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Perfect Buoyancy Control and Trim
divebumNsandyeggo - 9/11/2012 3:19 AM
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Category: Educational
Comments: 3
Perfect Buoyancy Control and TrimIt’s difficult to overstate the importance of buoyancy control as a dive skill – most dive magazines reference it, it’s a component of nearly all dive courses, and among all dive skills, it can make the difference between a dive that is memorable for all the right reasons, and one that you’d rather forget. Too, if you dive with someone that seems to be part fish, uses less air than everyone else, and is a picture of grace beneath the surface, you can be certain that it is buoyancy control that sets them apart.

Some of the benefits of managing buoyancy are obvious, while others may be less apparent. At the top of this list, exercising good buoyancy control means that you’re a safer diver. At the top of list of the not-so-obvious benefits is that having good buoyancy control almost always means an improvement in your air consumption. When you combine buoyancy control with good trim you’ll generally reduce the amount of energy you use on a dive, which again means better air consumption. As a neutrally buoyant diver, you won’t have to work to stay off the bottom, and once you perfect it, you won’t be cycling air from your tank through the Buoyancy Control Device (BCD) and into the water.

Among the more subjective benefits, though almost as important, is that having good buoyancy control tends to improve the enjoyment of a dive, because you don’t have to concentrate as much on staying off the reef. Rather, you’re maintaining a constant distance off the bottom during the dive. Because you present a more passive presence in the water, you’re able to interact more with marine life, and if underwater photography is your thing you’re going to get significantly better shots.

If we look at the costs associated with a lack of buoyancy control, we simply have to take the converse of the arguments made above. Add to that, however, that in some locales (such as Bonaire), they won’t let you dive there until you’ve demonstrated good buoyancy control with a Park Ranger/Dive Master. The issue in Bonaire is that they’re averse to visitors descending onto their coral heads, and rightly so. One cost associated with buoyancy control worth restating, though, is that buoyancy control is inherently a safety issue. Scuba diving has matured to a point where dive incidents are exceedingly rare, but when dive incidents do occur, far and away the biggest contributor is a lack of buoyancy control.

The basics of buoyancy control are pretty well documented, and taught in every Open Water Course. Fundamentally, at that level it often amounts to: (1) making sure you’re properly weighted; (2) doing buoyancy checks anytime you change dive environments, gear or haven’t dove in awhile; (3) adding air to your BCD or dry suit as you descend, and venting or dumping air periodically as you ascend. In most Open Water Courses, your instructor is likely to have also addressed how fine tuning your breathing can aid your buoyancy control.

Many dive professionals claim that buoyancy control is one of the most difficult diving skills to master. While that may be the conventional wisdom, I’ve seen many students ‘get it’ during the Open Water Course. The majority of my students have it before they complete their Advanced Open Water Course, even if they take the Advanced Course the week after they’ve completed the Open Water Course. I do think it’s generally true that good buoyancy control is a skill that distinguishes newer divers from the more experienced, but it’s been my experience that with a basic understanding of the principles that govern buoyancy control, and diligent application of those principles, divers can develop both good buoyancy control and trim in 10 dives or less. Although it’s not my purpose here to sell courses, I will say that one of the best ways to perfect your buoyancy control is to take the Advanced Open Water Course, where you can elect Peak Performance Buoyancy as an element of your class, applying what you’ve learned.

So how do you perfect your buoyancy control? Many factors influence your buoyancy, including your body weight; the amount of ballast weight you’re carrying; your choice of steel or aluminum tank; the capacity of the tank; the thickness of the exposure suit you’re diving; whether you’re diving salt or fresh water; the depths you’re diving to; and your breathing. To a lesser extent, the lift capacity of your BCD and your trim will also impact your buoyancy.

The good news is that most of these factors remain relatively constant once you’ve set them up and begun the dive. Other factors are essentially variable, changing during the dive along with bottom time, depth, or both. Some you can control through preparation and knowledge, while others you’ll need to adjust for during the dive.

