These activities explore the relationship between depth and pressure, the causes of pressure, and the additional factor of compressibility present in the diver activity.

The diver activity takes advantage of the fact that air is compressible, while water under normal conditions is not. When the bottle is squeezed, the pressure within it is raised uniformly. This is called mechanical pressure, caused by the action of your hands. The increased pressure within the bottle compresses the air bubble trapped in the test tube. If the bubble size is adjusted correctly, as it contracts it reaches the point at which it displaces less water than the weight of the tube, so the tube sinks. Removing the pressure reverses the process.

Another type of pressure explored in this activity is hydrostatic pressure. Hydrostatic pressure is a function solely of the height of fluid above a certain point, and the fluid's density. So the pressure in water depends only on depth, not on the size of the body of water. It's the same pressure ten feet down in a swimming pool as in the ocean. (Actually it's a little more in the ocean, because salt water is slightly denser than fresh.) In space there is no hydrostatic pressure, because there is no atmosphere pressing down. It may be too subtle to detect, but as the diver sinks the hydrostatic pressure will also increase, in addition to the mechanical pressure applied by the hands, causing the tube to sink more easily.

In the flow rate experiment, there should be a direct, one-to-one relationship between the three flow rates and the heights of the holes -- assuming the holes are close to the same size, and also assuming the measurements are made with a roughly constant water level in the bottle. There should be the same linear relationship between the horizontal stream lengths and hole heights, because the different water pressures at each depth determine the stream lengths.

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NATIONAL SCIENCE STANDARDS