

You may have noticed that when you are floating on water if you breathe out, you will start to sink a little. This is because when we breathe in our lung volume increases, which in turn increases the amount of water that we displace. When we breathe out our lung volume decreases and our body no longer displaces enough water for us to float effectively and we sink a little. As a diver we need to utilise all the different levels of buoyancy control.

Buoyancy Control Device:

As most humans float on the surface you we need to use weights in order to be able to descend, however we would not be able to stay at the surface for very long if we could not make ourselves positively buoyant. This is why it is important whenever we are at the surface we need to keep our BCD (Buoyancy Control Device) full of air. When it is full we displace a large amount of water keeping us comfortably at the surface. Obviously as we want to go diving we need to be able to submerge ourselves, so for this we need to make ourselves negatively buoyant. This is simply done by releasing the air from the BCD, this results in our overall displacement reducing rapidly and we begin to sink. It is important not to wear too many weights or the rate that we sink may be too much, leading to an uncontrolled descent.

Perfecting Neutral Buoyancy:

Neutral buoyancy is the middle ground that all divers need to be able to find. Being neutrally buoyant allows you to keep yourself at any depth using only breathe control. Throughout the dive our buoyancy changes constantly. Because air compresses as we descend and expands as we ascend neutral buoyancy changes constantly with depth. This means that as we dive deeper we need to add air to our BCD to stay neutrally buoyant, and if we ascend even slightly the air in our BCD will expand and we will become slightly positively buoyant, and as a result begin to float up.

We need to constantly change our buoyancy throughout the dive, not only because of depth changes, but also because as we use the air from our tanks they become lighter, which means we need to remove some of the air from out BCDs. As a diver using open circuit SCUBA equipment we can never be truly neutrally buoyant as when we breathe in we will add air to our lungs increasing our displacement and as we exhale we remove air from our lungs which deceases our displacement. The best is to be slightly negatively buoyant with no air in your lungs so when we breathe in we become slightly positively, this means when we dive we don’t stay at one depth, but we rise and fall very slightly while swimming along.

The only way to be perfectly neutrally buoyant while diving is to use a CCR (closed circuit rebreather), this is because when you use a rebreather your exhaled gas goes back into a part of the rebreather called counter lungs, and when you inhale your gas comes from those counter lungs. As it is the same volume of gas is simply being transferred between the diver and the rebreather the diver’s buoyancy doesn’t change unless the diver changes depth.

Practice!

Buoyancy is the key to diving successfully, if you feel yours is not up to scratch then you should get some practice in. The PADI Peak Performance Buoyancy specialty is a great way to hone your buoyancy precision. Having good buoyancy will not only make you more comfortable in the water, it will also make your air consumption better as you will spend less time flailing about. It will also allow you to see more interesting marine life as you will be able to look under rocks and coral without the fear of smashing into the reef!

‘Buoyancy Control’ was written by Mike

Check out our Blog: ‘The Basis of Buoyancy