Topic

Description

A manometer is a pressure measuring device that uses the heights of static liquid columns to determine pressure. Manometers come in a variety of sizes and shapes and are used in many different applications.

A familiar manometer is commonly found in a physician's office for the determination of blood pressure. A cuff is placed around the arm and inflated to a pressure above the pressure in the arm (brachial) artery and slowly deflated. Appropriate readings of the column height of the attached mercury manometer give the peak (systolic) pressure
and the lowest (diastolic) pressure, typically in millimeters of mercury.

Many pressure measuring devices use the fact that a force (pressure times area) causes an elastic object to deform.

The Bourdon gage is a very common type of pressure gage. When the pressure within the hollow curved tube of the gage is increased, the tube tends to straighten. Through a set of linkages the resulting slight motion of the end of the tube is translated into the rotation of a dial that indicates the gage pressure.

For an incompressible fluid at rest the pressure increases linearly with the fluid depth. As a consequence, large forces can be developed on plane and curved surfaces in contact with the fluid.

Hoover dam, on the Colorado river, is the highest concrete arch-gravity type of dam in the United States. The water behind Hoover dam is approximately 715 feet deep and at this depth the pressure is 310 psi. To withstand the large pressure forces on the face of the dam, its thickness varies from 45 feet at the top to 660 feet at the base. (Video courtesy of U.S. Bureau of Reclamation.)

The hydrostatic force distribution on a curved surface can be quite complex because the force acts normal to the surface, and the direction of the normal varies from point-topoint along the surface. A pressure vessel must be designed to withstand the pressures generated.

If the weight of the pop is neglected, the net force on the complex-shaped bottom of a pop bottle is equal to the pressure times the projected area, a circle the diameter equal to that of the bottle. Thus, when the pressure is 40 psi, a force of approximately 580 lb is needed to fasten the bottom of a 4.3 in. diameter pop bottle to its cylindrical sides.

Archimedes' principle states that the net hydostatic pressure force on a submerged or floating body is equal to the weight of the fluid displaced.

By pressing the sides of the bottle, the pressure within it is increased and the air within the inverted test tube is compressed. This allows additional water to enter the test tube, thereby causing the average specific weight of the object (the test tube, air, and the enclosed water) to be greater than that of the surrounding water. The tube sinks.

The specific gravity of a liquid can be determined using Archimedes' principle.

A hydrometer is a glass bulb weighted on one end with a narrow stem at the other end. When placed in a liquid, such as water, it will float when the constant weight of the hydrometer is balanced by the buoyant force. Readings along the stem can be related to the specific gravity of the liquid. If the water is replaced with a heavier liquid (green salt water) the hydrometer will rise since a smaller displaced volume is required to balance the weight.

A floating body can be stable even though its center of gravity is located above its center of buoyancy.

In this model, the center of gravity can be raised or lowered by moving the weights on the mast. The center of buoyancy remains below the free surface. With a low center of gravity, the object is very stable. As the center of gravity is raised, the object becomes more tippy. Eventually it becomes unstable.