Introduction
When scuba diving you don’t want to overload yourself with weights or float too much so it’s tough to submerge. Understanding buoyancy is key to safe and easy scuba diving. While the concept of buoyancy may be confusing at first, it becomes clearer when we consider how buoyancy affects scuba divers and what divers need to know to properly control it.
What is Buoyancy?
Buoyancy or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pressure at the bottom of a column of fluid is greater than at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object. The pressure difference results in a net upward force on the object. The magnitude of the force is proportional to the pressure difference, and (as explained by Archimedes’ principle) is equivalent to the weight of the fluid that would otherwise occupy the submerged volume of the object, i.e. the displaced fluid.(Well that was very sciencee wasn’t it)
In simple terms:
Positive Buoyancy/ Positively Buoyant: The object or person floats upwards in the water or remains floating on the surface.
- Negative Buoyancy/ Negatively Buoyant: The object or person sinks downwards in the water or remains on the bottom.
- Neutral Buoyancy/ Neutrally Buoyant: The object or person neither sinks downwards nor floats upwards, but remains suspended in the water at a single depth.
To achieve negative buoyancy, divers who carry or wear buoyant equipment must be weighted to counteract the buoyancy of both the diver and the equipment.
When underwater, a diver often needs to be neutrally buoyant and neither sink or rise. A state of neutral buoyancy exists when the weight of water that the diver and equipment displaces equals the total weight of the diver and equipment. The diver uses a BC to maintain this state of neutral buoyancy by adjusting the volume of gas in the BCD and therefore its buoyancy, in response to various effects, which alter the diver’s overall volume or weight, primarily:
Why is Buoyancy so Important when Diving?
It’s important because buoyancy is what controls your movement underwater Bad buoyancy control has a big effect on your diving and your impact on the coral reef. A good buoyancy control will make you better at managing your air, give you a better position in the water and all in all give you much more control of your diving.
Although it is not difficult, it is important to learn how to master buoyancy skills. Keep in mind, it takes practice. As you gain experience, you will become better at mastering and controlling your buoyancy.
Buoyancy Compensator: Why So Important
How Does Buoyancy Work?
When a diver is submerged in the water, water is pushed aside to make space for the object. For example, if you drop your watch in a full glass of water, not only will you not know the time, unless it is waterproof, but you will have a nasty little spill from the water that overflowed the glass. The amount of water pushed aside to make space for the watch (now dripping onto the floor) is exactly the same volume as the watch. We say that this water has been displaced.
When an object or diver displaces water, the water surrounding it has the tendency to try to fill in the space the object now occupies. The water pushes against the object, exerting force and pressure on it. This pressure pushes the object upwards and is called the buoyant force.
Use the Proper Scuba Weights
When diving, you need to bring the right amount of weight to achieve proper buoyancy.
To do this you’ll need to do the following:
With the regulator in the mouth, release all the air from your BCD. You should be at eye level with the water.
As you breathe in and out and go up and down in the water. Remember when the dive is finished the tank is usually empty and therefore 1-2 kg/lbs lighter. You need to account for this and bring 1-2 kg/lbs extra for the dive.
Using the BCD for better Buoyancy
When you descend, you empty your BCD. After you reach your desired depth make yourself neutrally buoyant by adding small amounts of air.
Don’t overdo it, small bursts with the inflator button. You don’t want to over-inflate and risk a rapid ascent. If you feel there is too much air, release in small amounts.
When you release air, stretch the inflator hose upwards so the air does not get trapped in the BCD. And look at your deflator for bubbles, if there are no bubbles coming out turn a bit to release trapped air from your BCD.
Once you’ve achieved neutral buoyancy you’ll be moving slowly up and down, as you breathe in and out.
Breathe for better buoyancy
Breathing is where you affect your buoyancy the most so while diving, it is not necessary to constantly use your BCD for controlling your buoyancy. The best way to control your position in the water by breathing in and out. Your lungs are a natural buoyancy controller and you can utilize this perfectly when diving. Be aware of your possession in the water and control it with your breathing.
