Deep diving: What prevents divers from going deep?

It is natural for humans to be curious about their limits: What's the highest mountain there is to climb? How old can we get? How strong is the strongest human out here? and other questions that uncover an unsatiable desire to push through our limits.

This article deals with the limits of diving, or more accurately: what limits our people from going deeper?

First and foremost, I want to clarify what I mean by diving and make a distinction from other types of submersions as well as explain why it is important to make a distinction: When I talk about diving in this article, I talk about Open-Circuit scuba diving, as opposed to:

• Freediving, which is the practice of holding your breath underwater; it obviously has its own limitations, but not the limitations I'll talk about in this article
• CCR diving or Closed Circuit Rebreather diving which uses a rebreather to scrub out the carbon dioxide exhaled and reuse the unused oxygen of each breath. This diving method also has its own limits, which will not be talked about in this article. Open Circuit Scuba Diving denotes diving using tanks that get depleted as the diver breathes its content. This being said, most limits affecting open-circuit divers also affect rebreather divers to an extent (I'll detail everything below)
• diving in a submersible, where the environment remains pressurized to 1 atmosphere. Most diving limitations occur from the increasing pressure affecting our body either directly or indirectly through the physiological effects of breathing hyperbaric gas. It makes sense that we can reach greater depth using a submersible, but that is not what interests us in this article.

Now that that's all cleared up, let's get to the heart of the matter!

As this article is being written, the current depth record for an Open Circuit dive belongs to Ahmed Gabr: In 2014, He reached a depth of 332m in Dahab, Egypt. What are the factors one would have to go through to even think about reaching such a depth? Are you thinking of trying to beat that record? Here is all the problems you would have to face:

Increased Air consumption

Everyone has their own Surface Air Consumption Rate, or SAC Rate. It will differ depending on each person and can be affected by factors such as weight, fitness level, gender (women tend to have a lower SAC rate), etc...

Something that we learn early on to take into account when diving deeper is that the quantity of air that we breathe is proportional to the depth we are at: this is because to overcome the outer pressure pressing on our lungs, we need to breathe air/gas at the same pressure. This is possible thanks to our regulator's first and second stages, which keeps the pressure of the gas we breathe constantly the same as the surrounding pressure. And of course, there is more pressure to overcome the deeper we go, therefore more air is required as we descend.

This doesn't make much of a difference the first few meters, but as we go deeper and deeper, this becomes crucial: at 40m, we breathe 5 times as much as we do on the surface (This is because there is 5 times as much pressure at 40m -5 bar- compared to the surface -1 bar-)! Now think about the diving depth record I talked about earlier: being at 332m, means being subjected to about 34 bars of pressure, which means breathing through the gas 34 times as fast as you would on the surface.

To give you an idea, With an average SAC Rate of 15L/Minute (which would be an acceptable SAC Rate for an average Male), you would breathe through a standard 12L tank filled to 200 bars (0.42 Cu ft tank filled to 2900 PSI) in 2 hours and 40 minutes on the surface. At 332 meters of depth, that same tank would only last you approximately 4 minutes and 40 seconds.

Therefore, diving with an open circuit at increasingly deeper depths requires thorough gas planning and bringing additional cylinders. For this specific problem, rebreathers are a very good solution! However, it isn't the only issue divers are facing, let's move on to the other limiting factors.

Nitrogen Narcosis

The exact mechanisms of Nitrogen Narcosis are still not perfectly understood, but its effects are said to be very similar to those of alcohol. It is a state of consciousness drawing parallels to those under anesthesia. The strength of this narcosis is proportional to the depth.

The effects of Nitrogen Narcosis range from mild euphoria to unconsciousness with everything in between: Overconfidence, dizziness, loss of memory, etc... The effects start to be seriously noticeable at 30m (98 feet) of depth. Everyone experiences it differently and has a certain tolerance for it. Regular divers, on top of usually being more tolerant, know how to deal with it better. Outside factors such as general tiredness may play a big role in the experience of Nitrogen Narcosis.

Although we usually say Narcosis itself isn't dangerous, the actions we take as a result of being under the influence may put us in danger: Surely, being overconfident at 40 meters underwater cannot be good! Divers who require a clear mind -such as cave divers, following strict protocols- cannot afford to be narced and have their judgment impaired, in this case, they would add helium in their gas mix to reduce the nitrogen content and therefore reduce the narcotic effect.

One problem: Helium is expensive, very expensive (because it is a non-renewable resource). As I write this article, I can buy a liter of Helium where I live for 0.08€, which doesn't seem like much at first, but when you realize that a simple mix containing 25% Helium in a 2x12L twinset filled to 200 bars comes out to 1,200 Liters of Helium (Costing 96€), it makes you appreciate each breath you take.

The Mix I just used as an example might be a standard mix to go to 45 or 50 meters of depth and stay relatively lucid. Now imagine going below 100 meters, or even 200 or 300 meters for that matter, with the increased gas consumption on top, requiring even more gas and Helium, a dive at that depth would cost more than an entire month's salary just to pay for the gases!

