Buoyancy and ballast

Tech Training

Despite its status as a core skill, buoyancy control often proves one of the most difficult aspects of diving to master.

For many divers – conditioned by their training and encouraged by an instructor to perform basic skills like mask clearing and regulator recovery while kneeling, heavily-weighted, on the bottom of a pool or seabed – being over-weighted is the norm. For them the concept of neutral buoyancy is just a theoretical and elusive ideal; one that has little relevance to the way that they’ve been taught.

Often unaware that they’re carrying far too much lead, (or too little.) the breathing patterns of an over-weighted diver will differ greatly from one who is neutrally buoyant. Rather than rhythmic, slow, deep breaths, the breathing changes to short shallow breaths that can lead to a build up of carbon dioxide and, at best, a general feeling of mental discomfort. (Should the situation deteriorate it can even lead to the onset of a full-blown panic attack.)

Maintaining proper depth during a blue-water safety or decompression stop requires additional effort, as does the simple act of swimming against even a moderate current. And, because gas consumption is generally higher when overweighted, dive times are invariably shorter than for a neutrally buoyant diver.

Environmental considerations play an equally important role in buoyancy awareness. An over-weighted diver swimming over fragile coral reefs is more likely to cause damage than one who is correctly weighted, while in the more advanced arena of technical diving the consequences can, quite literally, be a matter of life and death. In the tight confines of a cave or wreck, careless fin contact with a floor can cause a silt-out that may lead to disorientation and a failure to find the exit point. And in the case of an aged and disintegrating wreck there’s always the possibility that even a casual bump might cause decks and bulkheads to collapse into the compartment, trapping or crushing the diver.

Although these are extreme examples, the fact remains that a proper appreciation of buoyancy skills is the key to pleasurable and enjoyable diving experiences.

Buoyancy Basics
Although all divers claim to understand the concept of buoyancy, it’s worth repeating them, beginning with Archimedes Principle: When an object is wholly or partially immersed in a liquid, the up-thrust it receives is equal to the weight of the liquid displaced.

The three states of buoyancy are: Positive – when the displaced volume of water weighs more than the object. Negative – when the displaced volume of water weighs less than the object. And Neutral – a condition of equilibrium when the weight of water displaced and the weight of the object are equal.

In simple terms, it’s the same principle that allows a steel-hulled vessel weighing many thousands of tons to remain afloat, (positive buoyancy) while a solid block of metal weighing just a few pounds will sink, (negative buoyancy). In diving, where the object of the exercise is to comfortably remain at a chosen depth, the aim is for neutral buoyancy.

The factors affecting the buoyancy of an object are its volume and weight. The greater the volume of the object when immersed in a liquid – as in a diver wearing either a dry-suit or wetsuit – the greater the amount of water displaced.

Because of its salt content 1 litre of seawater weighs 1.025 kg, whereas 1 litre of fresh water weighs 1 kg. The greater buoyancy characteristics of seawater means that – all things being equal – a diver who is neutrally buoyant when diving in the ocean will be negatively buoyant should they dive in fresh water when wearing the same equipment and carrying the same weight.

The Body
Composed mainly of liquid, the average human body is almost neutrally buoyant. When the lungs are filled, the almost imperceptible changes in volume will cause a motionless diver to float with their head just above the surface. When the lungs are vented, the body will sink.

As an equipment-dependent activity, it’s the buoyancy characteristics of the gear worn that will usually determine how much weight each diver requires.

Staying Afloat
While on the surface, the bulkier a wet- or dry-suit, the greater the amount of compensating weight needed to counter the increase in body volume. At depth, however, the increase in water pressure reduces the volume of the suit and causes the diver to become progressively less buoyant.

For divers clad in a dry-suit, the solution is to simply add more gas to the suit via the suit inflation system in order to maintain a constant volume. Divers wearing wet suits, however, must rely on their Buoyancy Control Device to offset the compressing effects of pressure on the neoprene.

