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TM 9-6685-202-14, C1
point and its image reflected off the mercury surface
(5) Using  the  micrometer,  measure  the
appear to just touch by observing at a slight grazing
cistern mercury height and record the value as "hc."
angle.
(6) Substitute the values recorded in (1), (2),
b.  The sleeve of the micrometer is calibrated in
(4), and (5) above in the following equation and add
millimeters from 0 to 25 millimeters. Only every tenth
algebraically:
line is numbered, making each line represent 0.5
millimeter.
The thimble is graduated into 250
increments with only every tenth line numbered from 0
(7) Solving the above equation provides the
to 48 (the 50 and 0 position is the same point). One
atmospheric pressure in millimeters of mercury.
complete revolution of the thimble advances the sleeve
NOTE
0.5 mm along the sleeve. Since the graduations on the
1
"C " is a factor used to compensate for deviatlons from
thimble divide each 0.5 millimeter increment on the
standard temperature and gravity. The value of "C1"will
sleeve into 250 parts, the micrometer can be read to
normally be furnished to each using activity by the
0.002 mm.
calibration activity. It this value is not available, It may
3-12. Operation as a barometer
be determined using the equation:
a. Type A-1.
After the instrument has been
prepared in accordance with paragraph 3-5 a, it is ready
for use as a barometer. Precede all readings with the
following checks:
(1) Insure that the temperature adjustment
agrees  with  the  temperature  indicated  on  the
thermometer provided.
(2) Insure that the base is level. The bubbles
must be centered within one division of the leveling
scales.
(3) Rap the table to adjust the meniscus.
(4) Read the mercury level in accordance
WARNING
When  using  externally  applied
pressure, check that all connections
NOTE
are securely sealed.
ha + hb is the average zero reading.
b. Type A-1 Modified.  After the instrument has
A zero reading (ha + hb) is taken
been prepared in accordance with paragraph 3-5b, it is
before and after the measurements
ready for use as a barometer.
and then averaged and applied to the
(1) With the barometer in a level position and
measurements per above equation.
no pressure difference indicated on the barometer,
3-13.
Operation  as  an  Altimeter  Calibration
observe the mercury height in the barometer tube and
Instrument
record the value as "hb." Read the scale and vernier in
a. Type A-1.  The barometer will measure the
accordance with paragraph 3-10 b.
range of -1000 feet to +80,000 feet when utilized as an
(2) Measure cistern mercury height, using
altimeter calibration instrument.
micrometer, in accordance with paragraph 3-11. Record
(1) Connect the barometer into the pneumatic
micrometer indication
as "ha."
(2) Level the barometer and adjust the
(3) Energize vacuum pump until vacuum
temperature index to the existing temperature.  It is
gage indicates 20 millitorrs (microns).  Maintain a
assumed that the gravity index was correctly positioned
vacuum on the barometer tube during measurements.
in preparing the barometer for use.
(4) Read the main scale and vernier and
(3) Adjust the vernier slide to the desired
record the value as "ht."
pressure-altitude by unlocking the vernier clamp nut,
sliding the vernier slide to approximate setting, locking
the vernier clamp nut, and making the final adjustment
3-14

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