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PRESSURE  INSTRUMENTS
NAVY DIGITAL ALTIMETER SETTING INDICATOR  (DASI)

Aerographers Mate, Module 01-Surface Weather Observations
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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. 2-14







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