At the most fundamental level, buoyancy is determined by two forces, one of which is a gravitational force, the other being a buoyant force. When these two forces are in balance, you are by definition neutrally buoyant. The gravitational force is determined by your weight, meaning your body weight plus the weight of all the gear you’re diving with. Opposing that force is a buoyant force, which is determined by the weight of the water that the diver (plus gear) displaces. When we’re doing a buoyancy check then, we’re trying to put exactly the amount of ballast weight that will put our diver in equilibrium at the surface.

In determining the optimal amount of weight to carry, the best place to start is to consider your body weight and the exposure suit you’ll be wearing on the dive. With a 3 mm full wet suit, and assuming you’re diving in salt water, you should start with ballast weight equal to 5% of your body weight. If you’re diving a 7 mm suit, start with 10 -12% of your body weight. Two people with the same body weight may need different amounts of ballast weight, because we all have different amounts of fat, muscle and bone, which have different densities and therefore different buoyancy characteristics. If you’re fairly lean, start with 10%. If you’re more on the not so lean side of things, start with 12%. Professional athletes and special forces divers I’ve trained can often get away with as little as a 7 or 8% weighting factor, but they’re a bit outside the norm. To that I’ll add 6 pounds of weight if I’m diving an aluminum 80 CF tank, and nothing more if I’m diving a steel 80 CF tank.

If you normally dive fresh water and you plan to do some salt water diving, add one pound of weight for every 28 pounds of body weight, and you should be in the ball park. Conversely, if you’re used to diving salt water and plan to dive fresh water for a change, subtract one pound of ballast weight for every 28 pounds of body weight.

Clearly, it isn’t quite this simple, as your choice of tank, capacity of the tank and a few other factors will affect your buoyancy to some degree. Even a change of gear as innocuous as changing booties or fins can alter your buoyancy; some fins are positively buoyant, while another pair may be negatively buoyant.

It’s not unusual for new divers to carry more weight than they really need. This is primarily because they’re not able to get all of the air out of their BCD’s and wet suits. The breathing pattern of many new divers compounds this, causing them to bob at the surface as they inhale and exhale. Getting off the surface on the first dive of the day can be a bit difficult for divers in general. Here are some techniques that may help:

• Many BCD’s tend to trap a bit of air in the right shoulder as well as behind the neck. If you raise the inflator hose and dip your right shoulder it helps move most of the air to the dump. If your BCD has a right shoulder dump, that one will work even better, but in that case you’d want to dip your left shoulder. Another technique that works well with Jacket Style BCD’s is to bring your free arm across the front of the BCD and squeeze it against your chest. Finally, if you rock back a bit, that will help move any remaining air to the dump valve.

• Get the inside of the suit wet early. The lining of a dry wet suit traps air bubbles between your body and suit, which adds buoyancy that has to be offset with more weight. Opening up the wet suit at the collar and letting some water in prior to descending will help flush out any remaining air bubbles. Another technique I use is to shower off before the dive, spraying water down inside the torso and arms to flush any bubbles.

• A lot of newer divers have a tendency to move their feet at the surface while they’re trying to descend. Moving your feet while trying to descend is going to put the fins to work, requiring more weight to offset the upward push generated by the fins. If this sounds familiar, the solution here of course is don’t do that; rather, keep your legs still, and as you begin to descend lay out in a sky diver position in order to manage your descent as you go deeper.

• Many Open Water students tend to ‘bob’ at the surface as they inhale and exhale. When we breathe, the normal tidal volume (the amount of air exchanged with each breath) is slightly less than a liter, but that still translates to a couple of pounds of buoyancy. What I normally do is exhale fully and hold it until I start sinking, and then I take shallow breaths until I’m down about 10 feet.

Under the heading ‘NOT a tip’, I’ve seen a number of articles that recommend ‘duck diving’ down to get off the surface, but I don’t subscribe to this as a technique for getting off the surface. My objection is this – if a diver has to duck dive in order to get off the surface at the beginning of the dive with 80 cubic feet of air in their tank, and they breathe their tank down to 500 PSI before surfacing, that diver is going to be 5 pounds lighter at the end of the dive. Assuming they were perfectly weighted at the beginning of the dive, they’ll be underweighted at the end of the dive, and they’re going to have a really difficult time holding 15 to 20 feet for a three minute safety stop.

Speaking of tanks, most divers today dive an aluminum 80 cubic foot tank, but many of the more experienced divers prefer steel tanks. Ask them why and most will tell you they dive them because they get to carry less ballast weight by diving steel. Although this is true, there is something of an urban myth regarding why – many believe they need less weight because the steel tank is more dense than an aluminum tank, or that the steel tank is heavier. Actually, neither of these statements is completely accurate, at least in terms of how they affect buoyancy.

The fact is that most high pressure steel 80’s are actually a bit lighter than most aluminum 80’s. Although Faber’s high pressure 80 weighs 32.5 pounds when empty, the comparable products by Worthington and PST each weigh right at 28 pounds when empty. Aluminum 80’s made by Luxfer and Catalina weigh 31.5 and 33 pounds, respectively, when empty. The reason you can get by with less weight with the steel tanks has more to do with the displacement side of things. In essence, with a steel tank, you have a thinner wall, but because the structural integrity is greater with the steel tank it can be taken to a higher pressure, which together allow the tank to hold the same amount of air in a smaller space. There’s a lesson here too in that anytime you can use smaller dive gear, you’re going to reduce your displacement, thereby minimizing the amount of ballast weight you need to carry, all else being equal.

The best time to perform a buoyancy check is at the end of a dive, when your tank is around 500 PSI. The reason for this has to do with the weight of air in your tank. It turns out that 80 cubic feet of air weighs exactly 6 pounds, so if we’ve breathed our tank down from 3000 PSI to 500 PSI, we’ve used 5/6ths of the air in the tank, or 5 pounds of air. It follows then that if we were to do a buoyancy check at the beginning of a dive, we’d be 5 pounds under-weighted at the end of the dive, and it would be difficult to conduct a safety stop. There is some debate as to whether you should be neutrally buoyant at the surface as most training organizations support, or neutrally buoyant at your safety stop depth of 15 feet as Diver Alert Network (DAN) seems to promote. In most Open Water Courses we work to get divers neutral at the surface at the end of a dive, but there is merit in being a little underweighted at that point. If you establish your proper weighting for neutral buoyancy at the surface with 500 PSI left in your tank and then go a pound or two lighter, you should be neutral for your safety stop at the end of a dive.

As you breathe your tank down you will slowly gaining buoyancy, but the change is so gradual that most divers don’t really notice it until the end of the dive. What I notice whenever I’m diving a aluminum 80 though is the difference in my trim - because the increase in buoyancy is occurring behind me, it feels as though I have a cork strapped to my back. I normally account for this through the distribution of weight, which I’ll discuss in detail below under the trim discussion.

Another big factor in our buoyancy is that of the exposure suit we’re diving. Exposure suits don’t so much keep us warm as they delay hypothermia long enough to get a dive in. The exposure suit accomplishes this in part due to the close fitting nature of the suit, and the minimal water transfer in and out of the suit. The larger part of this process though has to do with improved heat transfer characteristics of the thousands of gas bubbles that make up the suit, and not so much from the suit itself. This too accounts for most of the buoyancy generated by the suit.

Clearly, a 3 mm wet suit offers far less buoyancy than a 7 mm suit, and a 3 mm shorty offers even less. A dry suit, depending on the material it’s made from, normally generates more buoyancy than even the heaviest wet suits. Beyond playing a significant role in determining how much weight you need to carry, there is another important aspect to how your wet suit affects buoyancy during a dive. As you dive to greater depths the gas bubbles in the neoprene compress, and their volume is reduced. This in turn reduces the insulating ability of the wet suit. As it gets compressed, the overall displacement of the diver wearing it becomes progressively reduced as well. This accounts for the sensation of being heavier (more negatively buoyant) as we descend, and explains why we have to add small amounts of air to the BCD with increased depth. It further explains why we must vent air off as we ascend to shallower depths, as pressure decreases, and the gas bubbles expand back to their original volume.

As you probably recall from your Open Water Course, for each 33 feet you descend, the pressure increases by one atmosphere (14.7 PSI). If you’re diving a 7 mm wet suit, for example, at 33 feet, the pressure on the air bubbles in the suit has doubled, and your suit is going to be thinner than 7 mm. When we consider that part of the wet suit that doesn’t compress so much (the neoprene itself), it turns out that at 50 feet your 7 mm wet suit has been reduced to a 3.5 mm suit. At 99 feet, (four atmospheres) it would only be less than 2 mm in thickness. Bear in mind that any air in your BCD is going to behave the same way.

Your buoyancy is going to change most dramatically in the first few feet as you descend. Toward the end of the dive, the expansion from the gradual but steady reduction in pressure as you ascend is also most dramatic near the surface. If you’re new to diving, pay particular attention to your buoyancy toward the end of the dive as you ascend. A new diver isn’t always aware that they’re becoming more positively buoyant with each foot as they ascend. They can be distracted by equipment issues, or they can get enrapt from their surroundings, and by the time they finally realize that they’re too buoyant, it can be too late to stop it.

One other final comment about depth changes before we move on here – the increased pressure at depth is also why your BCD, dive watch, computer, or wrist mounted compass tends to loosen at depth. Once I get to my dive depth I normally snug up on my BCD straps to keep it tight, and then loosen them up a bit when I begin my ascent. It’s also why I often wear lanyards on my wrist dive computers, to prevent them from coming loose and falling off at depth.

With the rest of your buoyancy factors set, your lungs become the final buoyancy compensator in diving. When full your lungs contain approximately 10 liters of air. Even so, the tidal volume, the amount of air exchanged during respiration under normal conditions, is only about half a liter. Given that a half a liter of salt water weighs just over one pound, this means that when you inhale with a normal, resting breath, you add about one pound of positive buoyancy. When you exhale normally, you remove one pint, shedding one pound of positive buoyancy. If you think back to the confined water session during your Open Water Course, you may recall that during your neutral buoyancy swims you rose and sank as you swam along. Now you know why.

Also from your Open Water Course, you may recall that in order to go deeper, you can simply breathe deeper. Conversely, if you want to go shallower, you just breathe more shallow. The reason this works has to do with the original discussion about how buoyancy works; when I breathe deeper, my lungs are on average less full than if I’m breathing shallow (retaining more air in my lungs). Lungs that are on average more full means that my average displacement (through a respiratory or breath cycle) is greater, which given a constant weight tends to make me slightly more buoyant. Conversely, if I’m breathing deeply, I’m dumping more of the air in my lungs, and my lungs are thus less full on average, which reduces my overall displacement, making me less buoyant, and I tend to go deeper. Once I’m at my dive depth then, this becomes my principal method for changing depth. I very rarely add or dump any air between 40 and 70 feet, but rather just change the way I breathe.

As it relates to the current topic, BCD’s are for the most part self-explanatory, so I’ll only hit the key points here. First, make sure that you’re BCD fits you properly – it should essentially be an extension of your upper body when adjusted, and realize that you may have to adjust it at depth based on compression of your suit. Secondly, your BCD is designed to ‘compensate’ for changes in depth, and allow you to remain neutrally buoyant at all depths. It’s not designed to be used as an elevator. If you find that you have to add a bit of air to begin your ascent, you’re not neutrally buoyant. Finally, be familiar with and adept at exercising the various dump valves on your BCD. In any given position, there is a ‘best’ dump valve to exercise – the one where the air is. When divers follow the terrain into shallow water, they’ll vent air off every 10 feet or so during the ascent. As noted above, as you get shallower, the differences in displacement become more dramatic. Normally when we’re following the bottom terrain into shallower water, I’ll ask student divers to try and maintain a consistent distance off the bottom. It’s been my experience that divers ascending from 60 feet, if they are not managing their buoyancy properly, will begin to separate further from the bottom somewhere between 40 and 45 feet. As they approach 40 feet, their fin tips will begin to come up, and as they pass through 40 feet they’ll begin to orient in more of a head down, fins up position. At this point, the dump valves in the shoulders of the BCD are relatively useless, because that isn’t where the air is. The best option available at that point (other than the Dive Master I have positioned above and behind them, watching for this) is to reach back and pull the dump on the lower right (or left with some BCD’s) rear dump valve.

