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Page Title: Gravity Adjustment and Correction Chart.
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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
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
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.
(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

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