To understand how scuba divers can go up and down underwater, you have to understand a concept called buoyancy. Buoyancy is essentially an object’s capability to float. Divers can manipulate their buoyancy using something called a Buoyancy Control Device (BCD) as well as dive weights. Understanding buoyancy can be confusing at first, but this article will provide a brief overview of the concept of buoyancy and how divers can manipulate it to go up or down as they please.
What is Buoyancy?
Buoyancy is an object’s (or person’s) capability to float. If it helps, you can think of buoyancy as an object’s “floatiness”. In scuba diving, buoyancy is an all-encompassing term. It doesn’t just refer to objects that float, but also objects that sink, and objects that neither float or sink (they are suspended in place). There are specific terms that refer to each state.
Positive Buoyancy: When an object or person is positively buoyant, they will constantly be floating upwards while underwater and will remain floating by the surface once reached.
Negative Buoyancy When an object or person is negatively buoyant, they will constantly be sinking downwards while underwater, and will remain on the ocean floor once reached.
Neutral Buoyancy: When an object or person is neither floating upwards or sinking downwards, but remains suspended in the water.
At a basic level, divers start off positively buoyant. That is why they carry dive weights to help them be negatively buoyancy so they will be descending. Divers do not want to descend too quickly, because the rapid change in pressure can cause ear barotrauma if it has not been properly equalized.
As they approach their desired depth, they fill their BCD with air (which is positively buoyant) from their scuba tank to slow their descent. Once the desired depth is reached, they fill their BCD with just enough air so that they are neither going up or down. In other words, they have reached neutral buoyancy.
Divers maintain neutral buoyancy at their desired depth so that they can swim horizontally and explore without wasting energy trying to stay at that depth. Once it is time to ascend, divers will fill their BCD with so much air that they become positively buoyant and they begin to go up.
With that said, they have to make sure they don’t ascend too quickly otherwise they will put themselves at risk of decompression sickness (the bends). Read our article on how to prevent the bends to get some tips on how to avoid it.
How Does Buoyancy Work?
Now that you know what buoyancy is, you might be wondering how and why does this happen? When an object (or diver) is submerged underwater, they displace a certain amount of water equal to their volume. Imagine sticking your finger into a glass that is filled to the brim with water. Some of the water will overflow because your finger has pushed aside, or displaced, the water.
When a diver displaces the water in the ocean, the nearby water will try to fill the space that the diver now occupies. The force and pressure the water exerts helps push the diver upwards, and this is called the buoyant force.
Factors Affecting a Scuba Diver’s Buoyancy
In this section, we’ll go over the most common factors that affect a diver’s buoyancy.
Buoyancy Control Device (BCD)
The BCD is a vital piece of scuba equipment that can be inflated with air or deflated to control the diver’s buoyancy. While the other pieces of gear maintain a relatively constant weight and volume (displacing a fixed amount of water), the BCD will have a variable amount of water it displaces based on how full its air bladders are.
Inflating the BCD will cause additional water to be displaced, increasing the diver’s overall buoyancy by a positive amount, and deflating the BCD will cause it to displace less and cause the diver to become less buoyant. As the name suggests, the Buoyancy Control Device is how divers can easily control when they go up or down in the water by manipulating their buoyancy
We have written extensively about BCDs and how to use them. We also provide our recommendations, such as the best ones for travel, for beginners, back-inflate BCDs, backplate and wing BCDs, for women, jacket vs. back-inflate BCDs, and the best ones overall.
Generally speaking, divers with all of their gear will be positively buoyant in the beginning, even when the air bladders of the BCD are empty. How are they supposed to go down? By using lead dive weights, divers can overcome their positive buoyancy and descend downwards. As they approach their depth, they will start to fill the BCD with air until they reach neutral buoyancy.
In an emergency, the dive weights can be instantly dumped, causing the diver to become positively buoyant since they are no longer weighed down. In normal circumstances when a diver wants to ascend, they simply fill their BCD with enough air so that they overcome their negative buoyancy and start to go up.
Any piece of gear that covers the diver’s skin such as their wetsuit or drysuit, gloves, hood, and fins, are all positively buoyant. The reason is that these pieces of gear are usually made with a material called neoprene, which has air bubbles trapped inside its layers. It is these air bubbles that cause the gear to be positively buoyant. The thicker the exposure protection gear, the more buoyant the gear and the diver will be.
Additional Dive Gear
There are other pieces of gear that affect the diver’s buoyancy. Typically, pieces of gear like the regulator and scuba tank are negatively buoyant. Divers using heavier equipment will need less dive weights to descend than divers with lighter gear. One cannot prescribe an exact amount of weight for a diver to use. It is up to the diver to test their buoyancy with their scuba setup to determine how much weight they should use for their dive. Any time a piece of gear is swapped out, even if it’s the BCD, divers should do a buoyancy test.
Tank Pressure and Type
Scuba tanks are heavy; a fully filled scuba tank can weigh over 30 pounds or more on land. However, it’s a different story when they are underwater. A scuba tank’s buoyancy, like the BCD, is actually variable.
First, you have to understand that the compressed air in a scuba tank has weight. The difference in weight between a full and empty tank can be 6 pounds or more depending on the tank size. Thus, the size and pressure rating of the tank affects how much compressed air can fit in it, which affects its weight and buoyancy.
The next thing you need to know is that there are two main types of scuba cylinders: steel and aluminum. Aluminum tanks are heavier than steel ones. When underwater, both aluminum and steel tanks are negatively buoyant at the start. However, as the diver uses up the compressed air inside, the scuba tank will get progressively lighter over the course of the dive. Depending on the type of tank it is, this has different consequences.
