pressure-measuring instrument at some stations. Many
shore stations also have a Navy digital altimeter setting
indicator (NDASI or DASI).
The Automated Surface Observing System (ASOS)
equipment also provides pressure readings and
becomes the primary pressure-measuring instrument as
it is installed. Then, the ML-448/UM aneroid is
retained as a backup instrument. Use of the NDASI
may be discontinued.
PRECISION ANEROID BAROMETER
The ML-448/UM precision aneroid barometer is
shown in figure 2-20. The barometer is mounted in
either a brass case or a black, hard, plastic case.
The precision aneroid barometer is designed to
accurately indicate atmospheric pressure in inches and
hectopascals (hPa). The pressure element is a Sylphon
cell, which consists of a sealed, bellows-shaped canister
that expands and contracts with changes in air pressure.
Gears and linkage arms transfer changes in size of the
Sylphon cell into indicator movement. The gears and
linkage also correct the movement for changes in
temperature. The ML-448/UM has a range from 910
hPa (26.9 inches) to 1,060 hPa (31.3 inches), with an
acceptable accuracy of ±l .0) hPa.
Precision aneroid barometers must be calibrated
twice a year when used aboard ship, and once a year
when used at shore stations. Basic guidance for the
barometer calibration program is provided in
NAVMETOCCOMINST 13950.3, Naval Meteorology
and Oceanography Command Barometer Calibration
Program.
Instructions for use of aneroid barometers during a
surface aviation observation are provided in both
NAVMETOCCOMINST 3141.2 and NAVMETOC-
COMINST 3144.1. To obtain a proper pressure reading
using any aneroid barometer, use the following steps:
Figure 2-20.—ML-448/UM precision aneroid barometer.
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