Pressure Drop

Pressure Drop

The deepest anyone has gone into a natural cave is just over two kilometers. Cavers are a strange group and there's a lot of drama around it, but finally the Russians won.

Gold miners, who can bore their own caves, have gone much deeper, scraping gold from the underside of South Africa about four kilometers down. Heat is the limiting factor for this kind of descent, as the rock face at such depths is hot enough to burn bare skin.

The absolute depth record for diving in water is three hundred thirty meters, about the height of the Eiffel tower. Descending this fairly modest distance into water requires a level of insanity so rare that only five people have come near it. Counterintuitively, free divers dive deeper than scuba divers.

It comes down to water being deceptively heavy. If you want to be macabre about it, next time you're submerged imagine yourself under a pile of bodies, which are, after all, less dense than seawater. This jolly thought experiment drives home just how crushing the weight is above you. The top ten meters of ocean weigh as much as the hundred miles of atmosphere above. At four kilos to the gallon, it soon starts to add up.

Twenty meters does not feel deep. It's far enough that bubbles rise with a dreamy langour, taking their sweet time to reach the surface. Everything is tinted blue, but it's not noticeably dark unless the day is overcast. In California, twenty meters marks the limits of the great kelp forests, and if you're lucky enough to have clear visibility you find yourself suspended amidst Martian trees extending into the distance, with schools of fish hiding between the stalks, embedded in the water like raisins in a jelly.

Twenty meters is also limit for direct ascent. You can bolt to the surface from this depth without getting hurt, provided you keep exhaling. Your lungs will feel painfully empty, but as you rise, the residual air will expand threefold. The danger is not running out of air, but having too much of it.

Below twenty meters, you must be able to suppress your panic response and make a controlled ascent, even when you are out of air, or you will be badly hurt. Not coincidentally, twenty meters is the limit depth for recreational diving.

At thirty meters, it is noticeably dimmer. If you get an advanced certification, they make you take a color chart down to this depth, and you can see the red patch fade to black. A flashlight makes everything explode in color. For biomechanical reasons, your lung capacity is cut by half. It is easy to get winded, and hard to catch your breath once you do.

At this depth, you start to get a nice nitrogen buzz. Nitrogen is physiologically inert at the surface, but becomes psychoactive at higher pressures. Everyone is impaired by twenty meters, though no one will admit it; by forty meters most people notice it. Divers refer to the effect, facetiously, in martini equivalents. It's a happy, mellow high in warm water; in cold water you may get a 'dark narc', where a general feeling of dread may combine with an overpowering sensation of being hunted or followed

A dark narc is no fun, but a happy one can be just as dangerous. Divers have been known to sit down on the bottom and marvel to the great oneness of it all, oblivious to their dwindling air supply, until a clearer thinker above them in the water column notices and comes down to drag them higher.

The mechanism by which nitrogen becomes an intoxicant is still not well understood. Most gases have the same effect, and the severity depends in part on atomic weight. Xenon is an anaesthetic at atmospheric pressure, it's main drawback being that it is very expensive. Nitrous oxide is another good one.

Somewhere around fifty meters, a can of soda no longer fizzes if you open it, which gives you a hint about what your blood would do if you tried to shoot to the surface. The relationship between panicking divers at this depth and rescue divers is adversarial. When you run out of air underwater, the natural instict is to spit out the regulator and swim upwards with all your strength. From this deep, the reflex is suicidal. A dive buddy's job is to prevent rapid assent possible, knocking off the panicked diver's mask and hugging their legs if necessary to keep them from bolting. Water can be barfed up and friendships mended on the surface, but an embolism is forever.

At seventy meters, it becomes hazardous to dive on compressed air. The risk isn't nitrogen (although you would be high as a kite by this depth) but oxygen toxicity. High-pressure oxygen will give you seizures. These are not directly harmful, but will kill you by making you lose consciousness and spit out your regulator.

People who dive to these depths use a mixture of air and helium called trimix. The helium is about the least psychoactive thing anyone has found to breathe[*], and its light weight helps the lungs move the gas that is now eight times denser than what they are built to handle.

Eighty meters is the equivalent depth of the Byford Dolphin decompression accident, when a worker on an oil rig accidentally opened the hatch to the atmosphere and killed five decompressing divers. One diver,

{being exposed to the highest pressure gradient, violently exploded due to the rapid and massive expansion of internal gases. All of his thoracic and abdominal organs, and even his thoracic spine were ejected, as were all of his limbs. Simultaneously, his remains were expelled through the narrow trunk opening left by the jammed chamber door, less than 60 centimetres (24 in) in diameter"

Even helium goes only so far. At about one hundred sixty meters most people will experience a slight tremor called 'high pressure nervous syndrome', a name that sounds more appropriate to an office park than the ocean. This is reversible but gets worse as you get deeper, until it becomes incapacitating.

