SM0486
Figure 18.
Relationship of Force and Pressure
The preceding example was included to establish a relationship between density
(weight per unit volume), volume, and the balance.
c. Pressures Associated with Manometers.
(1) The pressure existing in, or exerted by, a fluid is (among other
factors) a function of the amount of fluid present. In the case a of
gas, it is the total effect produced by all the molecules of the gas
and the individual velocities producing impact on all sides of a sealed
tube containing the gas. As gas is removed from the tube, the pressure
in the tube decreases as the tube volume per unit of gas increases.
(2) In engineering and scientific work, two pressures are commonly used for
reference: absolute zero and atmospheric. Atmospheric pressure is that
pressure existing at the location on or above the earth and is the
total effect of all air above to the limit of the atmosphere.
It is
subject to change by reason of temperature changes and local weather
conditions.
It is measured and indicated by a barometer at the
location.
A graphic relationship between absolute and atmospheric
pressures is shown in Figure 19.
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