some permanent damage and a new barometer should be requisitioned.
c. Testing linkage. The linkage system of an aneroid barometer must
be as friction-free as possible if the barometer is to respond to small
changes in pressure. Anything which increases this friction decreases
the sensitivity of the instrument very rapidly. In regions of prevailing
high humidity the tests given in (1), (2), and (3) below, should be
made at least twice a month. In other regions, monthly tests should
(1) Tests for all models. The simplest test for the proper function-
ing of the linkage system is to note the change in indication of the
aneroid when its elevation is changed approximately 15 feet.
(a) Read the barometer very carefully at the lower elevation.
(b) Holding the barometer steadily in both hands and being careful
not to jar or move it suddenly, walk upstairs, or otherwise change its
elevation approximately 15 feet. The pointer should indicate the
changed elevation by a decrease in pressure as the elevation is increased.
(c). Now return the barometer to the original elevation and again
take the reading. The pointer should have returned to its original
(2) Amount of change. Generally, there will be a change of approxi-
mately one-tenth millibar for every 3 feet of change in elevation. Any
barometer that does not respond to a change of 15 feet in elevation
is probably defective and should be returned to the depot.
(3) Barometers ML- 102 -D and ML-316 /TM. (a) These models
are more sensitive than other types to. slight changes of pressure. A
change in elevation of as little as 3 feet should cause a slightly different
indication of the pointer. This is sometimes hard to discern and the
least vibration will destroy the value of this test.
(b) Another and easier test for these models consists of the follow-
ing: Pick up the barometer case in both hands. With the thumbs in
the center of the plastic window, suddenly press inward a slight
amount. If the barometer is functioning properly, the pointer will
quickly move a small amount to indicate a higher pressure, then as
quickly return to its original position. Remove the thumbs suddenly.
The pointer should move slightly in the opposite direction to indi-
cate a decrease in pressure and then quickly return to its original
Warning: Never open the case of Barometers ML-102-B, ML-102-E,
or ML-102 -F. Any adjustment that can be made in the field can be
made without opening the case. B a r o m e t e r ML- 1 0 2- D and ML-
316 /TM may be opened to place the mechanism in a new case or to
make adjustments in an emergency, as outlined in paragraph 23. Except
for these adjustments, the case must not be opened. These barometers
are precision instruments. Tampering with the internal mechanism or
attempting to adjust the linkage or magnification system will make it
necessary to recalibrate the instrument. This CANNOT be done by
Signal Corps repair facilities. Recalibration involves the use of special-
ized auxiliary recision equipment and highly skilled personnel avail-
able only at the manufacturer's plant. The adjustments and replace-
ments which are allowed on these barometers are given in the follow-
ing paragraphs. Using personnel are instructed to confine themselves
to these operations.