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The Gas Laws and Physics of Diving

By The ITS Crew

So now that we’ve got the diving introduction out of the way, let’s jump right into what we feel is a major issue. When it comes to SCUBA Diving, it is important to know your Gas Laws. In Dive Physics, during the Second Phase of BUD/s, they teach you that these laws are very important to a diver. A diver must be aware of the physics affecting him underwater. Without the knowledge of what happens to the human body underwater, circumstances could easily become disastrous.

In this article, we’re going to cover six important diving laws and principles that every diver needs to be aware of. We urge you to study this article and develop a full understanding of how these laws work and to draw your own opinions about why they’re important. The definitions are taken from documents provided in BUD/s training, and we can’t think of anyone better to talk to about the water than the SEALs.

Boyle’s Law

Boyle’s Law states that “given a constant temperature, the volume of a gas varies inversely as the absolute pressure.”

This law can be interpreted any way you like, but we should stress that this law is dependent on the absolute pressure. That means that as the pressure increases, the volume decreases. It would be silly to think that you could decrease the absolute pressure simply by increasing the volume of a gas, so it is fair to say that as pressure decreases, the volume will increase.

To illustrate this law, we take a bottle full of air down to a depth of about 100ft (30m), 3 ATM (atmospheres gauge) or 4 ATA (atmospheres absolute). As the bottle descends, the absolute pressure around the bottle forces the gas (Air) inside the bottle to compress therefore allowing for less volume. This happens because as you go deeper into water, the pressure increases at a rate of one ATM every 33ft (10m). This is the pressure that decreases the volume of gas inside your bottle.

Lets say we take that bottle up into the air 36,000 ft. (10,970m), cruising altitude for a standard commercial jet liner. The air in the atmosphere actually has a lower absolute pressure than we do here on the surface. You may have heard a little rumor about how space is a vacuum, well this is that transitional point from our perfect living conditions. Now, using Boyle’s Law we can conclude that the bottle will actually grow as the absolute pressure around it decreases. The difference here is that the pressure changes are not as drastic in the atmosphere of planet Earth as they are in the water.

Charles’s Law / Gay-Lussac’s Law


Charle’s law states that given a constant pressure, the volume of a gas varies directly as the absolute temperature. This law is sometimes referred to as Gay-Lussac’s Law.

This is probably one of the simpler laws. The same principles are in play most often in your daily life. Most of you know that as metal gets hot or cold, it expands and contracts (more notably than other things like plastic). Well gases do the same thing.

What Charles’s Law says is that if you take a volume of air, and keep it at a constant pressure, you can increase your volume simply by heating it up. You could also effectively reduce the volume by putting it in a deep freeze, or setting it outside if you live up north. So to illustrate this law, we use a SCUBA tank.

When a SCUBA cylinder is filled, the friction of the air entering the cylinder heats things up from the inside out. This makes the surface of the cylinder warm to the touch. Just about all of your shops will overfill cylinders by about 10%, which is the maximum allowed by the DOT. There is a reason for this.

Every cylinder model has a different “working pressure,” which means that they can handle being filled to lets say 3000 psi (206 bar), plus 10% of that pressure. So then we have a cylinder that gets filled to 3300 psi (227 bar). This is not a safe working pressure for the cylinder (theoretically), so why do we fill a cylinder to more than its safe working pressure? They are overfilled because of Charle’s Law. Friction heats the cylinder and that cylinder will eventually cool off, thus, decreasing the volume of air inside. So to counter this effect, dive shops put more air in the cylinder than would be necessary for a typical dive.

Dalton’s Law

TanksDalton’s Law states that the total pressure exerted by a mixture of gases is the sum of the pressures exerted by each of the gases.