Although the primary function of the BCD (Buoyancy Control Device) is to provide a diver with positive lift while on the surface, it serves a dual role in that it is also used to establish neutral buoyancy at any given depth.

All too often, however, a consistently overweighted diver will place a false reliance on the BCD. Basing their selection on a high lift capacity and the notion that ‘more is better’, they overlook the fact that a large BC adds to the diver’s overall volume; creates additional drag when swimming, and requires greater effort and skill in controlling the venting of gas during ascent.

The buoyancy characteristics of the cylinders play an equally critical role in determining the correct amount of weight to be worn. Steel cylinders, for example, are generally heavier than a similar sized aluminium cylinder. Divers switching between the various cylinder types need to take these differences into consideration and add or subtract weight accordingly.

It must also be borne in mind that towards the end of the dive, as the gas supply becomes depleted, some cylinders become more buoyant. An undesirable condition when carrying out a safety stop, or for divers who have incurred a decompression obligation. In those situations, the diver should begin the dive being marginally overweighted with the aim of being neutrally buoyant towards the end of the dive at a depth of, say, 3-metres with 500 psi of gas remaining and little or no air in their BCD’s.

Weighing In
While many divers determine how much weight they require by trial and error over several dives, the most practical method is to invest a little time in getting it right before plunging into the open ocean. (It’s something that also needs to be performed after every major equipment purchase.)

In a suitably deep pool or sheltered area of open water, and with all of the equipment in place except the weight belt, vent all gas from the BCD (and dry-suit if worn). Assume an upright position in the water and remain motionless while first filling the lungs with air and then fully exhaling through the regulator. If, after exhaling, the diver still remains on the surface then it’s time to begin adding appropriate amounts of weight.

Neutral buoyancy is achieved when the diver floats at about nose level with full lungs and begins to sink when the lungs are emptied.

Although the importance of breathing properly and continuously – and never holding one’s breath – is one of diving’s basic precepts, the movement of air into and out of the lungs is often used to good effect when making minor depth adjustments; as, for example, when photographing along a steep wall. It is, however, something that should be done with caution and based on complete awareness of one’s depth.

Weight Distribution and Trim
Where and how the weights are carried about the diver’s person not only affects their in-water comfort, but also their ability to maintain a streamlined position when swimming. Should the need arise, the ability to jettison weights quickly and easily is fundamental to safety and assists the ascent.

Regardless of whether the weights are carried on a waist-belt or incorporated into the BCD as an integrated system, (one that usually gives the diver greater control when ascending from depth by allowing them to jettison a portion of the total weight) these ‘personal’ weights should all be contained in a quick-release system. (Cave and wreck divers, bound by an overhead environment that prevents their direct ascent to the surface, have a somewhat different approach to the implications of accidental release than an open ocean diver.)

Where the weight is positioned is critical to the diver’s trim and their ability to position their body to best advantage. A diver who can maintain a perfectly horizontal position when swimming will experience less water resistance and drag with consequent savings in gas consumption. They will also be better able to respond to any critical situation that may arise, such as donating a regulator in an out of gas emergency.

The positioning of the cylinder(s) plays an equally vital role in achieving perfect trim. Cylinders worn too low down on the back, or too high, will alter the centre of gravity. This may cause the diver to swim in either a feet up/down or head up/down position. To overcome this effect many divers now incorporate non-ditchable trim weights that attach to the cylinder.

In some instances – particularly when wearing a dry suit – divers with ‘light’ fins may feel the need to employ ankle weights with Velcro fastenings that help prevent a ‘feet up’ attitude.

Although the traditional webbing belt and quick-release buckle continues to be the work-horse of weighting systems, a number of manufacturers have now developed a range of innovative products aimed at improving diver comfort and safety by giving them greater control over their buoyancy and trim; twin aspects of a neglected skill that adds immeasurably to the enjoyment of diving and the feeling of true weightlessness.


The above article was first published in 2000

Categories: General

Tags: , , , , , , , , , , , , , ,

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: