Tag Archives: buoyancy

Scuba 101 – How much weight

How much weight
Proper weighting is achieved when a diver can hold a 10′ stop with no air in the BC and with 500 psi remaining in the tank. This insures that the diver can hold his last stop at the end of a dive, without the need of a buoyancy device.

Why overweighted in OW?
If the proper weighting is significantly

Effects of Overweighting
Carrying additional weight than necessary means that the diver needs to carry more air (buoyancy) was well.

When a diver ascends, this extra air expands and needs to be managed.

Depth Proper Overweight Difference
100′ 2# 6# 4#
33′ 4# 12# 8#
Purge 2# 6# 4#

Let’s say that a diver is properly weighted and compensating for 2# of weight near the end of his dive at 100′. He ascends from 100′ to 33′ and now has 4# of lift in his wing, as the air in the BC expands from decreased pressure. In order to remain neutrally buoyant, the diver removes 2# of lift.

In the same scenario, let’s say that the same diver is over weighted by 4#s. At 100′, he is now compensating for 6#s of weight. When he ascends to 33′, the air in his BC expands to equal 12#s of lift. To remain neutrally buoyant, the diver must now remove 6#s of gas from the BC.

It’s definitely possible to manage the extra 4#s of gas. However, it requires more management and more diligence. For new divers, this extra 4#s of gas is additional work that is not necessary and only adds to make diving more difficult.

Requiring less weight the more I dive
As you gain experience, you shed add’l weight cause you improve your breath control, your body movement and buoyancy.

In addition, you’re be more effective in eliminating gas from your BC. Trapped gas or inefficient purging is one of the reasons that new divers need to carry more weight.

Scuba 101 – Controlled ascents

One of the most important skills for new scuba diver to master is a controlled ascent. It is not uncommon for newer divers to feel okay at depth (>30′) but then feel unstable on ascents and in shallow waters – “I can hold my 20′ safety stop, but when I move shallower, I quickly find myself on the surface.” This is especially true for new cold water divers and new drysuit divers.

Fortunately, controlled ascents can be learned.

Scuba diver ascent
Picture by Toine Peeters

Proper Weighting
Carrying the appropriate amount of weight makes diving easier. Carrying too much weight adds instability into the scuba system, as more air is needed to compensate for the additional weight.

Obviously, it’s possible to manage the extra weight. But why do so when it’s unnecessary and makes diving more difficult?

Proper weighting is achieved when a diver can hold a 10′ stop without any air in the BC and with 500psi remaining in the tank. This insures that the diver can hold his last stop at the end of a dive.

Be a Proactive Diver
The most common statement I make while teaching a basic scuba course is “be a proactive diver. And not a reactive diver.”

Instead of waiting to react to buoyancy changes, it’s better to anticipate and proactively manage. On descents, this means adding air as one descends and thus stopping a few feet from the ocean bottom. An example of being reactive on descents is the diver that touches down on the ocean bottom and then adds air to lift him off the bottom and into a neutral state.

On ascents, being proactive means dumping gas from the BC when the diver ascends, before feeling a positive buoyancy effect. On an ascent from depths greater than 30′, this generally means dumping gas every 5′-10′ as appropriate. For ascents from 30′, this may mean dumping gas every 2′ or less, depending how you are managing your ascent rate and ascent strategy.

Getting in Front of the Buoyancy Curve
To be proactive on ascents, this means that the diver must get in front of the buoyancy curve.

Instead of adjusting once you get to the desired depth, adjust en route.

For the ascent, this means adjusting during the ascent, before you feel the buoyancy lifting you up. To do this, the diver uses the combination of lung and BC to manage the ascent.

Simplified, the steps would be as follows:

  1. Start neutral in the water column
  2. Inhale to start the ascent
  3. Dump gas
  4. Exhale back to normal lung volume
  5. You should now be neutral in the water column
  6. Repeat 1-5

Remember that the recommended ascent rate is 30′ per minute. That means from a typical safety stop of 20′, it should take the diver 40 seconds to reach the surface.

A proper ascent and bubble management would look something like below. The bubble is appropriately sized for the ascent, and not pulling the diver up and out of control.

Ascent Bubble curve

If the diver waits too long to manage his buoyancy, and gets behind the bubble curve, then the extra air in the BC will cause a rapid and potentially uncontrolled ascent.

The excess bubble (buoyancy) causes the diver to rise. This expands the bubble more and in turn causes the diver to rise faster and expand the bubble even more. I think that all of us in our diving career has felt the sensation of being carried away to the surface.

Ascent Bubble curve

Body Position
While I discuss body position in the scuba ascents and descent article, its worthwhile to revisit again. The horizontal trim position offers the diver stability while moving up and down the water column. This is because the horizontal trim position provides the largest vertical drag possible.

