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Teaching Guide
Blowing Ballast

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  1. Stretch a balloon by inflating and deflating it about a dozen times. If necessary, pull and stretch out the balloon to make it more flexible.
  2. Obtain a length of plastic aquarium tubing. If you do not have tubing, you can construct such a tube by connecting straws. If you are using straws, secure them with waterproof, sturdy tape, and make sure to place a bend in the straw as shown above.
  3. Place the mouth of the balloon over one end of the straw/tubing. Use a rubber band to secure the balloon to the straw/tube end. Make sure that the rubber band does not squeeze off the air passage.
  4. Insert the balloon into the 1-liter clear plastic beverage container.
  5. Place the container in a large tank or tub of water. Let the container fill with water. As it fills with water, the container should sink. If it does not sink, add several fishing weights until the water-filled container settles to the bottom of the tank.
  6. Make a prediction. Suppose you blew a small puff of air into the balloon. How would that change the buoyancy of your "submersible"?
    (Although any air would increase the upward force, a small volume of air may not produce enough buoyancy to raise the sub)

    Suppose you inflated the balloon to a greater volume? Would that offset the sub's weight?
    (It depends upon the actual weight of the sub. Although a larger volume of air would create a greater upward force, you would still need to produce a counter force greater than the weight of the container.)


  1. Why was it necessary to "pre-stretch" the balloon?
    (The fabric had to be loose enough to expand when a less powerful push of air was forced into it through the straw.)
  2. Why was it important to keep the air passageway unblocked?
    (You needed an unobstructed path for the air to travel to the balloon. Otherwise the balloon would not inflate.)
  3. What was the purpose of the fishing weights?
    (The weights added an extra downward force that made the submersible sink when the tank was not filled with air.)
  4. What happened when you blew into the open end of the straw/tube?
    (Air moved down the tube and filled the balloon. As the balloon filled with air, the craft became lighter until it eventually floated to the surface.)
  5. Consider the balance of forces that are responsible for the surfacing and diving of your classroom submersible. How can you apply what you've learned to Alvin's operation? (Like the classroom sub, Alvin's depth is determined by the balance between its weight and buoyancy. But unlike the classroom sub, and most other submersibles, Alvin has a dual ballast system -- fixed and variable. First, fixed steel ballast weights are used to sink the Alvin down to just above the ocean floor. Then those weights are dropped to halt Alvin's descent. Next, the variable ballast system is used to finely adjust the sub's exact depth. Alvin's variable ballast system pumps seawater into or out of tanks to increase or decrease the sub's total weight, just like a normal sub and the one in this activity. But then when it's time to return to the surface, more fixed weights are dropped, and the sub rises. Using fixed weights for descent and ascent this way is very safe, because the weight release mechanisms are simple, and can be operated by hand if necessary. And using a variable system only for fine adjustments at the bottom saves on precious battery power to run the seawater pumps.)


A Biological Connection

Some species of seaweed have tiny air bladders that line their stem-like parts. Think about it. What survival advantage might these sacs of air offer?
(Like an air-filled ballast tank, the air sacs help keep the stem-like parts afloat. Floating higher in the water exposes them to more sunshine.)

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