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Name: Patrick L.
Status: Student
Age: 15
Location: N/A
Country: N/A
Date: November 2002


Question:
How do you measure water pressure underwater?



Replies:
There are several ways. Here is how one works. Think of a flat cylinder with solid electrically insulated wall, but flexible electrically conducting ends. For simplicity assume the inside is evacuated, but that is not absolutely necessary. If a voltage is impressed across the two ends, this makes an electrical capacitor. The capacitance of the device can be measured very accurately and precisely with an appropriate electrical circuit. The value of the capacitance depends upon the distance between the two end plates. the water pressure squeezes the ends closer to one another as the device is lowered in the water. This changes the capacitance in proportion to the water pressure. Look up "parallel plate capacitors" in an encyclopedia, physics book or on the Internet and you will find the actual equations for this type of electrical circuit.

Vince Calder


The simple answer is you go underwater with a pressure gauge. You can easily do it yourself by gluing a ruler to the side of a water glass. As you go further underwater, the water level will rise in the glass as the air in the glass gets compressed by the increasing pressure.

Since the ideal gas law is pV = nRT, at constant temperature and amount of gas, pV= constant and so p = c/V. As the pressure increases the volume decreases proportionately. You can easily evaluate the constant c since the pressure when the rim of the glass just touches the water is atmospheric pressure and V is the volume of the glass. If you chose a straight walled (non-tapered) glass, the volume of air is just proportional to the height of the air in the glass.

Best, Dick Plano...



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