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Scuba Diving



Scuba-from the phrase "self-contained underwater breathing apparatus"-refers to a type of diving in which an individual carries his or her own supply of air. This air supply is stored within a steel or aluminum cylinder called a scuba tank. A device called a regulator "taps" the pressurized air and adjusts its flow for breathing. Prior to the dive, a mechanical compressor fills the tank with a large volume of air. The pressure produced by this compressed air can exceed 200 times the standard atmospheric pressure! As a diver breathes, flow-adjusting devices called stages drop the flow pressure. Air that is exhaled does not return to the scuba tank. Instead, it is released and observed as the rush of rising bubbles. Sport divers can safely dive to a depth of about 39 meters (130 feet) or five atmospheres of pressure. Although the first crude scuba apparatus was invented over 150 years ago, it was Jacques-Yves Cousteau and Emile Gagnan who perfected the modern day Aqua-Lung. Unlike the "hard-hat" divers that relied on a surface air hose, Cousteau (in the early 1940s) had attained untethered freedom.


Imagine entering a freshwater pond or lake. Take a deep breath and you're likely to float. Exhale, and you'll probably find yourself sinking. These "ups and downs" depend upon the amount of air in your lungs. As the volume of this gas increases, you become more buoyant. As the gas volume drops, you lose buoyancy and begin to sink. In this main activity, you'll construct a device called a Cartesian diver. Like a floating person, this diver has a buoyancy that depends upon its volume of trapped air. As you explore its behavior, you'll uncover the relationship between pressure and volume. Materials
  • plastic 1- or 2-liter soda container with a screw-on lid
  • glass medicine dropper 1. Fill the container with tap water. 2. Lower the medicine dropper into the container. Squeeze the bulb slightly so that the glass tube becomes partially filled with water. 3. Set the dropper floating within the container. Add more water to the container so that the level of water rises to the brim. 4. Screw on the container lid. The seal should be tight enough to prevent the leakage of water. 5. Squeeze the center of the plastic container. What happens to the medicine dropper? Release your pressure. What happens now? Note: If the dropper remains afloat, you'll need to open the container and fill the dropper tube with more water. 6. Take a close look at the air bubble trapped within the medicine dropper. What happens to the bubble's volume as you squeeze the container? Can you explain the connection between this change in volume and the behavior of the medicine dropper? What happens to the bubble's volume when you release your grip? How does a change in volume relate to the movement of the medicine dropper? Extend the activity Can you modify the design of your medicine dropper so that it can recover items that are scattered at the bottom of the container? First, design a diver that can retrieve paper clips and other objects attracted to magnets. Then, redesign your diver to "recover" targets that have eyelet-like handles. Is it possible to make a Cartesian diver out of other materials, such as the plastic cap to a pen, weighted with a bit of clay? See what objects will work. Questions 1. Does squeezing the bottle force more water into the air or compress the air, making the diver heavier and causing it to sink? 2. How do these demonstrations relate to scuba diving equipment? How do they explain free divers' use of stones for weight as they dive?
  • Resources

      Frazier, R. (1995) A philosophical toy. Available at:
      Marx, R. (1990) The history of underground exploration. New York: Dover Publications, Inc.
      Computer Software:
      Chariot: Eco-Adventures in the Oceans. 3.5 disks for DOS or Macintosh.
      (619) 298-0202 or
      Edmark: Destination: Ocean. CD-ROM for Macintosh/Windows. (800) 320-8379 or
      The Learning Company: Operation Neptune. CD-ROM for Macintosh or Windows. (800) 852-2255

      Web sites
      Divers Alert Network (DAN)
      (800) 446-2671
      Scuba! On-Line Interactive Magazine