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TM 9-6685-202-14
for use as a standard for calibrating altimeters in the
position of the vernier slide assembly along the scale is
range of -1000 feet to + 80,000 feet. The barometer
then read by means of the scale index.  Refer to
accurately  measures  atmospheric  and  pneumatic
paragraphs 3-9 and 3-10 for sighting the mercury level
pressures from zero to 790 millimeters (mm) of mercury
and reading the scale and vernier.
absolute. The barometer is accurate within 0.2 mm of
mercury without the use of charts. Using a calibration
2-4.  Computing-Correcting Mechanism (Type A-1
chart, the tolerance is less than 0.1 mm, equivalent to
3.5 feet at sea level.  The accuracy is within 0.0015
percent of full scale in terms of barometric pressure.
When the temperature and gravity indexes are
The barometer is equipped with a compensating system
alined for standard conditions, the cam bar is in a
which automatically computes and applies corrections
vertical position.
When the indexes are set to
for deviations from standard temperature and gravity. It
nonstandard conditions, the upper end of the cam bar is
is also possible to check the functioning of the
displaced to the right or left of vertical depending on the
barometer at all points where error could be introduced.
direction of the deviation from standard conditions. The
For checking the level of mercury in the system or
displacement  of  the  upper  end  will  cause  a
checking the corrections for temperature and gravity by
displacement all along the bar which will be proportional
comparison with the computed values, see paragraph 4-
at any point to the distance of that point from the lower
end of the bar which is pivoted. The magnitude of the
displacement of the upper end is proportional to the
magnitude of the deviation from standard temperature
b. Type A-1 (Modified). The modified barometer is
and gravity as set on the temperature and gravity
designed  for  use  as  a  standard  for  calibrating
scales. The displacement from the vertical at any point
barometers and other pressure measuring devices with
along the cam bar will be proportional to the deviation of
the same or lesser accuracy. The barometer accurately
the temperature index from 25 ; the deviation of the
measures atmospheric and pneumatic pressures from
gravity index from 980.665 cm/sec ; and the height of
zero to 790 millimeters of mercury absolute and
the mercury column represented by the distance along
differential. The barometer is accurate within + 0.014%
the cam bar from its pivoted end.
Since the
of reading +0.092 millimeters of mercury (torr).
temperature and gravity errors of the barometer are also
proportional to these same factors, the displacement of
the cam bar from the vertical at any point is a measure
of the temperature and gravity errors of the barometer
The main scale is graduated to 820
at that point. Thus the errors have been computed by
mm.  However, due to mechanical
the cam bar and translated in terms of horizontal
limitations the slide will only move to
790 mm.
2-5. Application of Corrections (Type A-1 Only)
2-3. Mercury Column and Vernier Scale (All Models)
The horizontal displacement of the cam bar is
The height of the mercury column is read by
detected by the correcting lever whose pivot is on the
means of the sighting ring, scale, and scale index or
vernier slide assembly, and therefore, moves vertically
vernier scale (fig.  1-2).  The scale is parallel and
with the slide. The correcting lever is similar to a bell
adjacent to the mercury column.  The sighting ring
crank, with two arms arranged at right angles, pivoting
surrounds the column and is mounted on the vernier
at the elbow. The cam follower pins are attached to the
slide assembly which slides along the scale.  The
lower arm of the correcting lever and maintained in
vernier scale is also part of the vernier slide assembly
contact with the cam bar by a spring. The other arm of
and is used to read the position of the slide assembly on
the correcting lever carries the correcting pin which
the scale. The height of the column of mercury is read
engages the sight-
by positioning the lower edge of the sighting ring even
with the highest point of the mercury column.  The

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