This is pretty straight forward. It becomes most useful for anyone qualified to fill a scuba cylinder. Lets say your target working pressure is 3000 psi (206 bar) for a typical SCUBA cylinder fill. Normally, you would take the air from the room you are standing in, compress it down, and pump it into a cylinder. Some people prefer the use of different breathing gases. For simplicity’s sake, we’ll use the breathing gas “NITROX.” There will be an article on the different gases, so don’t panic if you are a little confused by this. The air we breath is an average 21% Oxygen 79% Nitrogen. Nitrox is a gas with an oxygen content of 22%-40%. The benefits of breathing oxygen rich gas are numerous, but we’ll cover that as well at a later date.

So, to reach a higher oxygen content, there are formulas that need to be followed. Many factors go into consideration, such as working pressure, cylinder volume, percent of O2 desired, etc. The basic formula says this: start with Xpsi of 100% O2, and then fill to 3000 psi (206 bar) with air. This gives the desired percentage of oxygen in your nitrox gas. So we have a hypothetical 400 psi (27 bar) of pure oxygen, and we will top off the cylinder with 2600 psi (179 bar) with air, leaving 3000 psi (206 bar) of pressure in the cylinder. 400 + 2600= 3000psi (27+179=206 bar). The total pressure is equal to the sum of the pressure of each part of the mixture.

Henry’s Law

Henry’s Law states that the amount of gas that will dissolve in a liquid is directly proportional to the partial pressure of that gas.

Here is a law that goes much deeper than mixing gases and heating SCUBA cylinders. This is the mother of dive laws in our opinion, but we believe others would agree. This law does not reference any of what has been discussed previously. We could think of ways to get gases to dissolve into fluids, but this law should be presented in the manner it was intended.

If you’ve taken a diving class, you know that nitrogen is your biggest opponent underwater. Nitrogen happens to be a gas, but we’re not so worried about the nitrogen absorbing into the water at higher pressures. Not hardly. You see, while you’re on the surface, nitrogen absorbs into your skin. This is not a real issue while you’re walking around, driving your cars, etc. Take that body down to 100 ft (30 m) of water and you change the circumstances dramatically.

During your time underwater, the pressure around your body forces nitrogen into your blood stream. This can (but not always) lead to a rather pleasant narcotic feeling at depth we refer to as “nitrogen narcosis.” Nitrogen has a narcotic effect on the brain which can be alleviated by ascending to a shallower depth until symptoms pass. The longer you stay, and the deeper you go, the more nitrogen will be forced into your blood stream. If you don’t get narcosis easily, then you will most likely not notice the build up of excess nitrogen in your blood. We know this gas is present because of very extensive research conducted by Navies around the world, more notably the US Navy which releases nearly a thousand page manual on diving to certain jobs.

The nitrogen at depth is not the issue, but rather when you complete your dive and begin your ascent to the surface. As you ascend, the nitrogen gas that has dissolved into your blood stream will begin to form bubbles as the gas expands. If you were to acquire an excess of nitrogen and immediately get out of the water, within about 15 minutes you would begin showing symptoms of DCS (decompression sickness) which can be fatal if not treated. This sickness has been dubbed “the bends,” or “Cassien’s disease” by divers in the past. This build up of nitrogen is removed from the body by ascending to a shallower depth and allowing the nitrogen to pass through your blood stream, and into your lungs. Once in your lungs, the nitrogen returns to its undissolved, gas state and passes from your body as you exhale. The depth and amount of time required for “off-gassing” is directly related to the amount of time, and depth of your dive.

Dive Tables

To prevent death or nerve damage, the US Navy released what is a called a “dive table.” If you are a certified diver, you will have been required to dive from a table to acquire your certification. For those who may be unfamiliar, a standard air diving table basically lets you know how much nitrogen you will build up at any depth for any amount of time. This basic table is a “no-decompression dive table,” which means that at any depth (to 130 ft or 40m) for any time allotted on the table, you will not be required to stop and off-gas nitrogen since you have not acquired any excess in your bloodstream. Tables can be quite confusing if you have never used one before so I will break it down for you.

