Archimedes Principle

Scuba 3

Learning about the physics of the underwater world can help divers to have a more complete understanding of their sport. One essential concept is Archimedes’ Principle, which states:

Any object wholly or partially immersed in a fluid will be buoyed up by a force equal to the weight of the fluid displaced.

Archimedes’ Principle in Plain English:

When an object is placed in water (or any other fluid), the water will exert an upward force on the object. The strength of the upward force is equal to the weight of the water displaced by the object.

To visualize this concept, imagine dropping a marble into a full glass of water. Some of the water will be displaced by the marble and will overflow the glass. If the displaced water is weighed, the weight of the water will be equal to the upward force.

Some objects, such as ping pong balls, will float partially above the water. In this case the upward force is equal to the weight of the water displaced by the submerged portion of the object.

How Does Archimedes’ Principle Help to Determine an Object’s Buoyancy?

Archimedes’ Principle describes the upward forces acting on an object immersed in water. However, an object’s weight (a downward force) also affects its buoyancy. To understand an object’s buoyancy, it is necessary to consider both the downward and upward forces present, and to determine if they are equal or whether one is greater than the other.

If the downward force on an object is stronger than the upward force, it will sink and is said to be negatively buoyant. If the upward force on an object is stronger than the downward force, the object will float and is said to be positively buoyant. Finally, if the two forces are equal, the object will remain suspended in the water and is said to be neutrally buoyant.

In the case of the marble dropped into the water glass, the marble will sink because the weight of the marble (the downward force) is greater than the weight of the water it displaces (the upward force). A ping-pong ball, on the other hand, will float when placed in water.

Archimedes’ Principle reveals why the ping pong ball and other air-filled objects tend to float. An air-filled object weighs little but displaces a relatively large amount of water. One example is a boat, which is basically an air-filled shell. Even a metal boat can float, provided that the water it displaces weighs more than it does.

What Does Archimedes’ Principle Have to Do With Scuba Diving?:

Archimedes’ Principle explains how various factors including a diver’s size, weight, and dive gear affect his buoyancy. Here are a few examples of Archimedes’ Principle in action:

Buoyancy Compensators (BCs):

In its simplest form, a BC is an inflatable air cell that a diver carries with him underwater. He inflates and deflates the air cell during the dive to adjust his buoyancy. When the diver inflates his BC, the air cell expands, displacing a greater volume of water, and increasing the upward force on the diver. When the diver deflates his BC, the air cell loses volume, displaces less water, and weakens the upward force on the diver. Underwater, a diver uses his BC to maintain neutral buoyancy. On the surface, a diver inflates his BC almost completely to allow him to float on the surface.


The use of lead weights in scuba diving is also justified by Archimedes’ Principle. When fully geared up, most divers are positively buoyant on the surface. A scuba diver displaces quite a bit of water! To counteract the upward force, a diver wears lead weights, which are small and heavy; they increase the diver’s weight but barely increase his water displacement.


A wetsuit increases a diver’s water displacement without significantly increasing his weight, which makes the diver more buoyant. Even a thin wetsuit will increase a diver’s buoyancy. The thicker his wetsuit, the greater a diver’s water displacement, and the greater the upward force on his body will be. Dry suits are bulkier and much more positively buoyant than wetsuits


Aluminium tanks also have an interesting affect on a diver’s buoyancy. When full, a standard 80-cubic-foot aluminium tank (Al 80) is negatively buoyant. However, the shape and volume of an Al 80 is such that when the tank is empty, its water displacement is greater than its weight (aluminium is very lightweight). As compressed air is breathed from a full tank, the tank becomes lighter and lighter, until it eventually becomes positively buoyant. A diver must weight himself to counteract the buoyancy of the aluminium tank at the end of a dive, which means he will begin the dive slightly overweighed. He must inflate his BC to compensate for this negative buoyancy when he starts the dive, and then gradually release air from the BC throughout the dive to maintain neutral buoyancy.

The Art of Equipment Configuration

Diver 1

1. Accessibility:

The single most important concept in scuba equipment configuration is accessibility. A diver should be able to deploy, use, and stow dive gear easily and efficiently. Any configuration that makes accessing a piece of dive gear difficult or complicated should be re-thought, even if it fulfils all the other tenets of optimized equipment placement. Whenever possible, configure all dive gear so that it may be deployed and operated with one hand. This ensures that a diver can use his other hand for emergency management, such as maintaining physical contact with an out-of-air buddy.

