http://en.wikipedia.org/wiki/Category:Breathing_gases

With all due respect, I've already been there and my question still isn't answered.

Mike

If you breathe 'normal' air at 'normal atmospheric pressure'. you would be fine.

Most underwater vehicles become pressurised at depth I believe....

At pressure it is lethal to breathe the wrong mixture.

Yes, but if the vessel is designed to withstand the pressure, why would you need to pressurize it?

The pressures are rather high - but you are correct that, if the vessels have thick enough walls and there are no leaks, a standard atmosphere would be quite satisfactory.

There are several possible explanations, some already given - I'm not in a position to discriminate, here are a few (almost certainly not comprehensive, and some being repeats):
that the mixture used in the original Benthoscopes has been misreported (it was clearly necessary in Cousteau's aqualungs)
that Nitrogen affects the rate of absorption of the CO2 in the soda-lime system that was in use (unlikely, I think)
that the internal pressure was made as high as practical to provide tests for leakage or reduce the wall thickness
that the users didn't have complete confidence in the durability of the seals at the point they started using them - so they used a helium mix in case there were significant (but slow) leaks, so they would have time to reach the surface before physiological problems could arise
that it became very hot in the Benthoscope - increased pressure would increase convection cooling, and high thermal conductivity of He would increase conductive transfer
that Cousteau used the Benthoscope as a starting-point for aqualung exploration (if so, there's a question why he used a sealable vessel - unless he only pressurised immediately before exiting - which could halve the time for which he was exposed to high pressures).

http://en.wikipedia.org/wiki/Decompression_sickness

I am really surprised no one has mentioned this.

Elementary, Watson! However, this is only if you go outside the vehicle at depth and then decrease pressure (decompress) too quickly. If you stay inside and the atmosphere is regulated to STP - I don't think you would have a problem.

Still wondering why they use heliox...

Thanks for the input.

Mike

From what I understand you still need to regulate the pressure inside to ensure the difference between the inside and outside pressure is not too large (or else you may compromise the structural integrity of the craft), hence compression and decompression issues.

I believe it's because at the pressures present at such depths, a nitrogen-oxygen mixture becomes toxic, but oxygen heavily diluted with helium is relatively harmless.

As a PADI Divemaster, I might be able to clear things up for you...the answers are actually embedded in previous replies to your post, so I'll just try to pull them together for you.

If you keep your body at approx 1 atmosphere of pressure (approx 14psi), then you just need to breath normal air. As you descend in water, the pressure increases by approx 1 atmosphere per 10 metres (33 feet). So submerging to 100 feet you would have a pressure on your body (or the exterior of the submersible) of 4 atmospheres.

In a submersible, if the interior is to be kept at 1 atmosphere (allowing the occupants to breath normal air as they would on dry land, then it needs to be constructed strong enough to withstand the much greater pressure outside from crushing the poor souls inside :-) - this is normally how a military submarine is built.

There is a different type of submersible often used by commercial divers where the internal air pressure is increased to match the external water pressure, this allows divers to enter & exit at will (works like turning a glass upside down & putting it in water - as you push the glass further under, the water level inside the glass begins to rise as the balance between the water pressure increases.

The use of different gas mixes applies to both scuba divers and the second type of submersible and counters two separate issues faced by divers.

The first is associated with a build up of nitrogen in the body. As a diver goes deeper, the pressure of the air he/she breathes is increased to match the water pressure (by the scuba regulator). Oxygen is used by the body, but the 79% nitrogen is actually absorbed into the body. The longer & deeper the diver dives, the more it builds up until there is pressure equilibrium with the external supply.

This is not an issue in itself, except for when the diver wants to surface. It takes time for the nitrogen to be released safely from the body. If the diver comes up too quick (sudden drop in external pressure), then the nitrogen comes out too fast and can form bubbles in the blood & other areas of the body - this is "the bends" or one form of decompression sickness. Worst case, the bubbles enter the blood stream & can cause brain damage.

The way to avoid this is to not stay down long enough for this to become a problem or to come back up very slowly - assuming you have enough air. Using a Nitrox or "enriched air" mix, where the % of nitrogen is reduced (by having a higher % of oxygen) increases the time it takes to absorb too much nitrogen so you can stay down longer.

The second problem however is that even oxygen is "toxic" at a high enough pressure. When the partial pressure of the oxygen gets to between 1.40 & 1.6 of normal atmospheric partial pressure, your central nervous system can go into spasm & you can loose control of many bodily functions. Not necessarily fatal in itself, but when you are 100+ ft under water it can be fatal.

