Measuring Depth Pressure
Country: United States
Date: March 2006
For a science experiment my 10 year old son
modified a demonstration from his scuba class.
He inverted a hard plastic pitcher (marked with measurements) with
a floating frog inside, and placed it at the bottom of the pool at
varying depths (using a net with weights).
He noted that the water inside the pitcher rose 2 centimeters for
every 2 feet of increased depth.
Is there an easy way to measure the water pressure, air pressure,
air volume, and compression rate?
If the pitcher were a cylinder with a ruler that you could easily
read from outside, you could easily make the measurements you desire.
The air volume is just the cross sectional area of the cylinder
times the height of the air column. The air pressure is identical
to the water pressure at the bottom of the cylinder and is easily
calculated using the equation pV = constant. Here p is the pressure
(atmospheric pressure is roughly 15 pounds per square inch) and V is
the volume. When the cylinder is above the water, the air pressure
is atmospheric. As the cylinder is lowered into the water, the
water level rises and the volume decreases indicating an increase in
pressure. Plotting the pressure increase versus the depth will give
you the compression rate.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
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Update: June 2012