2. Durability:

The manner in which a diver configures his gear should ensure that it will stay in place for an entire dive. A common equipment configuration failure is the alternate air source regulator coming free of its quick release.  The first time a piece of gear comes loose, the diver should reconfigure his gear to prevent the problem from reoccurring.

If it happened once, it will happen again!

3. Streamlining:

All dive gear, including accessories and hoses should be streamlined. In no circumstance should a piece of dive gear dangle more than a few inches below a diver’s horizontal body line . Hoses should be adjusted so that they do not loop above or beside the diver. Keep in mind that regulator hoses are available in a variety of lengths. Proper streamlining reduces the chance of entanglement, damage to the environment, and drag.

A common failure of streamlining in standard recreational scuba diving is the octopus-style alternate air source, which necessarily must be attached to a longer hose to facilitate donation to an out-of-air diver. The hoses on octopus-style regulators typically loop out to the side of the diver, presenting an entanglement hazard and creating drag. As far as streamlining goes, integrated alternate air sources and long hose/necklace regulator configurations are both preferable.

All types of diving require some level of redundancy in the gear. Recreational open water diving requires that a diver carry at least one redundant second stage for his buddy to use in an emergency. More advanced or technical types of diving may require additional redundant gear for safety. A diver should carry the minimum amount of accessories and gear that allows him to dive safely. At a certain point, additional gear no longer increases dive safety. The trick is to balance redundancy and simplicity when configuring dive gear.

5. Trim and Comfort:

Once a diver has optimized his equipment configuration, he should test it in a forgiving environment such as a pool or shallow water dive site. When evaluating changes to his gear configuration, a diver should notice whether the changes upset or improve his trim and whether or not position of the gear is comfortable. No matter how fantastic it seems on the surface, the final test of an equipment configuration is whether or not it is comfortable and functional underwater.

Evaluating and adjusting your scuba diving equipment configuration can make diving easier, safer, and more comfortable.

Hands and Buoyancy Control! Advice on Establishing Proper Neutral Buoyancy While Scuba Diving – By Natalie Gibb, Guide

Every movement a diver makes underwater effects his position.  When a canine dog-

Paddling Dog

paddles, he propels water downwards to keep his head above the water. A diver who dog-paddles also moves himself upward in the water. Some divers use their hands to adjust their buoyancy instead of adding air to their buoyancy compensator (BCD). Every time the diver moves downwards in the water (causing him to become more negatively buoyant) he would compensate for his decrease in buoyancy by pushing water downwards.

When a diver swims with his hands, his breathing rate increases from the extra exertion and he empties his tank more quickly. If the diver dog-paddles, he may also stir up bottom sediment (decreasing the visibility) and he runs the risk of accidentally slamming his hands against coral or other objects.

One of the most important techniques a diver can learn is to properly control his buoyancy using his BCD and his lungs. New divers tend to struggle with buoyancy until they learn to notice small buoyancy changes as they ascend and descend. These small changes alert a diver to the fact that he has changed depth. By responding quickly and effectively to small buoyancy changes, a diver maintains his desired level in the water and avoids an uncontrolled ascent or descent.

A diver unconsciously uses his hands to counteract the small buoyancy changes. As a result, he does not notice that he is descending, and continued to become more negatively buoyant until his dog-paddling is no longer sufficient to keep him off of the ocean floor. At this point, he completely loses buoyancy control and suddenly plummet downwards, unable to inflate his BCD quickly enough to counteract his descent. Furthermore, every time he stopped swimming, he sank downwards once he stopped moving his hands. “Cheating” by using his hands prevents him from noticing small buoyancy changes and fine tuning his buoyancy using his BCD and lungs.

Many divers unconsciously use their hands to make small adjustments to their buoyancy. This seems to be a natural response, similar to kicking on the surface. A good way to break the habit is to have the diver consciously hold his hands still by placing them in a pre-determined position. I recommend hand positions such as clasping the hands in front of the diver, crossing them on the diver’s chest, holding onto the BCD shoulder straps, or “superman-ing” in front.

