Water Pressures at Ocean Depths

Water pressures in the deep is one of the many phenomena researchers must contend with when exploring deep-sea sites. The ocean is deep. If we shaved off all the continents and filled the trenches in the oceans with the earth from the continents, the entire globe would be covered with water about 2 miles in depth. The average ocean depth is 12,566 feet about 3800 meters. The greatest ocean depth is 36,200 feet over 11,000 meters! What effect does this great depth of water have on things living in the ocean? The answer depends upon where in the ocean it lives. A fish or a plant near the surface feels little effect from the great depths. It matters little if there is six feet or six thousand feet beneath a swimming fish. An animal living at 10,000 feet depth, however, is greatly influenced by the depth of the water over it.

We often speak of pressure in terms of atmospheres. One atmosphere is equal to the weight of the earth's atmosphere at sea level, about 14.6 pounds per square inch. If you are at sea level, each square inch of your surface is subjected to a force of 14.6 pounds.

The pressure increases about one atmosphere for every 10 meters of water depth. At a depth of 5,000 meters the pressure will be approximately 500 atmospheres or 500 times greater than the pressure at sea level. That's a lot of pressure.

Research equipment must be designed to deal with the enormous pressures encountered in the depths. Submarines must have reinforced walls to with stand pressures. Instruments that work well at the surface may be collapsed or rendered useless by the pressure.

Calculate how much pressure (pounds per square inch) the equipment used on NeMO cruise must withstand.

Depth of Axial Caldera - 1540 Meters
(One atmosphere of pressure on one square inch of surface is subjected to a force of 14.6 inches. Pressures increase about one atmosphere for every 10 meters of water depth)

How many pounds of pressure per square inch will the NeMO cruise equipment experience???

Dr. William Beebe was a pioneer in deep-sea exploration. With support from the National Geographic Society and the New York Zoological Society, Beebe constructed the Bathysphere (bathy = deep). In this steel sphere he would be lowered to depths of over 2,500 feet. The thick walled sphere was designed to withstand the great pressures of the ocean deep. The sphere had two thick quartz windows for viewing. To test the windows the bathysphere, unoccupied was lowered to 3,000 feet. When the great steel ball was hauled up, Beebe wrote.

"It was apparent that something was very wrong, and as the bathysphere swung clear I saw a needle of water shooting across the face of the port window. Weighing much more than she should have, she came over the side and was lowered to the deck. Looking through one of the good windows I could see that she was almost full of water. There were curious ripples on the top of the water, and I knew that the space above was filled with air, but such air as no human being could tolerate for a moment. Unceasingly the thin stream of water and air drove obliquely across the outer face of the quartz. I began to unscrew the giant wingbolt in the center of the door and after the first few turns, a strange high singing came forth, then a fine mist, steam -like in consistency, shot out, a needle of steam, then another and another. This warned me that I should have sensed when I looked through the window that the contents of the bathysphere were under terrific pressure. I cleared the deck in front of the door of everyone, staff and crew. One motion picture camera was placed on the upper deck and a second one close to, but well to one side of the bathysphere. Carefully, little by little, two of us turned the brass handles, soaked with the spray, and I listened as the high, musical tone of impatient confined elements gradually descended the scale, a quarter tone or less at each slight turn. Realizing what might happen; we leaned back as far as possible from the line of fire. Suddenly without the slightest warning, the bolt was torn from our hands and the mass of heavy metal shot across the deck like a shell from a gun. The trajectory was almost straight and the brass bolt hurtled into the steel winch thirty feet across the deck and sheared a half-inch notch gouged out by the harder metal. This was followed by a solid cylinder of water, which slackened after a while to a cataract, pouring out of the hole in the door, some air mingled with the water looking like hot steam. Instead of compressed air shooting through ice-cold water. If I had been in the way, I would have been decapitated. "

The pressures are great, indeed.

From: Half Mile Down by William Beebe, Published by Duell Sloan Pearch (New York) 1951.

Creatures who live at great depths do not have air in their bodies such as the swim bladders found in fish that live in more shallow waters. Without air in their bodies, the pressure problem is solved. Fish, crab, octopus, worms, limpets and clams are just some of the creatures found in the depths of the oceans.

When man enters the world of water he encounters a number of problems. The average scuba diver becomes incapacitated at 250 feet of depth. This is a far cry from the 11,500 foot depth at which deep sea fishes have been found.

Scuba divers need oxygen to survive. Oxygen makes up 21% of the air we breathe. About 78% of the air we breathe is nitrogen gas. Nitrogen is relatively inert; it is more or less chemically inactive. The oxygen and nitrogen are carried in the bloodstream. At sea level the nitrogen presents no problem for man. But what happens to these gases as we descend into the ocean depths.

The increased pressure allows more oxygen and more nitrogen to dissolve into the blood. At about 100 feet the pressure will cause enough nitrogen to dissolve in the blood for the nitrogen to become a danger. Nitrogen narcosis results from too much nitrogen being forced into the blood stream. It will eventually result in stupor and sleep, not a good condition 100 feet below the surface. Before the stupor stage, divers become dizzy, their ability to make even simple mental decisions (like tell time) is reduced. Sometimes they decide they no longer need to breathe through their mouthpiece. The precise symptoms and the depths to which the symptoms appear vary with each individual and with each dive. Diving below 100 feet requires special skills and is dangerous. Returning to the surface reduces the nitrogen content and reduces the symptoms.

If one atmosphere equals about 14.6 pounds per square inch pressure, and the pressure increases 1 atmosphere for every 10 meters of depth. How many atmospheres are forcing the nitrogen into the blood stream at 30 meters (about 100 feet) and at 75 meters (about 250 feet)?

Divers limiting the time and depth of their dives can avoid nitrogen narcosis. Coming to the surface in stages with a pause at each stage allows the nitrogen to diffuse out of the blood.

Pressures adapted from Project For Sea by Jim Kolb.