Beyond using the BCD to help you manage buoyancy and trim during a dive, the other thing to know about your BCD is the lift capacity. Lift capacity is essentially a measurement indicating how much negative buoyancy the BCD is able to offset. Without going scientific on you, I maintain that the greatest lift capacity you’ll ever need is when the BCD is floating freely on the surface, fully loaded with weight and a full tank. In essence, the amount of lift capacity you need in a BCD is determined by the negative buoyancy of your tank when full, plus the amount of weight you’ll carry on a dive, plus some margin of comfort to enable putting the BCD on in the water (donning a fully inflated BCD in the water can be a fairly challenging experience). If you only dive tropical waters, you shouldn’t need more than 20 to 30 pounds of lift capacity, but if you’re diving colder waters like ours here in Southern California, you may need as much as 40 or 50 pounds of lift. I’m often carrying 10 pounds or more extra for students or clients that may need extra weight (beats swimming back in to get more weight), so I dive the Zeagle Tech, which has over 60 pounds of lift capacity. If the lift capacity is too low for the conditions you’re diving, you’ll find yourself having to kick upward just to remain at the same depth. This not only makes for an unpleasant dive, but is also a safety risk.

Which brings us to trim – the term trim refers essentially to maintaining a body position in the water that offers the least resistance as you move through the water. Most references to trim also include the configuration of your gear, meaning that your alternate air source is clipped to your BCD, your console is clipped to a D ring on your BCD, and any other gear is similarly attached such that it’s not dangling.

There are a couple of ways that divers typically manage trim, the most important of which is to be properly weighted. Divers that are over-weighted tend to hang feet down in the water column. Once you’ve established proper weighting, you can then move on to the distribution of your weight. Most BCD’s available today have trim pockets either integrated into the rear of the BCD, or positioned as pockets on the tank strap(s). If your BCD lacks trim pockets, it’s very likely that you can add them as an after market accessory and mount them to your tank strap(s). If you’ve already determined that you are properly weighted, and your feet are still hanging down as you swim along, try moving a larger percentage of the total weight to the trim pockets on your next dive.

I normally start by putting 25 – 35% of my total ballast weight in the trim pockets, depending on whether I’m diving a steel tank or an aluminum one. Given that aluminum tanks are generally more buoyant than steel tanks, and that this buoyancy is positioned to the rear, if I’m diving aluminum I’ll load more of my total weight up front, and less in the trim pockets. If I’m diving a steel tank, I’ll position a larger percentage of my total weight to the rear. Another way you can adjust your trim is to move your tank straps up or down. Moving the tank strap(s) up on the tank slightly will shift your weight lower, and bring your feet down. Conversely, moving the tank strap(s) down slightly will tend to bring your feet up. If you’re moving the strap down, though, you want to be careful not to bring the tank up too much or you’ll be rapping the back of your head against the regulator every time you look up

When setting your trim, one thing that can get easily overlooked is the placement of the scuba unit as a whole. With regard to donning the scuba unit, what I teach is to have students get into the scuba unit, and then bend over at their waist at a 90 degree angle while securing the BCD straps. What this does is that is takes the weight off the shoulder straps, allowing the diver to tighten the straps more easily and get them good and snug. It also pulls the scuba unit higher up on the back, giving the diver a consistent set point for getting their trim dialed in.

As you’re getting your ideal total weight and trim (distribution) established, be sure to put this information in your dive log. It will come in handy later when you’re changing to a thinner wet suit or changing from a steel tank to aluminum. If you’ve kept good notes in your log book you won’t have to guesstimate the proper weight and distribution.

You’ll generally be able to tell whether your trim is correct or not, but if you want to check, the easiest way is to establish neutral buoyancy at depth, hover a few feet off the bottom, and then float parallel with the bottom with your legs extended out. If your legs sink, move a couple of pounds to the trim pockets on your next dive, or move the tank strap down slightly. Trim is largely a matter of where on your body the buoyancy is coming from. Based on that, you might also consider the buoyancy generated by your fins and booties. If your legs hang down slightly, going to fins or booties that are slightly more buoyant will tend to raise your feet. Bear in mind, however, that adding or subtracting buoyancy by changing out gear needs to be countered by an overall change in the amount of weight you’re carrying.

