Mercury Column and Vernier Scale (All Models)

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

Only)

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

NOTE

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

displacement.

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

2-2