sure in whole millibars. While the range of the millibar scale is from
1 0 85 to 745 millibars, these barometers are calibrated only to 1066.7
millibars (or 31.5 inches). This does not mean that higher pressure
readings will be inaccurate, but only that the accuracy has not been
checked above 1066.7 millibars.
(2) In covering the range of the millibar scale, the pointer makes
almost two revolutions of the dial. Unlike the inch scale, the same
scale is not used on the second revolution of the pointer. Pressures
b e t w e e n 108 5 and 895 millibars are read on the center scale, while
pressures between 895 and 745 millibars are read on the inside scale.
(3) The millibar scales are graduated in whole millibars and esti-
mated readings may be made to a tenth of a millibar.
(4) In figure 2 the reading on the outer scale is 101 5 .0 millibars.
The reading on the inner scale is 81 1.8 millibars.
c. Station pressure. (1) The pressure, as read on the dial of Barome-
ter ML-1 0 2-B or ML-1 0 2-F is the station pressure.
(2) The pressure, as read on the dial of Barometer ML-102 -E must
be corrected for temperature (see par. 11 below) before the station
pressure is obtained.
11. CORRECTING READING
OF BAROMETER ML-102-E
Note. Barometer ML- 102 -E must have a temperature correction applied to each
reading. Barometers ML-102 -B and ML-102 -F are equally affected by temperature
but temperature correction charts were not required, hence pressure data obtained
from these two barometers generally will not be as accurate as that obtained from
B a r o m e t e r s ML- 102-E, ML-102-D and ML-316/ T M .
a. Chart. The effect of temperature upon the indicated reading of
B a r o m e t e r ML- 1 0 2-E has been determined by actual test for each
instrument and the corrections are shown in the form of curves on
the temperature calibration chart fastened to the back of each instru-
ment (fig. 5 ) . These curves also include a small residual error for
each instrument (par. 1 5 C (2 ) ) .
b. Method. (1) Three temperature curves are given on the graph:
one for a temperature of 80 F., another for 40 F., and a third for
o F. These curves are plotted on a grid on which the pressure is
shown on the horizontal scale and the corrections on the vertical scale.
(2) To compute the temperature correction, determine the tem-
perature to which the barometer has been exposed and locate the
temperature curve nearest to that temperature.
(3) Find the indicated pressure on the horizontal scale. Move ver-
tically until the pressure value meets the correction curve. From the
point on the correction curve move horizontally to the left until the
correction scale is encountered. If the pressure reading does not meet
the temperature correction curve on a horizontal line, it will be nec-
essary to estimate between the two tabulated correction values. Deter-
mine the correction at that point, noting whether it is plus or minus.
(.4) Add or subtract this sum to the observed pressure reading.
Caution: If the temperature is changed very suddenly, wait at least
1 hours before making a reading.
c. Interpolated correction. (1) When the temperature falls between
the values for which correction curves are given, the corrections must
be determined for both curves and the true correction by interpolation.