Oxygen toxicity

Oxygen is what keeps us alive, we need it for our metabolism to work properly, however, as it turns out, too much of it is detrimental. That is going to be another big factor to take into account as you go deeper and deeper.

Oxygen toxicity becomes more serious as the partial pressure of Oxygen increases. In a recreational setting, the maximum operating depth of a gas would correlate to where its oxygen content would reach a partial pressure of 1.4 Bar. When breathing a gas for accelerated decompression, we breathe up to 1.6 Bar of Oxygen partial pressure.

For instance, while breathing Air (with an oxygen content of 21%), if we aim to stay below an Oxygen partial pressure of 1.4 Bar, we would have to stay shallower than 57m (187 feet); this is because, at 57m, the air you would breathe would be at the same pressure as the outer environment, or 6.7 Bar. 21% of that pressure would be induced by the 21% oxygen content, which would equate to ~1.407 Bar of partial pressure.

If you are using a 50% Oxygen mix for accelerated decompression, you won't go deeper than 21m to breathe it, to respect the 1.6 oxygen Partial Pressure limit.

Oxygen toxicity mainly affects the lungs, eyes, and most importantly, the Central Nervous System. The main symptoms are spasms that can progress to full-on convulsions. This is obviously a problem underwater as an unattended convulsing diver will basically be paralyzed and drown.

To remedy to this problem, divers going below 57m will breathe a gas mix with an oxygen concentration lower than 21%. This is known as hypoxic diving, requiring special training.

Decompression/Inert Gas saturation

Decompression is one of the major issues of deep diving as the longer and deeper you are, the more decompression stops you'll have to go through, potentially turning a short dive into a dive lasting multiple hours.

To explain the necessity of those decompression stops without going too far into all the details, imagine that you store inert gas (Nitrogen, Helium, and other Metabolically inactive gases) in your blood and tissues. The deeper you go, the more amount of gases will be able to be stored in your tissues due to Henry's Law (stating that the solubility of a gas is proportional to its partial pressure), but it takes some time for them to get from your lungs to your tissues. So the longer you stay at a depth where the tissues can still on-gas, the more saturated in inert gas you will become.

Now, if you ascend, the surrounding pressure will diminish and the inert gas will want to "come out of solution", meaning come out of your tissues. If you are too saturated or come up too fast, small bubbles in your tissues or arteries/veins may occur, leading to complications. For this reason, we ascend slowly and have those so-called decompression stops, to let the inert gas slowly diffuse out through the blood and then through the air we exhale instead of forming potentially life-threatening bubbles.

This is why ultra-deep dives last longer; it may only take a few minutes to descend, but multiple hours to go through all the deco stops. To give you an idea, Saturation divers -divers working in diving bells and, due to the prolonged exposure, have their tissues saturated in nitrogen- require around 1 day of decompression per 30m (100 feet).

So if you want to dive very deep, be mentally prepared to stay in the water for some time!

HPNS

As I have mentioned before, when going deep, Helium is added to gas mixes to reduce the unwanted physiological effects of Nitrogen and Oxygen. A problem that arises is that below 150m (~500 feet), helium has its own unwanted effects: Known as High-Pressure-Nervous-Syndrome, or HPNS, those tremors induced by breathing gas containing Helium at high pressure are one of the limits of ultra-deep diving.

Both the speed of compression and the total quantity of breathed Helium play into the severity of the tremors.

There have been solutions to reduce or take Helium out of the equation, such as reintroducing some Nitrogen, despite its narcotic effect (which has also been shown to reduce HPNS symptoms), or substituting a part of helium for hydrogen.

Going further

Before reaching the end of this article, let's talk about pressure itself. We might imagine that pressure would be one of the biggest problems of diving in the sense that it may "crush" our body. In reality, the main issues described previously all arise from breathing gas under high pressure, not pressure itself. But just for the sake of our curiosity, imagine we weren't subjected to all those limitations, how far could we go until the ambient pressure would just crush our body?

Our bodies are made up of around 60% water, which isn't really affected by pressure as water is effectively incompressible. The air pockets of our bodies (lungs, ears, sinuses, etc...) "resist" the outer pressure by equalizing and counteracting it with the same pressure from the inside (preventing "squeezes").

One direct result of pressure, more prevalent during fast compressions -meaning fast descents- is compression arthralgia, which is a pain in the joints. Slowing down during the descent may prevent this pain from developing but it may still arise regardless when doing ultra-deep (200m / 650 feet) dives and deeper.

The compressive strength of Human Bones is between ~1300 and 2050 Bar (19,000-30,000 PSI) which corresponds to around 13,000-20,500m of depth (43,000-67,000 feet). The deepest point in the world is the Mariana Trench at a depth of 10,916 m (35,814 feet); therefore, even going to the Mariana Trench wouldn't be enough to crush human bones!

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