Please refer to the linked article above for a more detailed discussion of body position.

Scuba 101 – Ascents and Descents

In many basic Open Water scuba courses, divers are taught vertical body position for ascents and descents. While there are some benefits to this technique, a horizontal body position is safer, more effective, and much easier to execute.

Body Position
The prone body position, preferred while diving, is also the preferred body position for ascents and descents.

Below is the prone/horizontal/skydiver body position:
scuba diver ascent in prone position

Below is the vertical body position that most divers use for ascents and descents:
scuba diver ascent in vertical postion

Drag
The horizontal diver position provides a significant drag in the vertical water column. The wider profile helps control buoyancy as well as slows the rates of ascents and descents.

scuba diver horizontal ascent drag

The vertical diver position provides less drag in the water column. As the result, it takes more effort to maintain position and rates of ascents and descents are faster.

scuba diver vertical ascent drag

The above illustrations are from the side profile. However, viewed from the top, the profile differences between the horizontal and vertical positions is even more dramatic.

For descents, it’s usually easiest to descend the first 2′-3′ in vertical position since it’s the most steamlined. Once the surface tension is broken and compression starts, then the diver switches to a horizontal position to help control the descent rate and allow for maximum mobility.

Platform
Because of the benefits of drag on the horizontal driver, this position offers the most stable platform. This stability is particularly useful if the diver becomes task loaded or needs to resolve an issue during descents, ascents, or safety stop.

Most diving issues occur during descents or ascents. Even with the appropriate equipment and bubble checks, the descent is when your gear is first being tested. In our local waters, descents are also a common time for buddy separation. During ascents, issues include OOAs and gas switching mishaps. In these situations, the diver needs to be able to maintain neutral buoyancy while resolving the issue. The horizontal position makes this much easier.

Field of View

During the descent, a horizontal position provides optimal field of view. Both positions allow for looking forward at your teammates, but the horizontal position provides a great birds eye view of the bottom.

The horizontal position does limit your ability to look above you. But teams should descend and ascend together, at the same rate.

scuba diving descent field of view

scuba diving descent field of view

During ascents, in areas where a total horizontal position ascent may be dangerous (e.g. ships overhead), then switching to a more vertical body position in the last few feet may prove helpful. However, the best ascent strategy is to ascend in teams and have your teammates watch overhead and behind you.

horzontal ascent as a team

Kicks
In addition to vertical drag benefits of the horizontal body position, the prone position allows quick access to all kicks. These could include the small kicks for positioning the diver with the team and the environment. It also includes large kicks to quickly reach a teammate if there is an issue.

A diver in the vertical position has less horizontal mobility. Kenn (Gombessa on ScubaBoard) notes the vertical position also reduces vertical mobility. In the horizontal position, tilting up or down offers quick adjustments. In the vertical position, moving up is easy but moving down requires a full inversion.

The biggest issue with the vertical position is the use of fins to maintain position in the water column. Not only does this require work (consuming more gas), kicking to control buoyancy is not a stable position. In order to maintain buoyancy or control, the vertical diver must manage the BC and kick a consistent cycle. Alternately, the horizontal diver simply uses the BC or breath control.

In addition, a diver kicking in vertical position has an impact on the environment. On descents, silt and sand can be kicked up by a vertical diver’s fin movement. It’s not uncommon to see great viz, until divers descend onto the ocean floor. Fortunately, this can be eliminated if divers descend in horizontal position with their fins parallel to the ocean floor.

vertical diver kicking silt

Rate of Descent and Ascent
While there’s generally prescribed rates of descent (slow enough to allow sufficient equalization) and ascent (30 ft/min), the overall goal is control.

Upon reading this article, Ben (ben_ca on ScubaBoard) made a good comment about the need to arrest your descent/ascent with relative ease. He recommends a range for beginning divers of 4-5 ft and advance divers of 1-2 ft.

Not only is this control helpful in managing ascent/descent related issues such as blocks, but it is useful in keeping buddy teams together and being available to help if required.

A good way to practice is to make predetermined stops on descents and ascents. For example, instead of descending immediately to the bottom, agree with your buddies that everyone will stop at 10′ and 20′. On ascents, safety stops can be done at 30′ for 1 minute, 20′ for 1 minute, and 10′ for 1 minute.

How to Descend

Below is high level descent strategy, the diver will need to adjust to local conditions.

  1. Signal descent, get confirmation
  2. Inhale
  3. Dump gas
  4. Exhale
  5. Confirm legs are not kicking
  6. Transition to horizontal position
  7. Look at buddy
  8. Look at environment
  9. Equalize
  10. Add gas to control descent speed
  11. Repeat 7-10 until hovering comfortably off the bottom

How to Ascend

Below is high level ascent strategy, the diver will need to adjust to local conditions.

