SM0486
b. From an examination of Figure 17 you can reason that as a plane goes
higher, the pressure on the partial vacuum chamber is less, thereby allowing the
spring tension of the metal to return the chamber to its original shape.
The
apparent increase in volume causes a slight upward movement of arm A. This upward
movement of arm A is multiplied by Levers B and BC, causing lever arm BC to rotate
in a counterclockwise rotation about the pivot point.
You can see that a
counterclockwise rotation of the pointer (lower pressure--higher altitude).
The
last section of this lesson presents the operating principle for typical
manometers. The basic pressure principles which affect the operation of barometers
also affect the operation of manometers.
10. Manometers.
a. Information in this section is limited to the well type manometer.
We
will discuss the well type manometer which is direct reading (not necessary to add
fluid deflections above and below normal zero as is the case with U-type
The useful range is 61 inches for temperatures between 4.44 and
manometers).
48.89 C. (40 and 120 F.). We are primarily concerned with the principle on which
the manometer design is based and the operation of the instrument when it is used
for pressure measurements.
b. Since the manometer uses the fundamental principle of the balance of
hydrostatic forces to measure pressure, we will include an example here to explain
the balance of hydrostatic forces. Place a block of material with the dimensions
shown in Figure 18 on a scale platform. When the block of material in Figure 18 is
placed on the platform, the following relationships exist:
(1) The force exerted by the block on the scale platform equals the weight
of the block (8 pounds).
(2) If the block rests on side 1, the weight or force per unit area is
8,
2x1
or 4 pounds per square inch.
This is the pressure developed by the
block against the scale platform.
Similarity, if the block rests on
side II, the pressure is 2 pounds per square inch (total 8 pounds), and
for side III, 1 pound per square inch (total 8 pounds).
Since the
scale platform exerts an equal force in the opposite direction,
regardless of the block position, there is a static balance.
Without
such balance, motion occurs in the direction of the unbalanced force.
(3) If the volume remains fixed, then the pressures develop by materials of
different densities (weight per unit volume) vary in direct proportion
to the density.
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