Chapter 2. THEORY OF OPERATION

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