The bends scuba diving gas law: Diving gas laws – The Diver Clinic

Diving gas laws – The Diver Clinic

The Gas Laws are particularly relevant to diving. Having a good understanding of the implications can help make you a safer diver. We frequently come across divers who seem not to be aware that the greatest proportional pressure change happens in the final 10 metres. Diving physics and the diving gas laws can seem a little dry during training but it is fair to say that your safety really does depend on you learning the lessons well.  Once you have a sound understanding of what is happening to you and your equipment underwater then you can concentrate on learning and practising the practical skills to keep you safe.

Boyle’s Law

If the temperature is constant and air pressure increases, the density of the air increases also, while the volume decreases and vice versa.

As a diver, Boyles law affects you every time you enter the water. Air spaces in the body are subjected to pressure and volume change, in direct proportion to your depth.

Without doubt, understanding Boyle’s Law is very important in scuba diving.

Note that Boyle’s law also relates to gas density. This part of the law becomes particularly important on deep dives; inhaled air will become denser the deeper one goes. It follows that increased gas density increases gas absorption.

Some applications of Boyle’s Law in action:

  • Running out of air at depth (not taking into consideration the increased gas usage at depth)
  • Rapid ascent, caused by not adjusting buoyancy quickly enough to allow expanding air to escape the BCD/ Dry suit
  • Sinus, ear and mask squeeze on descent – sometimes resulting in bleeding. Sinus or inner ear pain may be experienced on ascent.
  • From 10msw to surface the pressure halves. If you breath hold, the air in your lungs will double in volume causing a ruptured lung.

Charles’ Law

Given a constant volume of gas, the higher the temperature the higher the gas pressure, and vice versa.

Suppose you have a 10lt steel scuba tank holding 200 bars of air this equates to 10 x 200= 2000 litres of available air filled when the air temperature was 20°c . Now you take the tank into water that is 10°c. Before you take your first breath of that tank’s air, Charles’s law predicts that the tank pressure will be lower than 200 bar.

Since the water temperature is less than the air temperature the law predicts that the pressure now will be less than at 20°c. Dive shop owners know about Charles’s law, which is why they often fill tanks in water where the temperature is kept lower than the surrounding temperature.

So what problems can occur with Charles law?

  • probably only one that will significantly affect the diver which is having less gas volume in the cylinder at the lower water temperature than you expect.

Dalton’s Law


The total pressure of any gas mixture is the sum of it’s parts. For example, with air:


GasPressure on surfacePressure at 50msw
Air1 bar absolute6 bar absolute
Air made up of:
Nitrogen, 79%0. 79 bar absolute4.74 bar absolute
Oxygen, 21%0.21 bar absolute1.26 bar absolute


In the case above, you can see that on surface the partial pressure of nitrogen is 0.79 bar absolute and oxygen 0.21 bar absolute, which relates directly to the percentage. At 50msw the percentages stay the same, but the partial pressure increases.

The amount of gas absorbed by the diver at depth is directly proportional to the partial pressure of the gases breathed.

So what problems can occur with Daltons law?

  • Nitrogen narcosis traditionally becomes a problem in the 20 to 30msw range, equivalent to a partial pressure of 2.37 to 3.16 bar absolute breathing air. The feeling of euphoria and well-being can lead to confusion and cause a risk to the diver. Building up dive depths at the start of the season can reduce this effect.
  • As depth increases, more inert nitrogen is absorbed in the body and longer decompression is required to release the diffused nitrogen from the body tissues.
  • A range of gas mixtures, involving various proportions of nitrogen, oxygen and helium, are used to reduce the effects of nitrogen narcosis, increase dive time and reduce decompression. Breathing the right gas at the right depth is essential to avoid problems.
  • Ensuring the partial pressure of oxygen does not exceed 1.2 to 1.6 bar absolute minimises the risk of in-water convulsions caused by breathing too high a partial pressure of oxygen. In chambers we can go much higher for medical treatments and of course there is no risk of drowning.

Henry’s Law

When the ambient pressure is increased with depth, the partial pressure of oxygen and nitrogen in the body rises. There will be more molecules of each gas dissolved in the blood and tissues.

Dissolved gases will diffuse out via the lungs on ascent as ambient pressure decreases, until a new equilibrium is established. This will continue even after surfacing until all the dissolved gas is removed. A controlled ascent rate and the completion of decompression stops goes a long way to avoiding problems.

So what problems can occur with Henry’s law?

  • Insufficient decompression of dissolved gases.
  • Re-absorption of gas due to reverse profiles, or saw tooth profiles.
  • Avoid getting too cold as it effects solubility of gases.

No. 3019: Decompression Sickness

Today, decompression sickness, the bends and Boyle�s Law. The University of Houston presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Decompression Sickness is the modern term for the illnesses caused by a sudden reduction in the pressure around the body, today most commonly in recreational or commercial diving, but also with aviators, astronauts and compressed-air workers. French miners were the first to show the symptoms from breathing pressurized air in a mine in the 1840s, but it became famous in the 1870s during two high profile bridge projects using caissons for their foundations.