In a nutshell, you select the depth at which you plan on diving. Always use the deepest point of the dive, even if it’s only for a minute unless you are using a planning wheel which will be covered at a later date. The depths are typically listed in both feet and meters so everyone can safely understand them. Next, move down the row to the corresponding number of minutes you plan on staying. You may notice that the deeper you plan on going, the less time you have available for the actual dive. The numbers represent the maximum amount of dive time allowed per dopplar no-decompression diving limits. Any longer, and you are in a decompression dive, which is not necessarily a dangerous situation, but you must be prepared for decompression stops. Some dive tables have mild decompression diving information on them used generally for emergency situations.

The tables help you understand how much nitrogen has been absorbed into your bloodstream, and what precautions must be taken to prevent serious illness that may prevent you from diving in the future. Going beyond the dopplar limits without proper training could prove to be fatal.

Archimedes Principle

ArchimedesPrincipleArchimedes Principle states that and object wholly or partially immersed in a liquid is buoyed up by the force equal to the weight of the liquid displaced.

This one is very simple to understand. Lets say we fill a cup with water until it’s about to spill over the edge. Then we take a rock, and drop it in that cup, the water would begin to flow over onto the table. Now to find the value referenced by Archimedes Principle, we would simple take the water that spilled, and weigh it. Lets say for the sake of argument the displacement of the water weighs eight grams. That means, according to the principle in discussion, that the rock is being pushed back up with a force of 8 grams.

This law is similar to Newton’s law of relative motion which states: for every action, there is an equal and opposite reaction. This is basically the same theory, we are just talking about objects being immersed in fluid, rather than friction and motion.

We use this law for many different things while in the water. It is mostly used to calculate the specific buoyancy of an object. This will be important for anyone that spends time underwater. For instance, if your Buoyancy Compensation Device does not have the lift capacity to bring up your steel SCUBA cylinders, you will surely be stuck on the bottom. We can set values to plan these things because of Archimedes Principle.

This law doesn’t only apply to the underwater community. Naval Architects will use this principal to calculate many different requirements of a ship’s hull. An aircraft carrier is made mostly out of steel, but because of the way she’s designed, she floats. If you were to place any discrepancy in the hull of a carrier below the water line, the consequences could be dire. The same goes for the laws of density. If you have two pieces of bread, and you throw one in the water like it is, then it will float, but if you crush the other into a ball and toss it, it will surely sink. This is the issue with ships as well, if we use Archimedes Principle along with other amazingly simple laws, we have enormous metal structures floating and moving under their own power.


We’d like to thank Derek Gill for his help in getting these gas laws rounded up into an article for ITS, and appreciate all his help. Please let us know if you have any further questions regarding any of these gas laws, and be sure to throw up a comment. We look forward to bringing you more information in the future regarding diving, and would like to hear what you’re interested in!


ITS Tactical cannot be held responsible for any attempts at any form of diving without first seeking professional training and advice. The following article is not intended as a replacement for proper training and equipment used in any water sport activity. Diving is inherently dangerous, and introduces a unique set of risks not typically present in everyday life. We urge you to seek proper instruction from a qualified, and certified agency before attempting any sport requiring a life support system, namely: SCUBA. It is likewise very important that you contact your health care provider before attempting any training classes to ensure you are in good physical condition. Those with pre-existing medical conditions may be at a higher risk for certain complications that may become present while SCUBA Diving.

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  • Ashley Gill

    Well.. To begin with: This was a very interesting article. Confusing at times, but very well written. 🙂 Kudos! As far as diving goes, I want to go get certified, so This article did help alot with the different gases.. Thanks! 🙂

    • Thanks Ashley,

      Be sure to throw out any concerns or questions you have, we’d be glad to answer them.

      ~ Bryan

  • Jak

    Ahh, reminds me of my chemistry classes. Nice to see some of it is really very practical. Great article guys.

    • Thanks Jak! There’s a lot of of science behind diving, while it’s not always the most interesting thing to read, its very practical. A lot can go wrong underwater if you’re not careful.