So at depth both nitrogen and oxygen are a problem. This is where Trimix (and other exotic combinations) comes in. Often helium is mixed in to reduce both nitrogen & oxygen to allow deeper/longer dives. Trimix is not normally breathable at the surface, but is fine at depth where the delivered pressures are high enough to deliver sufficient oxygen

Nitrox or normal air is used until the diver gets to depth, then they switch over to the trimix - reverse the process on the way up.

Sorry for the length, but I hope this explains it.

Well said, from a cave diver who even uses travel gas to get where I want to be (breathing hard with slight more O2 to get down to depth), then breath off my back gas when I explore (longer bottom time at depth, not so much O2 and you will get tired if you do real work with little O2, but at depth if we have too much O2, we are dead).

Hey thanks G!

I was suspecting that thing about the difference in being able to go outside (need special mixture) or stay inside (standard atmosphere is fine).

The solubilities of gases in human body fluids changing depending on the ambient pressure I know about - you gave a very good explanation of that!

I appreciate your reply - Thanks

By the way, what does PADI stand for?

Mike

Well explained. Hardly matter for the length. Well done.

In WWII the procedure in diving a diesel electric sub was to apply a low pressure blower system for "pressure in the boat" before submerging to assure that all openings to sea pressure were shut and secured. This was a "ten pound" pressure.

When the submarine USS Squalus went down in 1939 or 1938 off the Maine/New Hamshire coast, in 240 feet, the hardhat rescue divers used a standard air mix and were subjected to "raptures of the deep" attributed to nitrous oxide (laughing gas) being formed by the pressure at depth by forcing the normal mixture of oxygen and nitrogen to form a molecule instead of a separate mixture.

We never experienced any problems with the 10 pound blower pressure except to occasionally have to pinch the nose and blow to "pop" the ears.

When I took a scuba diving course in 1974, "raptures of the deep" were mentioned with the sometimes fatal reaction of taking the mouth piece out to offer to the nearest fish.

I never questioned this comment because the course was being taught by NY State troopers on the state dive team, and if you can't trust your local trooper, who can you trust??????????????

We took a controlled ascent from 100 feet using a momsen lung at the escape tower in New London and the presumption was that it was a standard air mix that was used to inflate the lung at the 100 feet depth.

The nuclear subs go so much deeper than we ever thought of going that the pressure hulls are extremely strong and the inside pressure approximates standard atmospheric. The Russians used titanium.

So much for ancient history, but I wonder if the nitrous oxide theory is correct.

i really don't know if was nitros oxide , but i think i experienced rapture a couple times....it wasn't actually as if things were funny , as much it was a feeling of well........

rapture

nitrogen narcosis

ni·tro·gen nar·co·sis

noun Definition: confusion from nitrogen in blood: light-headedness, confusion, or exhilaration caused by increased nitrogen in the blood. This occurs in deep-sea divers exposed to pressures several times that of the atmosphere.

in my cases, it sort of made me feel weird, and i had to keep telling myself that everything was fine, and try to concentrate on something.......

The correct term is Nitrogen Narcosis (< See Link / Wiki).

Dangers are taught to all Navy divers, PADI / NAUI scuba divers and Commercial Divers alike. Search for, and you will find (pdf for download) the Navy Diving Manual.

With astonishing repetition, people STILL succomb to the 'rapture'. Guess they didn't want to wait.......

!Edit!...upon first viewing ("Guest's" post ONLY) I wasn't seeing these other posts...(!?) Saw that one following a "search"... and replied to IT. Returning, here, after "submitting", I trust this response isn't TOO redundant...

PADI stands for "Professional Association of Diving Instructors".

Glad I could help.

Cheers.

we are dreaming about a trip down under............but figure it'll be years until we have the time and money to do it right......we'll be diving the second largest barrier reef off of honduras this september.............cheers

Regarding PADI

PADI is one of the largest scuba diving training and certification groups internationally (if not the largest). http://en.wikipedia.org/wiki/List_of_diver_training_organizations

To go scuba diving and to get tanks filled dive shops normally require a certification by PADI or another group so they are comfortable with knowing you are educated enough (hopefully) to not get yourself killed (legal liability may also be a factor). PADI offers training sessions for anyone interested in learning to scuba dive and they are taught by a certified instructor. Growsey is certified to a very high level due to his classroom and "in the water" training and experience. His next step if he chooses is to become a certified instructor. You can learn more by visiting www.padi.com.

Best Regards

As has already been mentioned, nitrogen at high pressure is toxic, and desorbs slowly from the human body. Rapid desorption of nitrogen causes the dreaded bubbles and the bends. It is actually boiling of the blood at body temperature. It also explains why decompression times are rather long. In air, nitrogen is only the carrier gas for the oxygen we require to sustain life, so if we replace the nitrogen in our breathing gas with helium, we end up with heliox that feeds us the required oxygen but the helium leaves the body more quickly than the nitrogen. That gives us much shorter decompression times and means we can spend more of our time at depth.

