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TM 9-6685-202-14
CHAPTER 2
THEORY OF OPERATION
Section I. GENERAL OPERATION
terms of acceleration (centimeters per second per
2-1. Fundamental Principles
second).  The numerical value of standard gravity is
980.665. The magnitude of the temperature and gravity
a. Basic Principle. The basic principle of the type
errors at any given reading depends on the deviation
A-1 barometer is similar to that of any standard
from standard temperature and standard gravity, and on
mercurial barometer.  The atmospheric or pneumatic
the observed height of the mercury column. The errors,
pressure applied to the surface of the mercury in a
therefore, increase in magnitude as the length of the
cistern is automatically balanced by the weight of a
mercury column increases.
vertical column of mercury enclosed in a glass tube.
The space above the mercury in the glass tube is
evacuated to minimize any force opposing the pressure
c. Compensation Features
being measured, other than the force produced by
weight of the column of mercury. Since the weight of a
(1) The type A-1 barometer has a computing-
column of mercury of uniform cross section varies
correcting mechanism which applies compensation for
directly as its length, the magnitude of the pressure
temperature and gravity errors when the temperature
applied to the cistern may be measured in terms of
and gravity indexes are set for the existing conditions.
length (usually referred to as the height) of the column
The mechanism compensates for any temperature from
of mercury required to balance it.
The height
0to 40C (32to 104F), and for any value of gravity
corresponding to normal atmospheric pressure at sea
from 977 to 982 cm/sec2.
The accuracy of the
level is 760 millimeters of mercury, or 29.922 inches of
compensation limits the accuracy of the barometer
mercury or 1013.3 millibars.
readings.
Applicable  specifications  call  for  the
temperature and gravity compensation to be correct to
within 0.10 mm of mercury.  The scale, therefore,
b. Inherent Limitations of a Barometer.
It is
indicates  true  pressure  as  reduced  to  standard
evident that as the mercury column expands due to an
conditions.
increase in temperature, the length of a column of given
weight will increase. The same is true for a decrease of
(2) The modified barometer does not have a
gravitational force.  Part of the error caused by the
computing-correcting mechanism. A true-length, more
temperature expansion is canceled because the scale
finely divided scale; a precision micrometer; and a
used to measure the height of the mercury column also
vacuum gage have been added to the modified
expands. The two effects are not equal however, and a
barometer. By substituting the values indicated on the
net temperature error remains. Therefore, if the height
scale and micrometer along with local values of gravity
of the mercury column is to be a true measure of the
and temperature and standard values of gravity and
pneumatic pressure, the observed readings must be
temperature into an equation, the errors normally
corrected to the values which would exist if the readings
associated with mechanical corrective devices is
were taken with the barometer maintained at a constant
eliminated.
standard temperature and standard gravity.
Zero
degrees Centigrade (32F) is universally adopted as the
standard temperature. Standard gravity is expressed in
Section II. FUNCTIONAL DESCRIPTION
2-2. General
a. Type A-1. The barometer is designed
2-1

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