      ~ Bryan

  • Mike


  • Christian Nadeau

    Good article. When I got certified they touched briefly on Boyle’s and Henry’s Laws, although the Deep Diving class didn’t really bring them up. Nitrox class focused on Henry’s and Dalton’s Laws to a greater extent (mostly partial pressure of O2) but it wasn’t until I became an Dive Control Specialist (SSI Assistant Instructor, basically) that I really became familiar with the four gas laws mentioned above as well as Archimedes Principal.

    I don’t really want to split hairs, but technically “Nitrox” is a mixture of any percentage of Nitrogen with Oxygen, so air (21% O2, 79% N2) is still “Nitrox”. The term “Nitrox” is simply used to refer to a mixture with a concentration of O2 higher than 21% all the way up to pure O2 (100%). However, as far as recreational divers are concerned, 40% O2 is the highest mix they need to know about and will be certified to use after taking a Nitrox class, unless they plan on taking a Technical Diving class (which would involve Advanced Nitrox and Decompression Procedures, normally using a 50% mix).

    Anyways, still a great article. I’m looking forward to more articles about diving.

    SSI Dive Con, Master Diver, Nitrox. Certified in 2005

    • Chris,

      I also agree with you, and this was mentioned in my PADI Master Diver Pipeline as well. Thanks for the kind words, and you can be we’ll have plenty more on diving!

      Thanks for the comment,

    • Ronald

      To complete that….Nitrox can also mean less the 21% O2….which can/is used on some dives.

  • I agree 100% with you Chris. Keep in mind we are trying to focus on gas laws and not gas mixtures. There is a more in depth look at breathing gases in the works right now that will clarify this a little more. I think the shop attendants would be a little confused if you asked for EAN21 for your deep dive! 🙂

    I highly recommend the advanced nitrox class from PADI if you want to dive deep. I went the SSI route, but I took the PADI Advanced Nitrox, and it has been a great add on, since it covers decompression, picking up, dropping bottles, etc. I don’t feel like its cutting corners, but if you can get several certifications in one class, why not?

    • Christian Nadeau

      I’ve thought about doing that but was leaning more towards SDI/TDI so I could get my solo cert along with Adv. Nitrox and Deco. I think I’ll look into PADI more though.


  • Josh

    A little late to the game on this one – but i do have a relevant question.

    A long time ago in a vacation far far away, i was told that because i have asthma and the conditions of my lungs, i should not/could not handle scuba diving, but regular snorkeling is good to go.

    I guess my questios are these – Is it the depth issue that my lungs cant handle? is it the breathing-out-of-a-mixture issue? was i being lied to? whats up?

    • Josh,

      Frankly there could be some truth to that. Obviously I don’t know your medical history, but I have a buddy that I dive with all the time who has Asthma, and he’s perfectly fine. This is a great place to use one of the gas laws to explain! As Mike said SCIENCE! In this case Henry’s law is what could potentially harm you.

      It all really depends on how you do with exercise and if that increases your symptoms. The danger there (Henry’s Law) is that you could have air trapped in your lungs from an attack that could potentially expand during your ascent. If this happens it could cause the airways in your lungs to rupture (barotrauma). If that occurs, air can get into the bloodstream and form a nasty air bubble that can lodge in the organs or brain.

      This is called an Air Embolism, and unfortunately I’ve had one and getting pressed in a chamber is NOT fun at all. Having this does not make you prone to repeated embolisms, and I’ve been diving many times since, but I was lucky it wasn’t worse.

      The depth would only be a factor in that you could potentially bolt to the surface during an Asthma attack like a Pegasus Missile LOL. Sorry had to throw that in…. The breathing of an “out of mixture” deal wouldn’t seem to me to have any effect, as you breathe Nitrogen and O2 every day.

      At any rate, go see your doc and run it by him. Hopefully this info helps and you can start diving!
      ~ Bryan

    • diverdown


      A good resource for health-related questions is DAN – Diver’s Alert Network. They have doctors on staff specifically trained for diving related questions/issues. They can also suggest a qualified physician in your local community.

