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The Bends

 

Overview

If you want a sport that puts you under a lot of pressure, try underwater diving. You can even feel some of the effects of pressure in a swimming pool. Down just a few feet underwater, your ears begin to hurt. That's caused by pressure on your eardrums. Where does that pressure come from? It's the weight of all the water--and air--above you. At the surface of the water, a column of air--stretching hundreds of miles out into space--weighs 14.7 pounds per square inch. Scientists call this amount of pressure "one atmosphere." When you go underwater, however, you add the weight of the water to the atmospheric pressure. A 10-meter (33-foot) column of water weighs 14.7 pounds per square inch, so at a depth of 10 meters, the pressure is two atmospheres: half from the water and half from the air above it. Pressure also influences how divers use air. At ten meters, for example, the increased pressure means your lungs hold twice as much air as they do at the surface--and you'll breathe all the air in your tank twice as fast. The deeper you dive, the more quickly you use up the air in your tank. When you breathe compressed air through a regulator underwater, you ensure that the air spaces in your body are at the same pressure as the surrounding water. But if you breathe compressed air underwater and then ascend, holding your breath, the pressure around you decreases, so your lungs expand. Air sacs in your lungs could rupture, causing an air embolism , which means that bubbles of air enter your blood stream and block circulation to your brain. "The bends," ordecompression sickness , is another health hazard associated with pressure changes. The longer you stay down and the deeper you go, the more nitrogen dissolves into your body tissues. If you ascend too rapidly, the dissolved nitrogen comes out of solution too quickly and forms bubbles in your tissues. You could experience severe pain (particularly in joints), dizziness, blindness, paralysis, and convulsions. Although decompression sickness is rare, divers learn they must ascend slowly and, under certain circumstances, take "decompression stops" on the way up. This allows the dissolved nitrogen to come out of the body safely.

Activity

It's important for divers to be of neutral buoyancy. That means they tend to stay at the depth they are--they neither sink nor float. You can explore buoyancy in a sink or bathtub. Materials
  • several dozen washers, in a few different sizes if possible
  • some objects that float, such as wooden "kindergarten" blocks, Tinkertoys, or plastic film canisters
  • paper clips
  • duct tape
  • an inflated and tied balloon
  1. Fill a bathtub (or any other deep container, such as a sink or a waterproof wastepaper basket) with as much water as is practical.
  2. Select an object. Verify that it floats.
  3. Find a way to attach the washers to the object as weights. For a wooden block, make a hook out of a paper clip and tape it to the block with duct tape; put the washers on the hook. For a Tinkertoy structure, put the dowel through the washer's hole. For a film canister, put the washers inside.
  4. Add washers until the object just sinks. Record how many washers it takes to sink it.
  5. If you have different-size washers, try to adjust the weight on the object so that the object is of neutral buoyancy.
  6. If you have different-size objects (for example, wooden blocks or Tinkertoys in different sizes), see how the amount of weight it takes to sink them depends on their sizes.
  7. Look around your home for objects that float which you can sink with washers. Try some. Were you surprised by any of them?
  8. Try to sink the balloon. Tie a paper-clip hook to the neck of the balloon and add washers. What happens? Questions
    1. What did you notice about the number of washers and the size of the object? Is this what you expected?

Resources

  • Briggs, C.S. (1985) Skin diving is for me. Minneapolis: Lerner.
  • Griffiths, T. (1989) Sport scuba diving in depth: An introduction to basic scuba instruction and beyond. Princeton, NJ: Princeton Book Co.
  • Open water sport diver manual (4th ed.). (1989) Englewood, CO: Jeppesen Sanderson Inc.
  • McGovern, A. (1989) Down under, down under: Diving adventures on the Great Barrier Reef. New York: Macmillan.