which can also affect the altimeter reading if different from the 75° F. at which the instrument was calibrated. The
altimeters are calibrated so that the error is small. However, when maximum accuracy is desired, this correction should be
e. Barometric pressure changes will affect the altimeter just as altitude changes do. Since the atmosphere is
continually changing, it is necessary to take pressure changes into consideration. The success with which pressure
changes are evaluated will determine the accuracy of the altimeter survey. During inclement weather and when there are
high, gusty winds, atmospheric conditions are very unstable and altimetry will not yield accurate results.
If reasonable limitations and precautions are observed, a majority of the elevations will be found correct within
one meter and the maximum error will seldom exceed three meters.
Pressure Sensing Element
a. The pressure sensing element is a capsule which has one end secured in a fixed position by means of an
anchor block fastened to the mounting plate. The other end is free and is linked to the pointer. Since the case is vented,
the external pressure exerted on the capsule is the atmospheric pressure. Should the atmospheric pressure decrease,
the capsule expands; should it increase, the capsule contracts. In this manner, changes in atmospheric pressure result in
movement of the free end of the capsule. This movement is transmitted to the pointer through a linkage consisting