What are caissons? They are huge bottomless boxes sunk to a riverbed to build foundations for bridge towers. Water is pumped out of the boxes, and compressed air is pumped in to keep the water out. Workers dig inside the boxes, removing sediment down to bedrock, then fill the caissons with concrete to form the bridge tower foundations.

A diagram of an open caisson, devised by Jules Triger, dated 1846.
Proc�d� Triger” by Am�d�e Burat – G�ologie appliqu�e ou Trait� de la recherche et de l’exploitation des Mines. Licensed under Public Domain via Wikimedia Commons.

The Brooklyn Bridge and the Eads Bridge in St Louis used caisson foundations. In both projects, caisson workers developed a mysterious debilitating disease, with pain in the joints, a bent forward posture, blindness and sometimes death.

Scientists discovered that tiny bubbles of nitrogen caused the �caisson disease� or �the bends.� Bubbles come out of solution in workers� blood as they decompress from the pressurized air in the caisson to the surface. You can see this for yourself the next time you open a bottle of soda. In a closed bottle, the soda is under pressure and the dissolved gas bubbles are too tiny to see. Opening the bottle reduces the pressure, so the gas bubbles get bigger and foam out.

To understand how the bubbles form, it helps to know Boyle�s Law. Sir Robert Boyle was a 17th century Irish scientist — a pioneer of modern chemistry and the scientific method. Boyle did a famous experiment in 1662. He used mercury in a sealed U-tube to discover the effect of an increase in pressure on a set volume of air. Boyle�s Law states that a gas volume is inversely proportional to pressure; in other words, at twice the pressure, a gas will have half the volume.1 So, as pressure is reduced by opening a soda bottle, coming up from an underwater dive — or returning to the surface from a pressurized caisson — tiny bubbles in the soda, or bloodstream, get bigger. At half the pressure, twice the volume.

The principal features of a caisson are the workspace, pressurized by an external air supply, and the access tube with an airlock. Photo credit: Yk Times/ Wikipedia

Today, the most common way people breathe compressed air is in recreational or commercial diving. Decompression sickness is prevented by limiting time under pressure and cured by time in a recompression chamber. In recreational diving, limiting underwater time and depth minimizes nitrogen saturation. In commercial diving, divers are monitored carefully underwater, then enter a recompression chamber when they come to the surface. Commercial divers continue to breathe compressed air in the sealed chamber as the air pressure decreases slowly over several hours.

Doctors and dive physiologists are still trying to figure out the effects of dissolved gases in the body. They don�t fully understand the disease made famous by the caissons of the Brooklyn Bridge.

I�m Janeen Judah, for the University of Houston, and interested in the way inventive minds work.

Brooklyn Bridge. Photo credit: Wikipedia/

At constant temperature.


On Boyle’s Law: I have quoted the law assuming the constant temperature of the human body. Boyles’ Law is a special case of the ideal gas law, PV = RT and is true only when temperature is constant. P1V1 = P2V2.

Robert Boyle, Boyle’s Law and his famous U-tube (not YouTube) experiment are widely discussed in chemistry and physics textbooks. 

As detailed article on caisson disease during the construction of the Brooklyn and Eads bridges can be found here.

Instruction on decompression sickness is an important part of certification courses for recreational divers. For detailed information, see the PADI (Professional Association of Dive Instructors) Encyclopedia of Recreational Diving, 2008. Pp. 5-41 to 5-61.

DAN (Divers Alert Network) is the premier resource for both divers and hyperbaric physicians. DAN’s article on decompression illness can be found here.

For a video of a Navy decompression chamber in action, go here.

Engines episode 2691, The Body at Vacuum, also discussed what happens when ambient pressure is rapidly reduced, but in space.


The episode originally aired on August 21, 2015.

Why is scuba diving (diving) good for your health?

If you’re looking for an adrenaline rush, there’s no better way than diving. Many people spend years scuba diving without knowing the health benefits. Let’s look at some facts about diving and its impact on a person’s psychological and physical health.

      • Controlled Breathing An important skill that you learn while scuba diving is the ability to maintain a constant breathing rate, which in turn leads to a stabilization of the heart rate. This is due to the limited amount of air underwater. You need to breathe slowly and evenly so that the air does not run out quickly, and do not hold your breath so as not to get lung barotrauma. The skill of “smooth” breathing and knowing what to do in all emergency situations are the opportunities that you will enrich your experience during the Diver NDL course in our club.
      • Physical fitness. One of the benefits of scuba diving is to improve your overall fitness level. When you dive deep into the water, muscles that are not used in normal life have to work harder. This includes strengthening the back muscles. It improves your strength, flexibility and posture. Poor posture provokes various health problems, especially with the spine. Avoid long hours of sitting in front of a computer monitor and devote this time to diving to revitalize your muscles and return them to a state of healthy tone.
      • Low blood pressure

