Du er her:Start1/How Much Weight Do I Need to be Neutrally Buoyant?
How Much Weight Do I Really Need?
by Ryan Conery:
Bringing the right amount of weight with you on a dive requires practice. However, it is important to get this dialed in. Having the correct weight makes it easier to get neutrally buoyant. It also helps with air consumption and it means you have to haul less weight to and from the dive site. Before we can determine our weighting requirements, we have to look at what factors are affecting us.
In order to achieve neutral buoyancy, we must balance the upward force caused by water displacement with the downward force caused by our weight system. Archimedes’ Principle states that there is a buoyant force exerted on an object immersed in fluid. When an object gets placed in water, it will displace some of that liquid. Displacement means that the water is being pushed aside. That object is actually taking up space that was previously occupied by the water. Another way to picture this principle is by thinking about your bathtub. If you were to fill that tub up all the way to the top, what would happen if you tried to hop in? Of course, the water would overflow and be all over your floor, right? This is because you are displacing some of that water as you step into the tub. Now, the water that gets pushed aside wants to take its space back. This creates an upward buoyant force equal to the weight of the displaced water. For example, if we took a 1 cubic foot ball and submerged it in a bathtub, the ball would displace 1 cubic foot of water. Fresh water weighs 62.4lbs per cubic foot (salt water weighs 64lbs per cubic foot). This means that the ball is experiencing an upward buoyant force of 62.4lbs. If the ball weighs more than this, it will sink. If it weighs less, it will float.
In diving, we have the unique capability to utilize our buoyancy compensation device to ensure that we weigh the same amount as the water we are displacing. This allows us to achieve neutral buoyancy. Although we can get close to becoming perfectly neutral, we can never get rid of the slight rise and fall resulting from our breath cycle. As we inhale, we displace more water causing our surrounding environment to exert a larger upward buoyant force. So, we will ascend slightly as we breathe in. Vice versa, we displace less water as we exhale causing us to descend.
Archimedes’ Principle controls how much weight we have to add to our system to reach this neutral state. Our bodies, wetsuit and BCD are naturally buoyant. We require ballast weight to overcome the initial positive force of buoyancy. There are many factors that affect how much weight we will need to carry, but, as a general rule of thumb, you may need anywhere from 5 to 10% of your body weight in lead. After weighing yourself, determine what your range of weights might be. If you weigh 200lbs for example, you may need to use between 10lbs and 20lbs of lead. This range of weight gives us a good starting point, but we will have to look at the factors that affect our buoyancy to really dial it in.
Exposure protection plays a large role in the weighting process as our suit is inherently buoyant. A thicker suit means that we are displacing more water and require more lead to enable us to sink. A 3mm wetsuit may only require 6 to 8% of your body weight in lead whereas a 7mm, wetsuit, or dry suit could require 10% of your body weight or more depending on your undergarments. One good experiment is to try on your suit and hop in a pool. Start by holding on to a 2lb block of lead. Keep adding weight in small increments until you begin to sink. This way, you can begin to estimate your weighting needs in a controlled environment.
Just like your exposure suit, your buoyancy compensation device could also play a role in your weight requirements. A jacket style BC tends to have lots of additional padding for comfort. This extra bulk tends to be buoyant and will require more lead. Some of the minimalistic rigs such as a back plate and wing have no padding and can even be negative by themselves. These BCs utilize different sizes of plates to move some of the weight from the belt or pockets onto your back providing better trim underwater. Make sure to bring your BCD into the pool with you to determine how much additional weight is needed to sink your kit.
Other Factors to consider for Neutral Buoyancy
Water type is another factor that affects the weight we will need to carry. Salt water is denser than fresh water which causes it to weigh more per cubic foot. This means that an object placed in salt water will experience a larger upward buoyant force than if the same object was placed in fresh water. Due to the increased density, you will need to carry additional weight when diving in salt water. Anywhere from 2 to 6lbs extra may be required depending on the salinity of the water.
The material your tank is composed of must also be considered. Aluminum tanks start out slightly negative at the beginning of a dive when they are full. However, as you consume air they will slowly become more buoyant. At the end of your dive, an aluminum tank will be anywhere from 2 to 4lbs positively buoyant. If you are diving aluminum tanks, you may want to add a couple pounds to your kit to compensate for the additional buoyancy. On the other hand, steel tanks start out around 6 to 8lbs negatively buoyant. As you consume air, they too will become more buoyant; however, they will stay on the negative side of the scale. You can use the additional weight a steel tank provides and remove some of the weight on your belt.
Experience improves Buoyancy
The last factor that can affect your buoyancy is experience. As you begin to dive more, you will notice that you start to need less weight. Often times, inexperienced divers have trouble getting all of the air out of their BC as they start their descent. Becoming more familiar with your gear and more comfortable underwater produces a more relaxed breathing cycle. Just by going out and diving, you will soon notice that you need less weight than you thought. Another test you may see divers do is called a float test. In full gear, empty the air from your BC. With your lungs half full, you should sit at eye level with the water. As you exhale, you should begin to sink. This is a great way to get a ballpark estimate of your weighting needs. However, remember that you might need to add a couple extra pounds to compensate for the increased buoyancy of a tank at the end of a dive. The best place to ensure your weighting is correct is at your safety stop. You should be able to maintain neutral buoyancy with little to no air in your BC.
Make sure, after every dive, you record the weight you used in your logbook. Also, try to log every factor that affected your buoyancy on that dive such as what suit you used, what the water type was and what kind of tank you had. Getting your weight dialed in is a learning process and it will require some adjustment. Keep at it, practice makes perfect.
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