If you are moving upwards away from the bottom, you simply exhale all the way. This will empty your lungs of air – hence making your total volume smaller and making you negative buoyant. Once you’ve reached your desired depth again, assume normal breathing.
If you are too close to the bottom you breathe in, to rise a bit. Remember never to hold your breath, but use a big breath so move you up a bit. If it’s not working, adjust your BCD and try again.
What Factors Affect a Scuba Diver’s Buoyancy?
A diver’s buoyancy is determined by a host of factors. Some of the factors affecting a diver’s buoyancy are:
Weights: In general, a diver and his gear (even with no air in his BCD) are positively buoyant or become positively buoyant during a dive. For this reason, divers use lead weights to overcome their positive buoyancy. Weights enable a diver to descend at the beginning of the dive and stay down during the dive.
Buoyancy Control Device (BCD): Divers control their buoyancy underwater by inflating and deflating their BCD. While the rest of the gear maintains a constant weight and volume (displacing a constant amount of water) a BCD can be inflated or deflated to change the amount of water the diver displaces. Inflating a BCD causes the diver to displace additional water, increasing a diver’s buoyancy, and deflating a BCD causes the diver to displace less water, decreasing a diver’s buoyancy.
Tank Pressure: Believe it or not, the compressed air in a scuba tank has weight. The volume of the tank and the weight of the tank’s metal remains the same during a dive, but the amount of air inside the tank does not. As a diver breathes from a scuba tank, he empties it of air and it becomes progressively lighter. At the beginning of a dive, a standard aluminum 80 cubic foot tank is about 1.5 pounds negatively buoyant, while at the end of the dive it is about 4 pounds positively buoyant. Divers need to weight themselves so that they can remain negative or neutrally buoyant even at the end of the dive when the tank is lighter.
Exposure Protection: Any exposure protection, for example a wetsuit or drysuit, is positively buoyant. Wetsuits have tiny air bubbles sealed within the neoprene, and drysuits trap a insulating layer of air around the diver.
The thicker (or longer) the wetsuit or drysuit, the more buoyant a diver will be and the more weight he will need.
Salt vs Fresh Water: The salinity of the water has a huge effect on a diver’s buoyancy. Salt water weighs more than fresh water because it has salt dissolved in it. If the same diver is submerged in first salt and then fresh water, the weight of the salt water he displaces will be greater than the weight of freshwater he displaces, even though the volume of water is the same. Because the buoyant force on a diver is equal to the weight of the water he displaces, a diver will be much more buoyant in salt water than in fresh water. In fact, a diver in fresh water can use nearly half the weight he used in salt water and still be adequately weighted.
Other Dive Gear: The buoyancy of each piece of gear contributes to diver’s overall buoyancy. All other things being equal, a diver using heavier regulators or fins will be more negatively buoyant and need less weight than a diver using lighter gear. For this reason, divers need to test their buoyancy to determine the proper amount of weight to use on a dive whenever they change any piece of dive gear, even their BCD, fins, or type of scuba tank.
Air in the Lungs: Yes, even the volume of air in a scuba diver’s lungs will have a small effect on his buoyancy. As a diver breathes out, he empties his lungs and his chest becomes slightly smaller. This decreases the amount of water he displaces and makes him negatively buoyant. As he inhales, his lungs inflate and he increases the amount of water he displaces, making him slightly positively buoyant. For this reason, student divers are taught to exhale on the surface to begin their descent; exhaling helps a diver to sink. During the open water course, a diver learns to make small adjustments to his buoyancy using his lung volume with exercises such as the fin pivot.
Body Composition: This may sound a bit harsh, but fat floats. The higher a diver’s ratio of fat to muscle, the more buoyant he will be. Women generally have a higher percentage of body fat than men, and are therefore more buoyant and need more weight. This is the reason that body builders sink in a pool, while the average person can float!
Summary
Remember every moment of every dive is an opportunity to learn and improve your diving skills. Go back to the basics. As a first step, adjust your weighting and complete a proper weight check, both before and after the dive. Take some time to get into confined water or the local swimming pool. Open-water diver level exercises are specifically designed to help divers hone their buoyancy.