At two hundred meters, it is too dim for photosynthesis, and pretty much any and here begins the least understood stretch of dark ocean. Fourteen meters more and you reach the insane world record depth for a breath-hold dive. Getting down this deep requires a heavy weight sled, a coke bottle, and the ability to hold your breath for seven minutes. Pro tip: urinate in fear on your way *up*, not down, to give yourself a little boost. At three hundred meters, you're going to have a bad time. The pressure here is the same as on the surface of Venus. The gas you are breathing is so dense that you can no longer breathe harder to get more air. Get even slightly winded at this depth and you will go into a downward spiral that ends with you smothered in your own carbon dioxide. Scuba tanks have a clever system for providing compressed air at ambient pressure, so there's no direct way to tell how deep you are. But the body does respond to high pressure. As the gas in the lungs grows denser, it takes more work to breathe in and out, limiting the amount of exertion you can sustain. More insiduously, plain old nitrogen becomes a narcotic at relatively low pressures. Disregard the narcosis and the next hazard is oxygen toxicity. At a partial pressure of xXX, oxygen can provoke a seizure. The seizure itself is not dangerous, but it will kill you underwater unless someone (hopefully breathing a safer gas mix) is nearby to force your regulator into your mouth. Once under water there are nothing but limitations. You can't really talk underwater, since you have a big regulator in your mouth, so communication is by hand signal. GPS doesn't work underwater, so you navigate by compass and counting kicks. And the air on your back limits you to about an hour underwater, depending on how big an air hog you are and how deep you go. Imagint that you can't talk, you can only walk at a slow shuffle, that you will feel woozy and black out if you descend more than five flights of stairs, and that you must take extreme care in climbing back up or you are liable to die. These are roughly the limitations of being a scuba diver, except that you must also carry and manage all the air you need. If you are not familiar with scuba diving, the limitations seem somewhat extraordinary. People have walked deeper into the earth than they have dived into the sea on scuba equipment. In fact, South African miners are digging for gold right now ten times deeper than the absolute depth record for scuba diving (303m). It's the crushing weight of water that keeps us so near the surface. The entire column of air above you, extending over a hundred miles, weighs as much as ten meters of sea water. This means pressure goes up quickly with depth, and twenty meters (three atmospheres) is the limit for recreational divers. At thirty meters, nitrogen starts to have a pronounced narcotic effect[x]. In warm water you can feel a sense of euphoria or boundless confidence (the last thing you should be feeling at depth), while cold water with poor visibility can give you a 'dark narc', with feelings of being hunted, or impending doom. Thirty meters is also about as deep as you can go Radio works poorly underwater, and it's rare for divers to use full-face masks. That means there's no way to communicate other than hand signals or writing on a slate. It also means GPS is useless below the surface, so the only way to navigate, especially in poor visibility, is by compass and counting kicks. Scuba gear is heavy and uncomfortable on land, but this melts away as you step into the water. There are few sensations more pleasant than walking in to warm water on a shore dive and feeling the big metal tank on your back become weightless. Everything about diving is awkward, at least until you get underwater. Divers simultaneously wear an inflatable vest and a system of lead weights, either on a belt or in quickly removable side pockets. The goal - a distant one for a novice diver - is to achieve neutral buoyancy, so you are neither plummeting into coral or popping out like a cork. The regulator is something like a bit between your teeth and takes getting used to. Compressed air has no moisture in it, so the tank can't rust, and this gives you cotton mouth very fast underwater. There are fancy ways to de-fog a mask, but my instructor prefers to use a dab of green gel toothpaste, spread thinly over the glass and then rinsed out. So as we descend my eyes sting a little with minty freshness. Going down means a complicated set of head and jaw motions, to try to equalize pressure in the ears. Old divers do this automatically, but it's not easy to do at first, and I'm relieved every time I hear the squeak or long whistle of air getting through my eustachian tubes and balancing the pressure of the water. Going up, provided it's done slowly, is much more fun - there's a lot of crackle and fizz to it. In Hawaii walk into the water anywhere and you're in a tropical aquarium. There's no need to look for wildlife, as it tends to swim over to look at you. The happiest thing that can happen to a tropical fish, we learn, is if a diver vomits through the regulator. With due respect for hte majesty of the deep, Hawaiian fish look pretty ridiculous. It takes ten minutes or so to get past concentrating hard on the act of breathing and having time to notice things. The little fish are quite territorial. There's a handsome spotted creature that looks like a Louis Vitton handbag with fins patrolling its few