  1. Start neutrally buoyant in horizontal trim
  2. Signal ascent, get confirmation
  3. Inhale to start ascent
  4. Look at buddy
  5. Look at environment
  6. Dump gas to control ascent speed
  7. Repeat 4-6 until the safety stop
  8. Conduct the appropriate safety stop(s)
  9. After safety stop(s), continue to slowly ascend until the surface

Horizontal Trim 101 – Uses

Before we put the basic levers for trim in use, it’s very important to review the horizontal trim position. For the majority of scuba divers, the horizontal trim position is not natural.

Horizontal Position

Consider the body in the horizontal plane. Now consider where the head needs to be for the body to be in the horizontal position. Yes, the head needs to be back against the 1st stage or tank manifold.

In addition, the legs (or more appropriately the knees) need to be up. This requires the flexing of the glutes and consideration of not “dropping the knees.”

Lastly, your back will be in a slight arched position (as shown in the diagram below.) A fellow diver, Lynne, makes a good recommendation regarding this position: “With regards to the part about body positioning, I always have a little bit of an issue with telling people that they should arch their backs. Too often, they try to make the whole movement with the small of their back, which results in significant back pain. I tell people to make their body as long as it can possibly be from the shoulders to the knees, and then contract the glutes to flatten out the hip joint. That seems to avoid the hyperlordosis issue.”

horizontal trim arch

Getting familiar with the horizontal position on dry land is the first step. Using either a bench, a bed, or the floor, assume the prone position and look straight ahead. Do not cheat, and lift the knees off the ground. Remember what muscles are used, since these will be the ones needed in the water as well.

While on dry land, having an instructor or a knowledgeable buddy review your position will be beneficial. In lieu of a buddy, a mirror can be used.

Below are scuba divers in an Essentials class getting into horizontal trim.

Essentials class, practicing trim on the ground

Once in the water, it is initially difficult to tell if one is in horizontal trim. As the result, video taped class sessions are often eye opening for many divers. Besides video, a good buddy can simply signal the degree of attack that he is seeing. Some instructors employ mirrors in pools to aid the diver as well.

If possible, free dive a swimming pool or the ocean. Drop down and hover in horizontal position (a few pounds on a weight belt will be sufficient). Without the extra buoyancy of wetsuits and additional weight of gear, getting into the correct positioning will be easier. Once this position becomes familiar, then don all the scuba gear.

Proper Weighting

Proper weighting cannot be stressed enough to new (and some experienced) divers. Overweighting is the root of many problems in scuba, and trim is definitely one of them.

The more weight that a diver carries beyond keeping him neutral, the more air must be carried. More air equals larger bubble. Larger bubble equals more management and exaggerated vertical movement. For example, you rise a little, the bubble expands generating more lift causing you to rise more. As the result, it’s best to minimize the amount of air carried.

Appropriate BC Lift

Another common mistake is selecting a BC with too much lift. The actual issue isn’t the lift, but it’s the excess material of the BC that’s needed for generating the lift. In order to provide more lift, the larger BCs need to be wider and or longer.

While more material generates more drag, it impacts trim as well. The extra material ultimately requires more active management from the diver and in some cases lead to trapped air.

Use of Levers

Once the diver knows and feels what horizontal trim feels like, then we can begin using buoyancy, weight and leverage to aid him.

Step 1: Find a comfortable spot and drop to 20′ of water. Get into horizontal position and do not move. Do not move the hands, do not move the fins. If need be, cross the hands and fin tips so there is no subconscious movement. The body should simply be pressing in the horizontal position.

Step 2: Count to 5. 1-2-3-4-5. Is there instability in pitch? Are the feet lifting you to the surface? Are the feet sinking even though you are flexing to keep the knees up? Is the head being driven into the ground? Is the torso being lifted up?

Step 3: Repeat 1 and 2, adjusting arm and leg position as levers to correct trim issues. For scuba divers in warm water (minimal weight and exposure protection), body positioning is often sufficient to correct trim issues.

Step 4: If these adjustments do not resolve the issue, then check buoyancy and weight locations. These will need to be adjusted.

For example, let’s assume that a properly weighted diver is in head heavy doubles rig. What are the options?

  1. Move tanks down
  2. Move wing up
  3. Get longer tanks
  4. Get a tail weight
  5. Move to an aluminum plate and use a heavier tail weight
  6. Weight belt
  7. Get heavier fins
  8. Ankle weights (*gasp*)
  9. If diving dry, dive with air in your arms
  10. If diving dry, add a vest
  11. If diving dry, use fin keepers and/or gaiters (maybe it’s floaty feet)
  12. Use a wing with lift at the shoulders

There are pros and cons with each of these approaches.

