Measuring Water Pressure
Name: Patrick L.
Date: November 2002
How do you measure water pressure underwater?
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.
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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Update: June 2012