Gravity Adjustment and Correction Chart.

TM 9-6685-202-14

clockwise rotation moves the temperature index toward

Fasten the mounting platform to the barometer by

a lower temperature, while a counterclockwise rotation

inserting the two 5/16-24 socket head screws through

raises it. With the temperature and gravity indexes

the holes in the barometer base assembly and into the

properly set, the reading of all models of the barometer

tapped holes in the platform assembly.

will be true pressure, reduced to standard temperature

(25 ) and standard gravity (980.665) cm/sec2. The

(b) With the pump still running at the end of two

barometer is now ready for operation.

hours, return the entire instrument to its normal vertical

position. The mercury will rise in the barometer tube to

(3) Gravity Adjustment and Correction Chart.

the same level as in the cistern.

The purpose of the gravity adjustment is to set the

(c) Slowly open the clamp on the tube connected to

correct gravity correction for the location at which the

the cistern nipple. This will allow the mercury to rise in

instrument is used. The proper value of gravity to be

the barometer tube until it attains a level depending

used in making the setting is sometimes specified for

upon the existing atmospheric pressure.

the station at which the instrument is used. If the value

is not available, obtain it from the gravity correction

(d) Connect hand blower 6640-5104-521 (found in

chart, figure 3-2, which is a plot of the values of gravity

the mounting platform drawer) to the cistern nipple and

against latitude. Since gravity varies with elevation,

pump air into the cistern until the mercury in the

several curves are given, each representing a different

barometer tube rises to the level of the orange

elevation. The elevation is marked on the curve for sea

calibration line etched into the tube (visible through the

level, 5,000 feet, and 10,000 feet; with intermediate

cut-out in the top of the barometer). Close the tube seal

curves for each 1,000 feet elevation. Values of latitude

valve by unscrewing tube seal knob at the top of the

are found at the bottom of the chart, and values of

barometer. Remove the hand blower, and the mercury

gravity are found at the left of the chart. To use the

will fall to the atmospheric pressure level. Remove the

chart, the latitude and elevation must be known. Find

vacuum pump from the tube-evacuation nipple. Install

the latitude at the bottom of the chart. Follow a vertical

the meniscus magnifier on the post of the sighting ring

line until it intersects the proper elevation line. Then

slide.

follow the horizontal line to the left to read the value of

gravity to which the gravity index should be set.

(e) Level the mounting platform by means of the

three leveling screws, using the indication of the two

levels in the mount platform. Clockwise rotation of the

b. Type A-1 (Modified). A power-driven rotary

leveling screws raises the platform.

vacuum pump and a thermoconductivity-type vacuum

gage are supplied with the modified barometer. See

paragraph 3-7 for pump operation. The barometer

NOTE

achieves its best accuracy and ease of reading when

installed in a well-lighted, well-ventilated, or air-

The instrument must be maintained

conditioned room. The instrument should be placed at a

level within 30 seconds of arc to

point of minimum vibration near a wall or supporting

reduce leveling errors to below 0.02

column, allowing at least 10 square feet of floor space.

mm of mercury. This is done by

A sturdy table about 30 inches high is desirable.

centering the bubbles within one

division on the level scales.

NOTE

(f) Loosen the two screws which hold

If required, fill the barometer in

the temperature-gravity scale (fig. 1-3)and slide it to

accordance with paragraph 4-5b.

align the GRAVITY index with the value of gravity at the

location of the barometer. If the latitude and elevation

(1) Removal of Moisture. To remove traces

are known, the value of gravity can be determined in

of moisture which may have entered the barometer tube

accordance with instructions in paragraph 3-5 a(3).

during shipment, proceed as follows:

Tighten the two screws securely. Once properly set, the

gravity index is not changed unless the barometer is

shipped to another location.

(g) Align the temperature index to the

existing temperature, as indicated by the thermometer,

by rotating the temperature adjusting knob (fig. 1-3). A

3-4