Archimedes Principle

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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.

Underweighting can be just as dangerous as overweighting.

scuba diver in lotus position

Equipment and the Environment Will Both Affect Your Buoyancy

Wetsuits, tanks, and even accessories and tools will affect your buoyancy. Whenever you change one of these items, it is necessary to complete a buoyancy check to determine the effect of the new item on your buoyancy. The salinity of the water will also affect a divers buoyancy. The obvious example is buoyancy in fresh water vs salt water, but keep in mind that the salinity of the ocean may also vary in different regions of the world, and you may need slightly more or less weight depending upon your dive location.

Conducting a weight test before a dive can make the difference between a miserable dive and an enjoyable one.


Conduct a Buoyancy Check Before DivingScuba 2

Test your buoyancy in a new location or whenever diving with a new piece of gear. Most of the time, diver’s have a limited number of dives on vacation, and it is worth the effort to make every dive comfortable and safe.

In fact, most dive operators will be thrilled if you ask to wade into the ocean or hop of the pier before your first dive to double check your weighting.


The general rule of thumb for proper weighting is that with all of his gear in place, with a nearly empty tank, a diver who completely empties his BCD and holds a normal breath should float at eye level. When he exhales he should sink.


Remember!! As the tank empties, it will become positively buoyant.

If you can empty your BCD, hold a normal breath, and float at eye level at the beginning of the dive, you will not have enough weight to comfortably maintain neutral buoyancy at the end of the dive.

The problem here is that most dive shops are not in the practice of providing nearly empty tanks for buoyancy tests. There are two solutions to this:


  1. Conduct the buoyancy test with a full tank as outlined above, and then add the appropriate amount of weight to offset the buoyancy swing of your tank as it empties.
  2. Conduct the buoyancy test as outlined above with a full tank, but instead of checking that you float at eye level, check to see that you sink slowly while holding a normal breath.


Double Check Your Buoyancy at the End of a Dive

Once you have successfully completed a dive with enough weight to keep you comfortably below water the entire time, it is a good idea to double check your weighting at the end of the dive.

To do this, purge your regulator gently until you are down to about 500 psi or 30 bar. Then, on the surface, conduct a buoyancy check before exiting the water. Do you float at eye level and sink when you exhale?Scuba 3

Do you have to swim to get down? Float like a balloon on the surface? Add a little weight on your next dive.

Do you still sink while holding your breath? Remove a few pounds and try again on the next dive.


Adding a Small Amount of Air to Your BCD During Descent Is Good

Many divers seem pleased that they can descend and arrive at the planned depth without adding air to the BCD.

Again, this is not a desirable situation. Because most tanks become more positively buoyant throughout a dive, divers who do not need to add air to their BCDs during descent to establish neutral buoyancy are likely to be underweighted.


I Don’t Need to Add Air During Descent. I Am Fine at the End of the Dive!

These are the people who have exceptionally low air consumption rates, and surface with at least half of their air remaining. Yes, they can be perfectly neutral during their safety stops, and they don’t go flying to the surface like awkward buoys at the end of the dive. Their tanks have not become positively buoyant because they haven’t breathed enough gas to cause a buoyancy swing.


The problem with this habit is that it does not prepare divers for an emergency situation, when they are low on gas because they over-breathed a tank due to a stressful situation, surface with less gas than normal due to an unforeseen occurrence, or are forced to share air with another diver.

In these situations, such a diver will have trouble staying down, and of course, these are the situations when having enough weight can make a big difference between an annoying situation and an uncontrolled ascent to the surface.

Plan for the worst case scenario: a low tank and increased breathing rates due to stress.


Diving with less weight does not make you a better diver.

Diving with the correct amount of weight does.

Slight overweighting is correctable: just add a little air to the BCD.

To be safe on dives, proper weighting is key.

Take the time to get your weighting right and you can avoid many potentially dangerous situations and dive more comfortably.

Buoyancy Basics – Seven Ideas for Adjusting Trim Without Adding Weight


Before making any adjustments to his gear, a diver must first check his trim. He should enter the water with all of his diver gear and attempt to hover in a horizontal position without moving his fins or arms. He can then note whether he has a tendency to hover head or feet up, or to roll to one side.

1. Move Weights Around

Changing the position of a diver’s weights may help to adjust his trim. Many buoyancy compensators (BCs) have trim weight pockets just below the diver’s shoulders. A diver who tends to float head-up may use the pockets to redistribute his weights, placing a few pounds in each pocket. For example, a diver who uses ten pounds of weight may want to carry six pounds on his weight belt and four pounds (two on each shoulder) in his trim weight pockets. If a diver’s BC does not have trim weight pockets, some divers will place a weight on the BC’s tank band for a similar effect.

Redistributing weights is not always possible, as in the case of a diver who uses very little or no weight, or a diver who uses a BC without trim weight pockets. Some divers simply find that redistributing weight does not correct their trim. Fortunately, it is possible to manipulate other pieces of dive gear to perfect a diver’s trim.

2. Tank Position

A diver’s tank position affects his trim. A tank can be lowered or raised in relation to a diver’s BC by changing the position of the BC strap on the tank. A diver who attaches his BC strap near the top of his tank will find that the tank sits low on his body. This may help to correct for a foot-up position by shifting the weight of the tank towards the diver’s feet. Attaching the BC to a lower part to the tank will have the opposite effect.

