SM0486
Figure 5.
Atmospheric pressure changes
f. The values in Figure 5 show that the increase in altitude is greater than
90 feet.
Therefore, the mercury level decrease in the tube is greater than 0.1
inch. To determine the drop (in inches), the total increase in altitude (500) is
divided by 90 and the result is multiplied by 0.1 to determine the decrease in the
mercury level. Now that you know the basic principles associated with force, fluid
pressure, and atmospheric pressure, we continue your study of pressure principles
by teaching the principles associated with pressure gages.
5.
Pressure Gages
a. Normally when we think of measuring pressure, the gage is the first
instrument that crosses our mind. Let's begin our discussion of pressure measuring
instruments with -the gage. Pressure gages, in spite of their intended use, have a
number of common characteristics. Almost all pages indicate the pressure by means
of a pointer on a graduated dial. As shown in Figure 6, A, the pointer turns about
a point located in the center of the indicating dial in a concentric type gage, or
the pointer may be located off center as in the eccentric type gage of Figure 6, B.
Another common characteristic of gages is that the pressure to be measured is the
sole source of power required to provide an indication.
This is accomplished
through the use of some sort of elastic chamber inside the gage
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