by Natalie Gibb
Updated May 25, 2017
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 is an object’s (or diver’s) tendency to float. You can think of buoyancy as an object’s „floatiness“. In scuba diving, we use the term buoyancy to describe not only an object’s ability to float in the water but its tendency to sink or to do neither.
Scuba divers use the following buoyancy-related 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.
HOW DOES BUOYANCY WORK?
When an object (or diver) is submerged in the water, water is pushed aside to make space for the object. For example, if you drop your new iPhone in a full glass of water, not only will you have a serious communication problem, 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 iPhone (now dripping onto the floor) is exactly the same volume as the iPhone.
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.
HOW CAN YOU TELL IF AN OBJECT (OR DIVER) WILL FLOAT OR SINK?
A simple way to determine whether and an object will float, sink, or do neither, is to use Archimedes’s Principle. Archimedes’s Principle explains that there are two forces at work to determine if an object will float or sink.
1. Gravity and the Weight of the Object – This pushes the object down
2. Buoyancy or Buoyant Force – This pushes the object up
Easy! If the force from the weight of the object is greater than the force from buoyancy, the object sinks. If the buoyant force is greater than the force from the weight of the object, the object floats. (Hint: iPhones sink).
Now all that is left is to figure out how much the buoyant force for a given object is. The simplest way to do this is to weigh the water that the object displaces. The buoyant force on a given object is the same as the weight of the water it displaces. It follows then that:
1. An object floats up if the weight of the water it displaces is more than its own weight.
2. An object sinks down if the weight of the water is displaces is less than its own weight.
3. An object remains suspended at one level if the weight of the water it displaces is exactly the same as its own weight.
In diving, we want to sink at the beginning of the dive to get down our desired depth, and then remain neutrally buoyant until we ascend. We can’t change from negative to neutral buoyancy on a whim because we can’t change the amount of water our bodies displace. Therefore, divers control their buoyancy using an inflatable jacket, or buoyancy control device (BCD) to displace more water (by inflating it and increasing their buoyancy) or less water (by deflating it and decreasing their buoyancy).
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:
1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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!
STEP-BY-STEP BUOYANCY FOR AN AVERAGE DIVE:
How do we apply buoyancy concepts to an average dive? Here is a step by step guide to how to adjust your buoyancy on a typical scuba dive.
1. Inflate the Buoyancy Compensator (BCD) and Jump in the Water:
Before leaping off the dock or dive boat, inflate your BCD so that you will float on the surface. This allows you to deal with any last minute problems before you descend, such as forgetting to open your tank valve or a mal-adjusted mask.
2. Deflate the BCD Just Enough to Descend:
To begin your descent, deflate the BCD just enough so that you can descend by breathing out. The trick is to descend slowly enough to have time to equalize your ears Completely deflating the BCD may cause you to sink like a rock and risk an ear barotrauma.
3. Add Small Bursts of Air to the BCD as You Descend:
As a diver descends, the water pressure around him increases. This causes the air in his BCD and his wetsuit (or drysuit) to compress, and he becomes more negatively buoyant. Compensate for your increasing negative buoyancy by adding a small burst of air to your BCD whenever you feel that you are beginning to sink too quickly.
4. Add Air to the BCD to Achieve Neutral Buoyancy:
Once you have arrived at your desired depth, add air to the BCD in small bursts until you are neutrally buoyant.
5. Deflate the BCD as Needed During the Dive:
Remember, as your scuba tank empties, it will become increasingly positively buoyant. It may be necessary to deflate the BCD in small increments to compensate for the increasing buoyancy of the tank.
6. Deflate the BCD as You Ascend:
This may sound counterintuitive, but remember that the air in your BCD and wetsuit (or drysuit) will expand and make you more positively buoyant as you ascend (because of the pressure decrease). The goal is to control your buoyancy during ascent by remaining neutrally buoyant and swimming – not floating – up.
7. Inflate Your BCD on the Surface:
Once your head reaches the surface, go ahead and inflate your BCD so that you can float easily on the surface before removing your regulator. This sounds obvious, but many divers are so excited about the dive that they forget to inflate and get a mouthful of water as a reward!
THE PROBLEM WITH TOO MUCH WEIGHT
Divers with an excessive amount of weight will have a more difficult time controlling their buoyancy. The more weight a diver uses, the more air he will need to add to his BCD to compensate for the negative buoyancy from his weights. As the air in a diver’s BCD expands and compresses with any small change in depth, the more air he has in his BCD, and the greater volume air that is expanding and compressing. This makes it more difficult for the diver to control his buoyancy as he changes depth. To avoid this problem, be sure to perform a test for proper weighting before diving.
Now you know the basics of buoyancy and how to apply it to your dives! Have fun!