when pressure is given in pounds per square inch,
BAROMETRIC PRESSURE is the term used
to describe the actual atmospheric pressure that
pounds per square foot, or similar units. The g
(for gauge) is added only when there is some
exists at any given moment. Barometric pressure
possibility of confusion. Absolute pressure, on the
may be measured by a simple mercury column or
other hand, is always expressed as pounds per
by a specially designed instrument called an
square inch absolute (psia), pounds per square
aneroid barometer.
foot absolute (psfa), and so forth. It is always
A space in which the pressure is less than
necessary to establish clearly just what kind of
atmospheric pressure is said to be under partial
pressure we are talking about, unless this is very
vacuum. The amount of vacuum is expressed in
clear from the nature of the discussion.
terms of the difference between the absolute
pressure in the space and the pressure of
To this point, we have considered only the
most basic and most common units of measure-
the atmosphere. Most commonly, vacuum is
ment. It is important to remember that hundreds
expressed in inches of mercury, with the vacuum
of other units can be derived from these units;
gauge scale marked from 0 to 30 in.Hg. When
remember also that specialized fields require
a vacuum gauge reads zero, the pressure in the
specialized units of measurement. Additional units
space is the same as atmospheric pressure--or, in
of measurement are introduced in appropriate
other words, there is no vacuum. A vacuum gauge
places throughout the remainder of this training
reading of 29.92 in.Hg would indicate a perfect
manual. When you have more complicated units
(or nearly perfect) vacuum. In actual practice, it
of measurement, you may find it helpful to first
is impossible to obtain a perfect vacuum even
review the basic information given here.
under laboratory conditions. A reading between
0 and 29.92 in.Hg is a partial vacuum.
ABSOLUTE PRESSURE is atmospheric
SUMMARY
pressure plus gauge pressure or minus vacuum.
For example, a gauge pressure of 300 psig equals
We have discussed the basic laws and
an absolute pressure of 314.7 psia (300 + 14.7).
principles covering electrical and mechanical
Or, for example, consider a space in which the
theory. This chapter was provided to give you only
measured vacuum is 10 in.Hg; the absolute
a basis on which to expand your knowledge of
pressure in this space is figured by subtracting the
electrical and mechanical fundamentals. It is
measured vacuum (10 in.Hg) from the nearly
important that you have a sound understanding
perfect vacuum (29.92 in.Hg). The absolute
of these laws and principles. The complex
pressure then will be 19.92 or approximately 20
electrical systems and the internal pressure-
in.Hg absolute. It is important to note that the
temperature relationships in a simple GTE make
amount of pressure in a space under vacuum can
it imperative that you understand the material
only be expressed in terms of absolute pressure.
presented. If you have problems understanding
You may have noticed that sometimes we use
this material, you should reread the pertinent
the letters psig to indicate gauge pressure and
portions until you have absorbed the basic
other times we merely use psi. By common
concepts.
convention, gauge pressure is always assumed
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