21. ADJUSTING BAROMETER. a. Comparative readings. (1) Before
adjusting the aneroid barometer, make at least six comparative read-
ings with an accurate mercury barometer. These readings should be
made at intervals during a period of 1 to 2 hours. If time permits,
even more readings should be made since the greater the number of
such comparative readings, the higher the degree of accuracy in the
final results. Take the average of the readings of each barometer.
(2) Subtract the average of the aneroid readings from the average
of the mercury readings. If the average reading of the aneroid exceeds
that of the mercury barometer, the difference should have a negative
(3) If the difference exceeds 0 .3 millibar (0 .01 inch) adjust the
aneroid by the amount of the difference (see b below).
(a) If the difference has a positive value, the reading of the aneroid
should be increased.
(b) If the difference is negative, the aneroid reading should be de-
Note. It is doubtful whether improved accuracy can be obtained by making an
adjustment of less than 0.3 millibar since the random error of the instrument is
probably as much as 0.3 millibar.
b. Adjusting Barometers ML-102-B, ML-102-E, and ML-102-F. (1)
Insert a screw driver in the slot which turns the worm of the gear plate
(fig. 10). Rotate
the gear plate
of this worm until
sure indicated by the aneroid has been changed by the amount of the
average difference as determined in a above.
(2) Turn the screw clockwise to decrease, and counterclockwise to
increase the indicated pressure.
c. Adjusting Barometers ML-102-D and ML-316/TM. (1) Remove
the plastic screw-plug from the plastic dial cover.
(2) Insert a small-bladed screw driver in the slot of the adjusting
disk. A clockwise rotation of this disk causes a like movement of the
pointer. This adjustment is limited to one-half revolution of the disk
either side of the index line marked on the face of the dial (fig. 3) or