        When you learn to breathe under water, your heart rate will become more stable. The coolness of the water slows down the blood flow in the peripheral vessels due to their service – blood pressure rises slightly in the first few moments of diving. This is facilitated by the release of adrenaline adrenaline, caused by excitement, interest and an unusual network of activities. As you continue to exercise, you learn to control your heart rate to stay calm, which lowers your blood pressure. The heart rate will automatically decrease underwater after a while, resulting in a drop in blood pressure. This is another reason why scuba diving is good for your health.
      • Therapeutic effects of water

        Water makes your body better both externally and internally. Full immersion in water for the body is, first of all, an imitation of being in the womb. A sense of security balances the psyche and makes it more stable. It evokes a feeling of well-being and happiness.
        Sea water, due to its high salt content, contributes to rapid dehydration of the body. To cope with dehydration, it is necessary to drink enough water to replenish the liquid supply of cells, which is also useful for the functioning of the body as a whole.
      • Interaction with marine life

        Under the water you can meet different types of marine animals that you have never seen before. How cool would it be if you ever accidentally discovered a new species of fish or mammal? Scientists also believe that bright colors, such as the colors on and around reefs, promote feelings of happiness and instantly change mood. Depressive mood will go away under the onslaught of this beauty.
      • Exposure to sunlight

        Your body needs extra sunlight, especially those who live in St. Petersburg, for example. Sunlight promotes the absorption of vitamin D, which strengthens the skeletal system. You will feel cheerful and the likelihood of bone diseases will decrease dramatically. The production of endorphins is also stimulated in the presence of sunlight. When you go scuba diving, you will most likely spend several hours or days in direct sunlight. The sun is very important for people of all ages.
      • Social benefits

        When you go scuba diving, you usually have a partner to ride with you. If you are a beginner and not familiar with this rule, or you don’t have a buddy (partner), you have nothing to worry about. Our club will find for you a diver who has similar interests and goals to you. Diving, as a tool of socialization, will allow you to meet new people, make friends, fall in love and discover new things in people and in yourself.

      • An excellent stress reliever With the constant stressful informational pressure of the surrounding reality, there comes a moment when you are not able to carry any further load – you urgently need a break. Get rid of stress by going scuba diving! You will meet different people and be able to travel with them through the most beautiful ocean and seas of our planet. This will give you the opportunity to relieve stress and escape from the hustle and bustle.

        Under water, your heartbeat and breathing slows down, while you are in silence and “solitude” in the water column, all this gives an effect similar to meditation. That allows you to come in complete peace and harmony with yourself while diving.

        Scuba diving provides an excellent platform for stress relief and socializing at the same time, which is rare.

        Before diving, it is important to take a training course to make your diving experience as comfortable and safe as possible. As soon as you start diving, you will feel that you are becoming a healthier and happier person.

      5 places to go scuba diving in Russia

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      Russia, the largest country in the world, borders not only on many European and Asian countries, but also on the Great Pacific and Arctic Oceans. That’s why you don’t have to go far to try diving and discover a new world with large mysterious caves, sunken ships, many whales and seals.

      Black Sea

      If you want to experience an amazing diving experience but are not yet ready to dive into cold water, the Black Sea is one of the best scuba diving destinations in Russia. The great thing is that even dolphins can be found there. Although at a depth of 50 to 200 meters in the Black Sea there are not so many aquatic organisms, but here you can find sunken ships and their cargo. To admire these objects, you will need an underwater diving lantern, since the light hardly reaches such a depth.

      One amazing place you should definitely visit is the village of Olenevka in Crimea, which houses an underwater museum of busts of Soviet leaders.

      Lake Baikal

      This is one of the most valuable objects of our country. Lake Baikal is not only the largest freshwater lake in the world, but also the deepest lake with a maximum depth of 1642 meters. Lake Baikal contains 20 percent of all fresh water on the planet. There are about 50 dive sites here.

      The crescent-shaped Lake Baikal is adorned with rich flora and fauna along with good visibility, making this already amazing snorkeling experience even more exciting. This place can be visited all year round, but you must remember that the water here is very cold, sometimes even dropping to 3-4°C in June and 7-10°C in August.

      White Sea

      This incredible sea is one of four seas named after colors, including Red, Black and Yellow. Rich in aquatic life, the sea is home to over 700 species of invertebrates, about 60 species of fish and 5 species of marine mammals. Temperatures here often drop very low, so this is another icy diving site.

      If you’re lucky, you might even encounter white whales, also known as sea canaries. The White Sea with an amazing variety of fauna and excellent visibility is a great place for diving.


      The Kamchatka Peninsula is one of the few places that have not been affected by traces of human activity. The untouched and beautiful peninsula with the Pacific Ocean on the east coast and the Sea of ​​Okhotsk on the west coast is one of the ideal places for diving. About 350 species of fish, seals, sea lions, sea otters, walruses, killer whales and whales live here. Here you can enjoy diving without going too deep. But if you want to study underwater life in more detail, then you need to buy an underwater flashlight and then you will see everything that excites the mind.