For in water training, I recommend UTD Essentials or GUE Fundamentals. Both courses have proven track records of cultivating the basic diving skills.

Horizontal Trim 101 – Levers

The horizontal trim video below is part of the UTD Essentials DVD:

Overview

The horizontal position is often called prone, superman, or skydiver position. For a scuba diver, the horizontal trim is the most efficient way to move through the water. In addition, the horizontal trim provides vertical drag, helping divers maintain a constant depth.

However, getting into this position is often difficult for most new divers. On the various scuba boards, there are daily questions regarding: “too head heavy”, “floaty feet”, “can’t stay trim”, etc. Fortunately, this problem is solvable.

In managing trim, there are two major forces that the dive must balance – buoyancy and weight.

  • Buoyancy is the upward force on the diver. This may include BCD, wetsuits, boots, fins, tanks, and of course our own personal floatation.
  • Weight is the downward force on the diver. This may include weight belts, trim weights, backplates, lights, fins, tanks, regulators, and any additional item on the diver that sinks in water.

A diver’s trim pivots around a point, and this is usually around the divers midpoint. In the diagram below, this is denoted by the red triangle. Think of the diver as a see-saw across this pivot. For example, if the diver has too little buoyancy or too much weight in the upper body, he will be head heavy.

Horizontal diver with pivot point

Buoyancy

The buoyancy compensator (BC) or buoyancy control device (BCD) is what scuba divers think of when required to manage their position in the water. Whether you have a jacket/vest, back-inflate, or a hybrid, it’s important to be familiar with the specific ways your BC manages air and learn to use it to its maximum potential.

Unless there is an obstruction, air will migrate to the highest point of the BC. As a consequence, the BC’s buoyancy is also concentrated at this point and this will have impact on trim. The BC’s shape, width, length, and of course location on the body can all come into play.

Also consider the buoyancy of the wing at the beginning of the dive and at the end of the dive. At the beginning of the dive, the wing will have its maximum lift – enough to offset gas in tanks and any exposure suit protection compression. At the end of the dive at 10′, the wing will be empty with 500 psi left in the tank.

The diagram below highlights the horizontal diver’s wing BC’s buoyancy in blue. Please note the position of lift compared to the pivot point.

Wing buoyancy

In addition to the BC, the scuba diver’s exposure protection also provides buoyancy. For a dry suit diver in particular, the suit can be actively managed to offset trim issues and fine tune his attack in the water.

The diagram below highlights the drysuit’s buoyancy in blue.

Dry suit buoyancy

Weight

While weight is needed for a diver to be comfortably submerged under water, the placement of weight can be used to a diver’s advantage regarding trim. Because weight is constant throughout the dive it is effective in managing trim and is often the first gear adjustment that divers will look to make.

Tanks’ buoyancy characteristics affects trim. Whether singles or doubles, moving the tanks up and down along the body will adjust trim, respectively. For example, if the diver is too head heavy, a tank can be moved lower on the body.

Hard weights are very effective in managing trim. Getting familiar with trim weights is beneficial, especially in cold water where more exposure protection and weight is used. Feature rich BCs often have trim pockets to help with adjustments. However, single weight pockets, such as the XS-Scuba weight pocket, can be used on cam bands. Some divers even thread weights into their shoulder harness.

Backplates are popular since it helps place weight in an area that promotes good trim. A stainless steel backplate will move up 6 lbs. With a heavy singles tank adapter (STA) or weight plates, even more weight can be moved up the diver’s body.

Doubles or twin tank divers also employ tail weights and v-weights to adjust their trim.

For an example of how different divers employ different weights, it’s worth skimming the drysuit weighting thread on Dive Matrix – backplates, weight belts, no weight belts, cam weights, tail weights, v-weights, STAs, etc.

Fins can also be a source of weight. Many drysuit divers prefer the negatively buoyant ScubaPro Jet Fins for this specific reason.

Lynne (TSandM) wrote a very good explanation of how to distribute weights to promote trim.

The diagram below highlights the areas of weight in green.

scuba diver weight

Leverage

To maximize the use of buoyancy and weight to affect trim, we must also discuss leverage. In addition to the obvious use of buoyancy and weights at the ends of the body, the scuba diver’s body itself can provide leverage.

It is important to note that many trim issues can be resolved by proper body position. In addition, improper body position exacerbates trim issues. Head looking down position often leads head heavy trim. As the result, the first fix to any perceived trim issues is body adjustment.

The diagram below highlights how body position (extensions and contractions) can change leverage and therefore impact trim.

Diver leverage

Next: Buoyancy, Weight, and Leverage to affect trim