Good buoyancy control not only requires the ability to properly use both the lungs and the BCD, but it also requires that a diver is able to recognize small buoyancy changes. The best divers make tiny adjustments to their BCDs and lung volume to maintain perfect neutral buoyancy at all points during a dive. If a diver uses his hands to keep himself up or down, he is depriving himself of the opportunity to fine tune his buoyancy techniques and experience the thrill of swimming effortlessly and weightlessly through the water


Dive Slowly

Scuba diving in Elba island, Italy

Scuba diving in Elba island, Italy (Photo credit: Wikipedia)

You will not be able to see everything the ocean has to offer in one dive, so why rush?

Why shouldn’t scuba divers race across the reef at top speed?

Won’t swimming quickly allow divers to cover more ground and see more?

Divers who speed through the water breathe more heavily than slow-moving divers because they exert themselves more. An increased air consumption rate leads to a shorter dive time. The amount of aquatic life a diver sees is proportional to the amount of time he stays underwater not the distance he covers. In fact, divers who swim quickly tend to pass over interesting smaller creatures.

Constant kicking frequently masks major problems, such as poor buoyancy control or improper weighting. A diver who won’t stop kicking frequently can’t stop kicking because he is using his fins to adjust his buoyancy. For example, a diver who is sinking may drop his legs and kick himself upwards to maintain a consistent depth instead of adding air to his BC.  Many divers have to force themselves to slow down and occasionally stop all movement before they learn to fine-tune their buoyancy control.

Swimming rapidly may also make diving unsafe on a physiological level.  A diver who swims hard enough to raise his breathing rate is doing the scuba equivalent of running a marathon underwater. Furthermore, breathing resistance increases underwater in proportion to depth. A diver who increases his breathing rate may not be able to exhale all of the used air from his lungs. This leads to an increase in carbon dioxide, which can cause a diver to feel starved for air, become dizzy or even fall unconscious!

All divers, novice and experienced, should avoiding racing through the water.

Swimming too quickly when scuba diving shortens dives, causes divers to overlook interesting aquatic life, masks serious buoyancy problems, distracts them, and leads to physiological issues.

Slow down and take it easy underwater — diving should be relaxing and enjoyable, not exhausting!

Controlled Descent

Descending from the surface should be as easy , but it sometimes can deteriorate into an exhausting struggle with our gear and our mounting anxiety.

The solution is to handle descents the same way you’d eat an elephant: one bite at a time.

The ability to make a controlled descent is important for three reasons:
1. If a diver experiences ear equalization problems and he cannot arrest his descent, he risks an ear barotrauma.
2. A diver must be able to descend without landing on the bottom because even a gentle fin kick can irrevocably injure coral or other aquatic life. Landing on a shipwreck or cave floor can not only destroy delicate historical information, it can stir up sediment to the point that visibility is dangerously reduced.
3. A diver should be able to stay close to his buddy during descent.

Avoid task-loading problems by breaking down your descent into discreet, manageable steps

On the Surface

After you and your buddy make your entries, don’t rush to descend. Relax, regroup and recheck. Make contact with your buddy, clear the entry area and float comfortably buoyant with your snorkel or regulator in your mouth and your face in the water.
Take a few moments to establish a slow, full breathing pattern and acclimate to the water temperature, especially if it’s cold. Then begin to methodically recheck your gear to make sure nothing has come loose on entry — your weight belt buckle or your mask strap, for instance. Listen and look for hose leaks and freeflows. If you’re wearing a dive computer, confirm that it is activated. Once you’re satisfied that everything is in order, visually check over your buddy’s equipment.
If surface conditions such as a current or wind chop make you uncomfortable, these checks can be performed while holding onto a current line or just under the surface on a descent line.
Practice your ear equalization technique a couple of times to loosen up the muscles. As a last step before descending, compare gauges and set your watches.
Begin Your Descent
Once the “thumbs down” signal is given, both buddies should begin their descent at the same time and remain together all the way to dive depth. Keep your right hand free to equalize your ears as you drop below the surface.
To start your descent, establish negative buoyancy by venting air from your buoyancy compensator (BC).