Hi All, some very interesting answers but not answering the original question I think. I don't know why the original deep water adventurers opted to use a mixed gas but I would venture the thought that it was more to do with the knowledge level of the day rather than actual need. Both the dives you mentioned were in the 50s and 60s and it was during that period that the effects of deep diving were only really being investigated. I would say that they were trying to be as safe as possible given what they knew.

Most current deep sea vehicles are built to operate at 1 bar internal pressure. for example Alvin is max rated to 5400m and has an operating depth of 4500m. an average dive is about 10hrs with 5.6hrs of 'travel' time and 4.4hrs bottom time.

This does not offer the time needed to decompress no matter what gas is used even at a modest 2 bar internal pressure.

I learnt on my tri-mix course that helium is really there to help reduce the narcosis effect (and that starts around 20m+ (3 bar) with helium usage starting generally around the 50m mark (6 bar)) but using helium carries a longer decompression penalty, hence the use of high percentage oxygen at the last stages of the dive.

I have never heard of the crew of a submersible having to go through this form of surfacing regime so I think that is a further indication that they are using air at 1 bar.

The only way to get a definitive answer would be to contact the people designing the deep sea vehicles.

you nailed it

One last post: Simply put, if you have an open hatch where you exit the vehicle, the pressure inside must equal outside plus 1-2 bars to keep the water from entering. The other reason is if you build a machine that can take the water pressure structurally, then 1 bar inside is fine at any depth. Military submarines. Failing that mechanical design, one must add pressure inside to equal outside so you don't crush the vehicle, so you calculate the thickness based on mechanical limits and viewing port limits, then when you exceed that you can only solve it with internal pressure. To just use structure only with 1 bar inside would result in a viewing port 3 meters thick for 300 mm diameter 'window' and hull thickness of 2 meters and special crush proof shapes limiting the interior design and costs for any type of super depth. So, for $7 million you get 1 bar, 3000 m vessel that you can't move inside, or $2 million you get same depth, can move around a bit, but need 12 hours decompression. Your choice. On the old movies of explorers, you saw them sitting around a open hatch (diving bell) talking like cartoon characters due to helium.

Hi Petro,

If you wanted to maintain a 1-bar pressure in the cabin, wouldn't the thickness of the hull and viewports depend on how deep you wanted to go? The 300mm dia "window" and the hull wouldn't necessarily need to be 3 and 2 m thick respectively. You would require these thickness only if you wanted to be able to go to a certain depth. The best geometry would be spherical. The hull thickness also depends on the size od the vessel. Where are you getting the $7,000,000 figure, what is the size of the vessel and what depth can it achieve (I'm assuming the 2m hull and 3m "window" thicknesses)?

Mike

Depth is everything, of course. Look at my 3000 m number. 10,000 feet depth. the $7m came out of my shorts. I'm part of a design team for a major oil company sub for sea equipment where we will put a 2 mega watt multiphase pump on the ocean floor including its electric motor. In 500 m the hull thickness is unbelievable (can't discuss further). I have a feel for $$ versus thickness. The mini-subs that go deep are small and you can't move. This is to keep the 'hoop strength' hull design compact and crush proof. Adding surface area inside tremendously affects the crush-ability factor exponentially. For a feel on pressure and thickness, go to a local aquarium. I'm in Melbourne this week and just went to their aquarium and for their main viewing tank 5 meters tall the Plexiglas is 1/5 meter thick. They have a chunk of it there to look at. I also know Dr. Silvia Earle a little bit (Google her) and she owns a ROV company in Oakland California. Email her.

Hey Petro,

Thanks for the info. I am a ChE and have worked with pressurized reactors and can appreciate the pressure/vessel thicknaess relationship (though it's been a long time!). It sounds like you have a really interesting job! I reallly have nothing more than a curious interest in this subject and probably won't contact Dr. Earl (unless I suddenly find the need for a submersible!). What prompted my original post was that I was watching the TV series "Surface". The episode where Rich and Dr. Daughtery made a submersible out of an old tank (looked like it might have been 1cm thick (at the most!) and was about 2m in diameter). They went down ~ 1300m in that thing and had only had bottled air to breathe! Can you say pancake?

Anyway, I appreciate all your info.

Cheers, mate!

Mike

Hmmm. 1 cm. I'd would have guessed 6 cm minimum with 1.0 SF. But, I work for a major oil company and we design for 2.5 safety factor and then when nobody is looking just double that again for good measure! Steel is cheap by comparison to the things inside the can plus in my world, lost production of 6 months at $7-8 M a day from one well.