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Click below to go to the Cruise Logs - (photo credit: Lophelia Coral - Open-File Report 2008-1148 & OCS Study MMS 2008-015)


Cruise Log: 8/16/2009    

Shrinking cups

Liz Baird

Liz Baird holds up the bag full of styrofoam cups before they are attached to the submersible (photo credit Art Howard) - click to enlarge
As we begin to prepare to end our adventure at sea, many of us are packing small Styrofoam cups into our luggage. These cups, decorated with "Sharpie" permanent markers, had been attached in an open mesh bag to the back of the submersible prior to a dive. When the submersible returns to the surface, the cups have shrunk due to the great water pressure at these depths.

When you are on land, you experience air pressure. It 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 on how deep a human can go before the pressure becomes too great. The Johnson Sea Link submersible is designed to go up to 3000 feet down, or the equivalent of about 90 atmospheres, or 1335 pounds per square inch.

The cups shrunken by the great water pressure next to a cup of original size (photo credit Liz Baird) - click to enlarge
How can the Johnson Sea Link survive this? 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 more deeply.

You can make a simple pressure demonstration using a two-liter bottle and a plastic eyedropper. Fill the two liter bottle with water, and then 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.

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