3. Reposition the Buoyancy Cell

Some equipment configurations, such as a backplate/ wing system, allow a diver to move his buoyancy cell (the part that inflates and deflates) upwards or downwards in relation to his harness. A diver can move the buoyancy cell towards his head to compensate for a foot-up position, or towards his lower body to correct a head-up position.

4. Change Your Neoprene

The thickness and distribution of a diver’s neoprene garments can have a huge effect on his trim. Thick, full-length wetsuits, especially wetsuits that have 5 mm or 7 mm legs, have a tendency to cause a diver to float feet-up. Switching to a wetsuit that has a 7 mm torso and thinner legs can remedy this situation. Similarly, thick wetsuit booties can cause a diver’s feet to float. Simply switching to thinner booties may solve a foot-up position. Finally, warm-water divers who find themselves floating foot-up may consider switching to a short wetsuit, as long as the suit still provides adequate thermal protection.

Adding neoprene layers to the torso may also have a slight effect on a diver’s trim. Thick vests and hoods add buoyancy to a diver’s upper body, and may be useful in compensating for a foot-up position.

5. Change Your Fins

Changing your fins may sound like a drastic step, but switching your fins can often be a helpful method of correcting trim. Different brands and styles of fins have astonishingly different buoyancy characteristics. For instance, SCUBAPRO Jet Fins are some on of the most negatively buoyant fins on the market. Other fins may be neutral or even positively buoyant. Heavier fins can compensate for a foot-up position, while lighter fins can compensate for a head-up position.

6. Change Your Regulator First Stages

Regulator first stages can be very heavy or relatively light. A heavy regulator first stage can help to fix a head-up position, and a lighter regulator first stage can help to compensate for a foot up-position. Of course, regulators should be selected first and foremost for their breathing characteristics, but given the choice between several similar first stages, a diver may want to make his final decision based on weight. The weight of the first stages is of particular interest to technical divers, who use at least two first stages on every dive.

7. Consider Your Accessories

Finally, a diver may want to take a look at his accessories and consider their effect on his trim. A diver who carries a heavy dive light may have a tendency to list to one side — in this case he may want to place other accessories (or counterweight in a worst-case scenario) on the side opposite side of his light. Divers who carry huge, heavy reels on their rear d-rings may notice that they float head-up. Switching to a lighter reel may fix this problem without any other adjustments.

In most cases, a diver’s trim can be adjusted without adding weight. Before making any adjustments, a diver should attempt to hover in horizontal trim with all of his dive gear in place (even accessories) to diagnose any trim problems. He can then begin repositioning or changing dive gear to improve his trim. It is advisable to change or reposition only one piece of gear at a time, and to get into the water after each adjustment to evaluate its effect. It is difficult to determine the result of an individual alteration when several adjustments are made at once.

 Take your time and make methodical adjustments to your equipment, one change at a time, until you are happy with your trim.

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


Aluminium vs. Steel Scuba Cylinders


What is the difference between steel and aluminium scuba tanks?  Divers never give much thought to what their air cylinder is made of, but it can affect your dive.

Cylinders are made out of two types of material: steel and aluminium

Aluminium is softer than steel.   Aluminium tanks must have thicker walls than steel tanks to hold air at a comparable pressure.   Aluminium is softer than steel, it scratches and dents more easily.   An aluminium cylinder is larger and heavier than a similar capacity steel cylinder.  They also don’t handle overfilling nearly as well.  Aluminium tanks are far more tolerant of corrosion from salt water.

Steel tanks may rust in the presence of moisture.   They are more likely to be damaged by improper fills containing moisture than aluminium tanks, and may require periodic tumbling (a process which removes oxidation from the inside of the tank).

Buoyancy Characteristics of Steel and Aluminium Tanks

The type of cylinder you use only has one major effect on your diving: your buoyancy.

As a diver empties his tank by breathing from it, the tank becomes lighter.   Aluminium has a particularly annoying characteristic; it becomes positively buoyant as they are emptied while steel tanks only become less negatively buoyant as they are emptied.   Whether a diver dives with a steel or an aluminium tank, he must compensate for the increased buoyancy of his tanks near the end of a dive.  A full aluminium cylinder will sink while an empty will float. This makes it harder to pin down a perfect weighting for the entire length of a dive.

This is why during a proper buoyancy check it is recommended to use a near-empty cylinder, or add weight to compensate.  The usual recommendation is to add about 2 kg to your base weighting to compensate for an aluminium cylinder.

Durability of Steel vs Aluminium Tanks:

When properly cared for, steel tanks generally last longer.   Steel is a harder metal  than aluminium, and is less likely to pit or dent.   Steel may rust, but with proper care most rust can be avoided. Any rust discovered during a visual inspection can be removed by tumbling the tank.

Aluminium tanks may develop cracks or fractures in the tank neck threads where the valve screws into the tank. These cracks can cause  gas loss, and a tank with a cracked thread is unusable. The tank neck threads of aluminium tanks are inspected during the standard visual inspection, and this problem is usually caught before it becomes dangerous

Steel tanks weigh less, are smaller and more durable, and require that a diver use less weight than standard aluminium tanks. However, aluminium  tanks are so much cheaper than steel tanks that they have rapidly become the industry standard.