By Marcus Gee
Image by Dr Michael Hindley
Buoyancy control is important for the perfect scuba diving experience and increasing the overall enjoyment of a dive, for you as well as those around you.
If you ask any dive professional what the most important dive skill is, the answer that you are most likely to get is having good buoyancy control. Most dive instructors will tell you that good buoyancy control is the essence of safe diving. The neutrally buoyant diver is never at risk of doing a runaway buoyant ascent and is also less likely to come into contact with sea urchins or other hazardous marine life. Good buoyancy control also improves your air consumption. A diver who combines neutral buoyancy with good trim control can significantly reduce energy expenditure during their dive.
The neutrally buoyant diver is never at risk of doing a runaway buoyant ascent. Having good buoyancy control improves the enjoyment of a dive and it will help you take better underwater photographs. It means that you do not have to concentrate as much on staying off the reef. Also, the neutrally buoyant diver has more dive buddy options as other divers appreciate the fact that you are not stirring up sediment as you explore the dive site and, because you are able to use your air effectively, everybody gets more bottom time.
Good buoyancy control can also mean that there are more dive sites available to you. With the degradation of the world’s reefs, some sites today (such as in Bonaire) require that divers demonstrate good buoyancy control before being issued their dive permits. When everything is said and done, buoyancy control is essentially a safety issue. The diver that is see-sawing up and down in the water column runs the risk of seeing the inside of a decompression chamber. Despite being one of the more complex skills to master in diving, I have seen many students get the hang of it during their entry-level courses with the majority of my students having mastered it before their advanced class is complete. With a fundamental understanding of the principles that govern buoyancy control, and the diligent application of those principles, divers can develop both good buoyancy control and trim pretty quickly.
Image: Robert Yin
Buoyancy control is pretty simple when you break it down – whether or not something (like a diver) floats is determined by two opposing forces. One of these forces is gravity (a downward force in the water) and the other is a buoyant force (an upward force in the water). When these two forces are in balance, a diver is neutrally buoyant. The gravitational force is determined by your weight, which is the sum of your body weight plus the weight of all the gear which you are diving with. As long as you do not lose anything during the dive, the only change in the weight over the course of the dive is the air in your cylinder (which I will elaborate on later in the article).
Opposing the gravitational force is the buoyant force which is determined by the weight of the water that is displaced by the diver and their gear. When doing a buoyancy check, we are trying to determine the exact amount of ballast weight that will put our diver in equilibrium at the surface. Saltwater weighs more than freshwater, thus its buoyant force is greater.
Fundamentally, buoyancy control comes down to the following:
- Carrying the right amount of ballast weight.
- Doing buoyancy checks anytime your diving environment or gear changes, or if you have not dived in a while.
- Adding air to your BCD as you descend.
- Venting or dumping air periodically as you ascend, or as you consume the air in your cylinder.
The biggest variable in determining how much ballast weight to carry is your diving suit – with a 3mm full wetsuit diving in saltwater, start with a ballast weight equal to 5% of your body weight. If you are diving with a 7mm suit in saltwater, start with 10-12% of your body weight. Start with 10% if you are lean and 12% if you are not – that will set you up pretty close to the ideal weight. Remember that two people with the same body weight will need different amounts of ballast weight; this is because we all have different amounts of fat, muscle and bone, and these have different densities and therefore different buoyancy characteristics. Professional athletes and special forces divers can often get away with as little as a 7% or 8% in weighting factor, but they fall outside the norm.
The best time to perform a buoyancy check is at the end of a dive when your cylinder is around 35 bar. The reason we do the buoyancy check at the end of the dive has to do with the weight of the air in your cylinder. It turns out that 2 300ℓ of air weighs almost exactly 2.72kg, so if we calculate that we have breathed our cylinder down from 210 bar to 35 bar, we have used five out of six parts (83%) of the air in the cylinder, or 2.26kg of air. It follows then that if we were to do a buoyancy check at the beginning of a dive, we would be 2.26kg under-weighted at the end of the dive and it would be difficult (if not impossible) to conduct a safety stop. If you want to do your buoyancy check at the beginning of the dive, just add 2-3kg in weights once you have completed the check.