With your left hand, raise your deflator hose straight above your head and depress the button while dipping your right shoulder. This position will ensure that the air has an unobstructed path to flow up and out. If you have previously performed a weighting check and are correctly weighted, you should immediately begin to descend slowly and effortlessly.
If you fail to sink, first stop all kicking and other body motions and exhale fully. This is often enough to trigger a normal descent. Next, try squeezing any remaining air out of your BC while keeping the deflator hose up and the button fully depressed. Only after you’ve tried the above should you consider adding weight.
Some divers try to deal with an inability to sink by “duck diving” — swimming head down to propel oneself under the surface. This takes a lot of effort and also makes equalizing difficult. A better option is to use a descent or down line, which often hangs vertically into the water near the entry area of the boat especially for that purpose. Or you can use the boat’s mooring or anchor line.
Try easing yourself down the first few feet (1 m) along the line while remaining in an upright position; remember to equalize your ears as you go. You may be properly weighted but just need that little bit of increased pressure to force out air trapped in loose spots between your body and wet suit and in your gear. It is also easier to relax and exhale fully once your head is under the surface. If you still don’t sink, you’re probably underweighted.
If you determine that you are either under- or overweighted enough that you can’t proceed with a safe, enjoyable dive, signal your buddy to abort the descent. Return to the boat to add or subtract lead.
To control anxiety, stop your descent and hold onto the line until your breathing and heart rate return to normal. Remind yourself to clear your ears often by equalizing every time you move your hand down on the line.

Allow yourself to float downwards until you can no longer easily control your buoyancy with your lungs. Once you reach the point that you continue to sink when you inhale, you are no longer neutrally buoyant. When you are neutrally buoyant you should rise slightly when you inhale fully. Remember, the goal is to maintain neutral buoyancy throughout the descent, not negative buoyancy. Add a tiny, tiny amount of air to your BCD. You should be able to stop descending or rise slightly when you inhale. Take some time to find this point of neutral buoyancy.
After descending a few feet and reestablishing neutral buoyancy, take a moment to check that your ears are properly equalized. Look at your depth gauge and notice if you are approaching or have reached your intended depth. Check on your buddy. If all is good . . .

Descend by Exhaling Once Again:
Once you have regrouped, continue your descent by exhaling fully. The goal is to control your descent by working your way slowly and carefully down through the water column using you lungs to descend and your BCD to keep yourself neutrally buoyant. When you arrive at your desired depth, you should have to do very little to fine-tune your buoyancy.

Equalize, Equalize, Equalize
As soon as your head sinks below the surface, equalize your ears and continue to equalize at least every couple of feet all the way to dive depth . The key to successful ear clearing is “early and often” — start early and do it often. This prevents pressure from building up against the eardrum, causing pain and difficulty equalizing.
Don’t continue your descent if you do experience ear discomfort; it won’t get better. Instead, ascend a few feet until you can equalize, then continue your descent more slowly, equalizing even more frequently . You can’t equalize too often.
Avoid looking down while trying to equalize; it can pinch the Eustachian tubes, making them more difficult to open. Raise your chin and stretch your neck to each side to help air flow through the tubes. Blowing harder is not the answer.

Slow Your Descent
The deeper you descend, the more negatively buoyant you’ll become as the increased pressure compresses the air spaces in your exposure protection. You may need to slow your descent by adding air to your BC with short bursts from your power inflator .
Stop your descent at least 1 m before you reach dive depth. Make yourself neutrally buoyant and hover. Confirm with your buddy that you’ve both completed a successful descent. Roll to a horizontal trim and begin your dive.

By taking it one bite at a time, every descent will start you off relaxed and ready to enjoy a perfect dive.

As you gain experience with controlling your descent, you will become more efficient and effective. Eventually, you will deflate exactly the correct amount of air from your BCD in one shot, exhale and float down, add air to compensate for the increased negative buoyancy at the correct moment, and continue quickly down.

Once mastered, a controlled descent is more efficient than dumping the all air from your BCD at the beginning of the dive because you do not waste time fighting with your buoyancy on the way down. You arrive at your desired depth neutrally buoyancy and ready to swim off on your adventure.

Be patient. Every diver can properly control his descent with understanding and practice.