square inches of coral. A mustachioed dignitary the size of a large koi roots through the sand like a truffle pig. The mustachioes are sense organs.. Some of the fish are very familiar from any tropical tank, while others are so ill-behaved in captivity, or hard to feed, or (like the goatfish) so fond of stirring up sand that they're impossible to maintain in captivity. at the only way you have to navigate the world is by using a compass and counting footsteps, and that you must carry the air you breathe on your back. Diving is safer than a lot of other sports, but with a little twist. When there is a problem underwater, it's your natural, instinctive reactions that are most likely to kill you. When you panic underwater, the thing you want to do most in the world is hold your breath and swim as fast as possible for the surface. This almost guarantees you'll die or be severely injured, so much of scuba training is built around teaching you to suppress that reaction. This is what it feels like to have your mask fall off, this is what it's like to run out of air (your instructor literally turns off your gas). The limiting example of this is combat diver training, where in order to pass you have to endure five minutes of instructors literally beating you up underwater, and tying your equipment up in knots, without bolting. You are not far from panic, and panic is what kills you underwater, since your most instinctive lizard brain reaction (hold your breath, sprint for the surface) is most likely to get you killed. So scuba training tries to familiarize you with things that could go wrong, so you don't cross that panic threshold. Here is what it's like to have your mask fall off or flood, here's what it's like if you regulator falls out of your mouth, or how to survive you run out of air. Diving is about extraordinary limitations. Every ten meters of water above your head weighs as much as the entire column of air above it. At thirty meters, the pressure is equivalent to four atmospheres, and the nitrogen in compressed air begins to have a noticeable narcotic effect. Divers call this getting narced, and measure the psychological effect in martinis. Depth also reduces the time you can spend in the water, both because you are breathing air at higher pressure, and because you must not stay deep long enough for significant gas to diffuse into your blood, at the risk of decompression sickness. Specialist divers can go deeper by breathing a mixture of air and helium, but even helium starts to have central nervous system effects past a few hundred feet. So the absolute record for scuba diving is a thousand feet, a descent that requires hours and hours of decompression stops on the way back up, so that the. So much for going down. But divers are humbled in other dimensions, too. There's no GPS underwater - the first few millimeters of seawater are enough to absorb the signal. You navigate with compass and by counting kicks 1.6 atmospheres - oxygen causes seizures 4 atmospheres - nitrogen narcosis noticeable And so though people dive in the most unlikely places, we are confined to teh thinnest upper skin of the world's oceans. One of the attractions of diving is this sense of limitation. As radio waves can't penetrate seawater, there is no way to communicate with other divers other than sign language, or writing on a slate. For the same reason there is no GPS underwater. Instead, like a 50's boy scout you are limited to magnetic compass and counting your steps. among scuba divers who are not crazy , James The effortless feeling of hanging weightless in water is a trick, disguising the fact that we are permanently confined to that thin layer under the surface. us to the surface far more implacably than even the earth. Despite the intimidating equipment, scuba diving is a very simple activity. You attach a pretty metal valve to an air tank, turn the knob, and suddenly you have super powers that allow you to survive in a completely alien world. The regulator is a slightly uncomfortable mouthpiece that gives you air every time you inhale. You blow back out through your mouth and release bubbles. The only reason that you can't go out and rent scuba gear without taking a certification class is that it's so very easy to get killed, even in shallow water, even with plenty of air. It's also easy to avoid getting killed, but that requires some practice and some training. When we talk about diving we think of 'the bends', but the real danger to recreational divers is lung expansion injury and air embolism. Basic scuba training is about teaching you not to lose your shit underwater. * This is often cited but not often explained. The burst strength of alveolae is about 1.5 psi. This pressure difference coresponds to rising x inches in the water column . ke the deepest possibel breath It is hard to sense this terrific mass directly, but it is the reason why you can't just go buy scuba gear at the local Wal-mart. A pressure gradient and there are some awful ways to get killed. But the effect is powerful. Because people are not designed to live in a sharp pressure gradient, we have no instinctive sense of its dangers. But it's easy to get injured from breathing compressed air if you don't know correct technique. With a little effort, you can die in four feet of water. Early in scuba training you are taught never to give a breath of air to a snorkeler or skin diver, for fear that they will keep their mouths closed on ascent and die horribly