Because of the way that a wetsuit is made, it also changes buoyancy considerations throughout the dive. Neoprene wetsuits are made up of thousands of nitrogen bubbles and these bubbles get compressed as we dive to greater depths. With greater depth, the volume of these bubbles is reduced. As the suit gets compressed, the overall displacement of the diver wearing it is reduced too. This is why divers feel heavier (more negatively buoyant) as they descend and why we have to add small amounts of air to the BCD with depth. It also explains why we have to vent air off as we ascend to shallower depths. At shallower depths the pressure decreases and the gas bubbles expand back to their original volume, displacing more water and making you more buoyant.
The other variable in controlling your buoyancy during a dive is your BCD. As the name suggests, your BCD is designed to compensate for changes in buoyancy as your depth changes. It also helps you to remain neutrally buoyant at all depths. As you descend, you will need to add small amounts of air to the BCD to balance the compression of your wetsuit from depth. Conversely, as you move into shallow water at the end of a dive, your wetsuit will return to its original thickness (making you more positively buoyant) and you will need to vent air off every 3-4m during the ascent to account for the wetsuit’s expansion.
With the rest of your buoyancy factors set, your lungs become the final buoyancy compensator in diving. Bear in mind that the following discussion is not intended as an argument in favour of holding your breath or practising skip breathing when diving. Breathing continuously when diving is still the most important rule in diving.
That said, skilled divers have an understanding of the variables that affect their buoyancy and know how to use these variables to their advantage and enjoyment. When full, the average diver’s lungs contain approximately 10ℓ of air. With that in mind, the amount of air exchanged during a breath is only about half a litre. Since a half litre of saltwater weighs just over 500g, this means that when I inhale with a normal, resting breath, I add about 500g of positive buoyancy. When I exhale normally, I remove 500mℓ, shedding 500g of positive buoyancy. If you think back to the pool session during your entry-level course, you may recall that during your neutral buoyancy swims that you rose and sank as you swam along. Now you know why.
Once you are at depth, you can use your breathing to control your depth. In order to go deeper, you simply breathe deeper. If you want to go a bit shallower, breathe more shallowly. When I breathe deeper, my lungs are on average less full than if I am breathing shallowly (retaining more air in my lungs). Lungs that are on average more full means that the average displacement is greater which, given a constant weight, tends to make me slightly more buoyant. If I am breathing deeply, I am dumping more of the air in my lungs, making them less full on average, ultimately reducing my overall displacement, further making me less buoyant and enabling me to dive deeper. Once I am at my preferred dive depth, this becomes my principal method for changing depth. I very rarely add or dump air in my BCD between 12-20m. Instead, I just change the way in which I breathe.
Image: Dr Michael Hindley
Now for trim, which essentially refers to maintaining a body position that offers the least resistance as you move through the water. There are a couple of ways that divers manage trim, the most important of which is to be properly weighted. Over-weighted divers tend to hang feet down in the water column. With proper weighting established, you can adjust trim through the distribution of weight. Most of the BCDs which are available today have trim pockets integrated into their rear. If your BCD does not have trim pockets, you can add them as an accessory and mount them to your cylinder straps. If you have determined that you are properly weighted, yet your feet are still hanging down as you swim along, move some of your ballast weight to the trim pockets on your next dive.
I normally start by putting one-third or more of my total ballast weight in the trim pockets. Another way in which you can adjust your trim is to move your cylinder straps up or down. Moving the cylinder straps up on the cylinder will slightly shift your weight lower and bring your feet down. Conversely, moving the cylinder straps slightly down will bring your feet up. If you are moving the strap down, be careful not to bring the cylinder up too much or you will be hitting the back of your head against the regulator every time you look up.
If you are a relatively new diver or if you are still getting your total weight and trim dialled in, be sure to put this information in your dive log. It will come in handy later. If you have kept good notes in your log book, you will not have to guesstimate the proper weight and distribution later.
Check your trim at depth by hovering a few metres off the bottom and floating parallel to the bottom with your legs extended out. If your legs sink, move a couple of kilogrammes to the trim pockets or move the cylinder strap down slightly on your next dive.
If you are new to diving, pay particular attention to your buoyancy toward the end of the dive as you make your ascent. A new diver is not always aware that they become more positively buoyant as they ascend. Newer divers can be distracted by equipment issues, or they can become ensnared by their surroundings. By the time they finally realise that they are ascending, it can be too late to stop it.
Finally, please note that the information presented here is not a substitute for proper training. I highly recommend that divers who are looking to perfect their buoyancy, trim, kicking style and other aspects of their diving seek out a qualified dive instructor to train with. Always dive within the limits of your training and experience. If you are interested in doing some type of diving that you have not been trained in (such as deep or night diving), please complete the course prior to attempting those types of dives.