9. ALIGNING POINTER. a. Tap the instrument lightly to see that the
pointer is free and in equilibrium. Have the dial lighted so that the
image of the pointer is clearly visible in the mirror ring between the
scales.
b. For a correct reading of the aneroid, when it is vertical, the eye
must be on a level with the pointer. Never stand on tiptoe or look
down at the dial of a barometer that is hanging on the wall. When the
instrument is read with the dial in a horizontal position, have the eye
directly above the pointer. To read the instrument correctly, have the
eye in a direct line so that the image of the pointer in the mirror ring
is obscured by the pointer itself (fig. 4) . This is important in reading
the aneroid since the pointer is approximately 3/32 inch above the
dial and unless it is aligned so that it completely covers the image,
parallax occurs and an erroneous reading results. In reading an aneroid
barometer, parallax is the apparent displacement of the pointer with
reference to the dial graduations when the pointer is viewed from the
side rather than from directly above. This effect can readily be seen
by moving the head to either side of the correct position and noticing
the difference in the apparent indication of the pointer.
Note. 1. If the barometer is transported by air, or there is a rapid change of pressure
of 100 millibars or more, wait at least 24 hours before taking a reading in order to
reduce hysteresis errors (par. l5a).
2. If the temperature of the instrument is changed suddenly by an amount exceeding
10 F., wait 1 hours before taking a reading.
3. These barometers may be read in inches or millibars.
4. Tap the instrument lightly with the fingers before reading, to reduce residual
friction in the linkage system.
a. Inch scale. (1) The outside scale is the inch scale. The inch scale
may be used over a pressure range corresponding to 31.5 to 22 inches
of mercury. The pointer makes more than one revolution of the dial in
covering this range. A pressure of 25 inches and less is not marked on
the dial. After the pointer has made one complete revolution, pressure
values are obtained by subtracting six inches from the indicated value.
Thus, on the second revolution 31 inches represents 25 inches of pres-
sure, 30 inches represents 24 inches, 29 inches represents 23 inches, and
28 inches represents 22 inches of pressure. The observer must know the
approximate elevation of the station in order to know which inch read-
ing to use.
(2) The inch scale is graduated in 0.02 inch intervals. Integral inches
an tenth inches are numbered. With practice, it is possible to make
an estimated reading of the barometer to the nearest thousandth of
an inch. Even with care, however, this estimated reading probably will
be in error by as much as 0.003 inch.
(3) In figure 2 the inch reading is 29.9, plus 0.06 (since the pointer
is beyond three graduations, each of which represents 0.02) ; the pointer
is also past the center of the next division which increases the reading
by an additional 0.01. Since it is slightly past the center, the reading
is increased by an additional 0.002 or 0.003. Thus the approximated
reading is 29.972 Or 29.973 inches.
(4) Assuming that the pointer has made one revolution and is
registering on the inner scale, the reading is 23.972 or 23.973 inches.
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