Operation as a barometer

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

"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

with paragraph 3-10.

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

system (fig. 3-8).

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