      Good luck!

    • Toni Douthwaite

      Hi Josh I am so sorry to hear you got told this wrong information. I am a dive master and have been put through a fitness test to manage students, i to have asthma. My asthma is controlled but have never been told i cant do something because i have it. When you sign up to a discover scuba class, they will ask you to get your doctor to decide if your fit enough if you declare that you have asthma in the first place…. Only GPS and prfessions can give you this information. I am so so sorry to hear someone with the lack of knowledge and experiance has given you this crule comment. As a diver, and can only speak for myself mind, divers NEVER excert themselves, diving is a relaxed sport. This sports NEVER gets me out of breath. xxx Toni Douthwaite. xx I do hope you wont let these types of people stop you doing anything you want to do. xxxx

  • tape an inhaler to your regulator…. (joking) 🙂

  • Bob Tennant

    Bryan hit the nail on the head. Also, breathing air for diving is dehydrated somewhat and that can cause an asthma attack as well. If you want to pursue diving then look for a Hyperbaric Physician. They can run tests to see if you are truly fit to dive. Warning it may be expensive though. I have to that medical every 2 years because I am a commercial diver. Family doctors dont really know much about diving and usually error on the safe side.
    Look on a Commercial Dive school website for doctors in your area.
    Best of luck.

  • Ashley Gill

    Wow. lol everyone’s throwing in their 2 cents on the matter. Hmm.. 🙂 Sooo Josh! I would agree with Gilligan0211 on this one.. just tape an inhaler to your mask. Or get a full faced mask and just tape it to the inside of that mask, then you can just rig a device so you can use it inside your mask. But then you have to think about the depth as they have said before. It might damage your lungs if you try to use it while you’re breathing O2 and Nitrogen. Soo.. as said before, I would talk to a doctor first before you do anything. Just to be on the safe side! 🙂 My 2 cents is in! 😀

  • Josh

    Awesome responses, guys! Ive had asthma all my life, and when i was a child, i would have full blown asthma attacks that would leave me hospitalized, but im 27 now, and havent had an attack since i was in middle school, if that.

    When i exercise or work out, i get winded and out of breath like anyone, but rarely do i resort to going to my inhaler. Ive ran 5K’s, done crossfit workouts and im winded, maybe more than everyone else, but i rarely feel the need to grab my inhaler to recover. Hopefully the fact that although i still have asthma, i can definitely function like a normal person when it comes to sports/exercise, is an indicator that i can in fact, engage in something like scuba diving

    For now, im gonna go consult some physician friends of mine and see what they say, and investigate for any Hyperbaric Physicians around my AO

    And Brian – i “lol’ed” at the “Pegasus Missile” comment.

    • Awesome Josh, you should be good to go on diving, and sound just like my buddy that I dive with. He also does Crossfit and has no problem with going for his inhaler.
      Be sure and let us all know what you find out! I can’t take credit for the Pegasus Missile comment, that came from one of my instructors in the military describing what he didn’t want to see during a water evolution LOL!

      ~ Bryan

    • Hey Josh, I want to throw this out there. If you are not certified, go about finding an agency first! Find someone you are comfortable with and let them know your situation. If you have asthma and you lie about it on the legal forms beforehand, you may run into problems with insurance. There will be a form they need your doc to sign off on to release them of liability. It’s not a big deal, but they are generally very picky about this. Its a full blown medical history questionnaire they will need from you. Anything major that you answer yes to is going to be an issue right off the bat. I would hate for you to have to miss any of your classes. Once the open water is out of the way, the other classes just roll on by. Good luck!

  • Lyne

    Oh god… the stories I can tell about diving. Here are a few pointers that can help make your dive a bit safer and please understand that most of this has happened to either me or an aquaintance of mine.

    The first thing to think about BEFORE hitting the water is: is your body and equipment up to specs? Yup, happened to me: Even if you rent your equipment from a dive shop, it CAN be faulty.