Buoyancy has a significant effect on the amount of energy you expend on a dive, and the rate at which you consume the air. Similarly, your trim also affects energy required and air consumption needed to provide that energy. Another cost of trim might not be quite as obvious though, because when a diver’s body position is more vertical in the water they tend to move up and down as they kick, which depending on the depth can further affect their buoyancy. In this situation, the typical reaction of the diver is to add more air to the BCD or dump air to compensate for the change in depth. The obvious solution here is to maintain a horizontal body position so that the drive generated by your fins only moves you forward.

There are several skills you can practice to perfect your buoyancy control and body positioning, but the most common ones are the Fin Pivots and Hovering skills you learned in your Open Water Course. As you may recall from that class, Fin Pivots involve laying flat on your stomach on the bottom (find an open area on the sand for this where you won’t be harming any marine life), and adding just enough air in your BCD to have your upper body float up slightly as you inhale, and down slightly as you exhale. If you’re properly weighted you should be able to perform Fin Pivots with two or three small puffs of air in your BCD at shallow depths. Add the air in small increments, give it a couple of breath cycles before deciding you need to add more air, and avoid pushing off the bottom to get the Fin Pivots started. Let your buoyancy do that.

Once you’re able to do Fin Pivots with ease, you’re ready to move on to practicing the Hovering skill. Hovering essentially works the same way as Fin Pivots, except that you’re free floating in mid-water as you inhale and exhale. Hovering is an essential skill for being able to conduct safety stops in mid-water. The trick with hovering is to exhale as you ascend, and once you reach a neutral buoyancy tipping point and begin to descend, start to slowly inhale until you reach the neutral buoyancy tipping point on the other side. Again, you don’t want to push off the bottom to get the hovering started – let your buoyancy do that for you.

Ultimately, there is no better way to improve your diving skills than by diving regularly. By doing so, you keep your skills fresh. Diving involves motor skills which, when repeated on a regular basis, become familiar and comfortable. Unless you dive a couple of times each month, your buoyancy skills, buddy skills and diving fitness aren’t likely to improve much.

If you want to chart your progress, one of the best methods of gauging your improved buoyancy is your air consumption rate. Air consumption increases as you dive deeper of course, but most dive computers are capable of calculating your Surface Air Consumption rate, or SAC. The SAC is based on your dive profile, and converts the amount of air you use at various depths to a surface rate. Most new divers have a SAC rate approaching 1.0 cubic feet per minute, while many experienced divers will have a SAC rate below .5 cf/min. Achieving perfect buoyancy and trim, mastering efficient fin kicks, and using your breathing to change depths on the reef should each improve your SAC.

If you have questions outside the information and opinions presented here, feel free to contact me through Dive Buddy (username is marcus0453), email or call me and I’ll do my best to help you out. Too, if you happen to disagree with something offered here I’d love to hear from you – as Dive Training magazine says, “a good diver is always learning”.

In closing, please note that the information presented here is not a substitute for proper training. I highly recommend that divers looking to perfect their buoyancy, trim, kick style and other aspects of their diving seek out a qualified dive instructor and train with them. The other closing recommendation here is to always dive within the limits of your training and experience. If you’re interested in doing some type of diving that you’ve not been trained in (such as deep or night diving), please take the class prior to attempting those types of dives. Dive S.A.F.E!

Marcus Gee

PADI MSDT #235938

Scyllis Diving



IhanaKuu - 10/22/2012 7:44 PM
Great article, thanks!

I wished I had read this before my live a board trip
divebumNsandyeggo - 10/22/2012 10:47 PM
thanks - glad you enjoyed it, and i certainly help the information helps divers become safer and more comfortable in the water!
LatitudeAdjustment - 9/13/2012 4:57 AM
Divers, ever notice on trips the first site is in poor condition, the DM is watching your buoyancy control before taking you to his favorite sites, if you or someone in your groups buoyancy control skills aren’t good you will never see them!

On a Honduras trip I told a woman she was over weighted, after she got done with her tirade about me being rude she realized what I was really saying and started getting the lead out. By the end of the week she was horizontal in the water and smiling up a storm, she was now able to get closer to the reef and see more.