08/16/09 – Shrinking Cups
As we prepare to end our adventure at sea, many of us are packing small Styrofoam cups into our luggage. These cups, decorated using “Sharpie” permanent markers, had been placed in a mesh bag, which was attached to the back of the submersible, and then taken down during one of the many dives that took place during this mission. When the submersible returned to the surface, the cups had shrunk to a fraction of their original size due to the great water pressure they experienced during the dive.
When you are on land, you experience air pressure. Air pressure is the weight of the atmosphere pressing down onto you. We rarely notice it, except for when it changes rapidly, such as driving up a tall mountain, riding an elevator in a tall building, or flying in an airplane. The change in air pressure is what makes our ears “pop.” One atmosphere of pressure is 14.7 pounds per square inch.
Diving into water creates a change in pressure too. Every 33 feet you descend is the equivalent of 1 atmosphere of pressure. SCUBA divers adjust to this when they dive, but there are limits to how deep a human can go before the pressure becomes too great. The Johnson-Sea-Link submersible is designed to go up to 3,000 feet down, or the equivalent of about 90 atmospheres, or 1335 pounds per square inch.
How can the Johnson-Sea-Link withstand the pressure? The Plexiglas sphere and metal aft chamber are both strong enough to withstand the pressure at these depths. The sphere does change shape — it shrinks about 1/8th of an inch. The support structure is designed to handle this change and actually becomes stronger as it goes deeper.
You can make a simple pressure demonstration using a two-liter bottle and a plastic eyedropper. Fill the two-liter bottle with water and half-fill the plastic eyedropper. Place the eyedropper in the two-liter bottle and put the top on. When you squeeze the plastic bottle, it changes the water pressure and the eyedropper will go down. More complete directions for making this type of “Cartesian Diver” can be found online.
Unlike the sphere of the Johnson-Sea-Link, the Styrofoam cups cannot withstand the water pressure at depth. As they descend, the air is squeezed out of the Styrofoam, permanently changing the structure of the cup. When the submersible returns to the surface, the cups are tiny. These shrunken cups are a souvenir of our exploration of the deep and a reminder of the technology that keeps us safe in the Johnson-Sea-Link.
Jim Sullivan and Craig Caddigan of the submersible crew provided background information for this log.
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