For a basic 80 cubic foot aluminum tank, it will start off with approximately 1.5 pounds of negative buoyancy. By the time it’s nearly empty, it will end up being about 4 pounds positively buoyant. In other words, at some point during the dive, the diver will start to float up with an aluminum tank. The diver needs to account for this by bringing more dive weights to counteract the positive buoyancy.
Steel tanks, on the other hand, start off negatively buoyant and will still be negatively buoyant when it’s empty, just less so. In other words, an empty steel tank will sink even when empty, but an empty aluminum tank will float. Thus, divers don’t need as many dive weights when diving with a steel tank since an empty steel tank will contribute some extra weight itself.
No matter which type of scuba tank one uses, divers should be aware of the variable weight and buoyancy of their tank and bring the appropriate amount of dive weights to account for their change in buoyancy.
Air in the Lungs
Even the air in a diver’s lungs will have a small impact on their buoyancy. As a diver exhales, his lungs contract and their chest size decreases. This small decrease affects the amount of water the diver displaces, making the diver negatively buoyant. Conversely, when the diver inhales, the increase in water displacement makes the diver slightly positively buoyant.
For this reason, student divers are taught to begin their descent from the surface by exhaling, causing them to sink. As part of the Open Water Course, divers will learn how to make micro-adjustments to their buoyancy by regulating their breathing.
Keep in mind, you should never hold your breath, especially while ascending. Always be continuously breathing to avoid lung overexpansion.
Freshwater vs. Saltwater
The salinity of water has a big impact on the diver’s buoyancy. The difference between the weight of saltwater and freshwater is 2.5%, with saltwater being heavier. This discrepancy is due to the salt content in saltwater. When diving in saltwater, divers are more positively buoyant, meaning it’s easier to float in saltwater. Divers will therefore need more dive weights to dive in saltwater.
Freshwater is lighter than saltwater, which means the amount of weight it displaces will be less. As a result, divers will sink more easily in freshwater, so they do not need as much dive weights. A good rule of thumb for determining the amount of dive weights needed to dive in freshwater is to use half the amount of weights that you would normally use in saltwater.
You can learn more about the differences between saltwater and freshwater diving by clicking here.
As harsh as this sounds, fat is positively buoyant. Thus, the higher your ratio of fat to muscle, the more likely you’ll float. Since women have a higher body-fat percentage relative to men, they float more easily and require more dive weights. This is also the reason why extremely lean individuals, such as male athletes, sink in the pool while it is almost effortless for the average person to stay afloat.
Putting It All Into Practice
Now that you know about the most common factors affecting a diver’s buoyancy, it’s time to put the knowledge into practical use. Here is how a diver would adjust their buoyancy during a typical scuba dive.
Inflate the BCD before jumping in the water
Divers want to start off positively buoyant so that they will float by the surface. This gives them time to make last minute adjustments or fix any problems before descending. For instance, maybe they forgot to open their tank valve, or their mask wasn’t properly sealed.
Deflate the BCD slightly to begin descending
Once the diver is ready, they will deflate their BCD slightly until they can descend just by exhaling. This goal is to descend slowly so that they are exposed to the increasing water pressure gradually. Deflating the BCD completely will cause divers to sink like a rock, subjecting them to a rapid increase in water pressure and leading to ear barotrauma.
Gradually inflate the BCD as you descend
As the water pressure increases during the descent, the air in the BCD, wetsuit, and lungs will compress, causing the diver to become negatively buoyant. To compensate for this, the diver should add a small burst of air whenever they feel that they are descending too quickly. Also, as they approach their desired depth, they should add air to the BCD in small bursts until they reach neutral buoyancy.
Deflate the BCD as needed
Remember that your BCD becomes more positively buoyant as it empties. You will find that you are slowly becoming positively buoyant over the course of the dive, so deflate the BCD as necessary to remain naturally buoyant.
Deflate the BCD gradually during the ascent
As counterintuitive as this sounds, you should be deflating your BCD on the ascent to continue to remain neutrally buoyant. This step is necessary because the decrease in water pressure will cause the air in the BCD and wetsuit to expand, making them more positively buoyant. However, the goal is not to float up, but to swim up while remaining naturally buoyant.
This allows divers to control their ascent rate which is critical if they want to avoid decompression sickness. They should follow the instructions provided by their dive computer to let them know how fast they should ascend and when to make a decompression stop.
Inflate the BCD at the surface
Once you have safely made it to the surface, inflate your BCD so that you are positively buoyant before taking off your regulator. Many inexperienced divers forget this final step and treat themselves to a nice mouthful of water.
Diving with Too Much Weight
It is crucial for divers to perform a buoyancy test so that they bring exactly as much weight they need and no more. Divers with too much weight will struggle to control their buoyancy. Since they are so weighed down, their BCD needs to be filled with more air to reach neutral buoyancy.
Having so much air causes the expansion and compression of air due to the water pressure to affect them more greatly. They will have to be more active about inflating and deflating their BCD as their depth changes.
Divers need to be careful not to bring so much weight that their BCD cannot provide enough positive buoyancy to help with their ascent.
To sum it all up, divers are constantly playing a balancing game based on their buoyancy underwater. If they want to go down, they deflate their BCD until exhaling causes them to descend. Once the desired depth is reached, they add a little bit more air to be neutrally buoyant. To go up, divers fin themselves up slowly and deflate their BCD as the air expands so that they are neutrally buoyant the entire time.