    I had a leaking buoyancy control device and could hardly control my depth ( my instuctor was also pissed till he reallised it was leaking), my regulator froze open and I had to work at it to keep it closed ( tapping and shaking it, still not fun 🙁 ), If your fins are not fitted right, you can lose them. If you are using a dive computer that is linked to you via an air hose and you compass underwater, make sure the hose is long enough; being off balance and in constant rotation can make you dizzy and it also makes for a dissatisfying experience (This is for the people that have longer arms) . Inspect your tank before each dive: there can be small air leaks around your valve that can cut your time underwater and it didn’t make me feel very safe.

    Now, for the Pegasus missile, there is more then one way it can happen, and it did happen to me. Make sure you inspect your weith belt or weight integrated systems. If you hit a sharp object or have a worn weight belt buckle that doesn’t lock properly, you can have a jetpack ride to the surface before you know it. Going spread eagle can maximise your drag, until someone holds your fin to try to keep you under ( hint: it don’t work), then you end up being vertical and going faster. You have a couple of vents on your bcd that helps if you open them, but not much. If you happen to be diving in cold water and have a 7.5 mm wetsuit, you go up faster then if you only have a 0.5 mm for warm water; more insulation equals more buoyancy. The trick to surviving the missile effect is to exhale like the mofo and to not hold your breath at all.

    Now these funny lil mishaps all happened to me during my test dives, when I trusted a dive shop for most of my equipment. The weight suit and weight belt were bought second hand. What I normally suggest is that you buy your equiment and keep it serviced on a yearly basis. Also do a thorough inspection each and every time you dive.

    Us Canadian divers tend to store our tanks over the winter. Remember to change the air in your stored tanks before going diving again, wouldn’t want to be sick from bad air.

    For the body part: Even having a flu can ruin a dive. Know the effect of the meds you are taking in an underwater environment ( also applicable to asthma puffers and every other ailments you would take meds for) . I almost had ear barotrauma after I got air locked in the Eustachian tubes when I equalized ( flu time) . Yes it hurt like hell and it wasn’t easy to fix at the moment. Instead of pinching your nose and exhaling to equalize, pinch and inhale to create a vaccum effect.

    This ia a story that happened to an aquaintance. If you have any dental cavities, no matter how big or small, get them filled by a good dentist and also let him know that you are going to dive and need it to be filled so that no air can get trapped. He dove and got air stuck in his tooth ( he had a cavity) . Comming up, he couldn’t fix the air lock and the trapped air expanded. They pulled him into the boat with a bloody mouth and a heck of alot of pain.

    Diving is really fun and rewarding as long as you don’t cut corners or overshoot your abilities. Best of luck to all from beginners to experts and have a great diving season!

    • Cheese Eater!!

      Hi, I’m a high school student from the Gold Coast and my school has decided for a chemistry assignment that we have to research about the uses on Enriched Air Nitrox in scuba diving. And after reading all the comments I was wondering if you could help me with a question. I’d like to know how Nitros compares to compressed air, and which one in your opinion is better.

      Thank you 🙂

  • Toni Douthwaite

    I didnt even declare i had asthma untill i went for my medical for DM ….. dont let things and illness put you off doing anything dude!!!! Love life, and live it!!!! xxxxx GO SCUBA… XXXXXX

  • gjdr9876

    best shit ever thanks heaps guys

  • ReuvenSaar

    May be you can help with the following question – a stabilized chemical reaction produces Hydrogen in a tank of volume V. The Hydrogen is being released to the atmosphere via a 5 mm hole in the tank’s wall. The conditions in the tank are – stable room temperature; 1.5 Bar pressure, and outside: Room temperature and atmospheric pressure. It takes for a V volume of hydrogen 3 hours to be released from the tank.
    How can I calculate the value of V?

  • This was a very interesting article. Confusing at times, but very well
    written. 🙂 Kudos! As far as diving goes, I want to go get certified, so
    This article did help alot with the different gases. Thanks for sharing with us.

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