AEROGRAPHER'S MATE 3 & 2 



instrument known as the aneroid barometer. 

 The term "aneroid" means without fluid. The 

 aneroid barometer , then, i6 a fluidless barometer, 

 utilizing the change in shape of an evac- 

 uated metal cell to measure variations in atmos- 

 pheric pressure. 



The aneroid barometer gets its name from 

 the pressure-sensitive element used in the in- 

 strument. It is an aneroid, which is a thin- 

 walled metal capsule or cell, sometimes called 

 a diaphragm, that has been either partially or 

 completely evacuated of air. The aneroid is 

 usually made of beryllium copper or phosphor 

 bronze. Most aneroid cells in the currently 

 used aneroid barometers are self-supporting, 

 and do not require external or internal springs 

 to prevent the crushing of the cell walls by 

 atmospheric pressure. 



In a common type of single aneroid cell 

 barometer, the top of the evacuated cell is 

 secured to a suitable linkage which transmits 

 the motion of the aneroid to an index hand or 

 pointer, which indicates the pressure. (See fig. 

 2-6.) 



Figure 2-6 <— Simple diagram 

 barometer. 



of the 



209.92 

 aneroid 



Precision Aneroid 

 Barometer (ML-448/UM) 



The Precision Aneroid Barometer (ML- 

 448/UM) is used aboard ship and at land stations. 



Of precision design and manufacture, the 

 precision aneroid barometer is constructed to 

 accurately indicate atmospheric pressure in 

 millibars or inches of mercury. (See fig. 2-7.) 



The pressure element of the precision aneroid 

 is a Sylphon cell, which consists of a bellows- 

 shaped metal cell having an internal spring to 

 provide pressure calibration. This element is 

 sensitive to minute variations in atmospheric 

 pressure. The Sylphon cell is connected to an 

 indicating pointer or index by means of a quadrant 

 gear and lever system in such a manner that 

 the movement of the cell, for a given change 

 in atmospheric pressure, is greatly magnified 

 by the linkage. This pressure variation is then 

 transmitted to the index hand or pointer with 

 a minimum of friction in the moving parts. 

 The instrument has a range from 910 to 1060 

 mb, and it is accurate to 0.67 mb. Outside 

 normal sea level pressure range it is still 

 accurate to within 1.0 mb. 



The precision aneroid barometer is compen- 

 sated for temperature changes; therefore, the 

 indicated readings require no temperature cor- 

 rections as are required for the mercurial 

 barometer. 



Aneroid barometers utilize spring pressure 

 to balance the effect of the air pressure on the 

 Sylphon cell. Therefore, no corrections for effect 

 of latitude (gravity) need be applied. 



The pressure element, dial, and linkage are 

 mounted on a sturdy metal frame that, in turn, 

 is shock-spring suspended from the aneroid case. 

 This spring suspension minimizes the effect of 

 jars or shocks that would otherwise affect the 

 linkage and index setting of the barometer. 



A screwdriver adjustment, located at the base 

 of the Sylphon cell, is provided to make adjust- 

 ments to the pressure readings of the instrument. 



The precision aneroid barometer, when prop- 

 erly calibrated and set to station pressure, is 

 used for observational purposes in lieu of the' 

 mercurial barometer. 



After the precision aneroid baromet.?- ,-. 

 installed in a permanent location, a series of 

 comparative readings are taken. These com- 

 parative readings are the differences between 

 the station pressure taken from the aneroid 

 barometer and the station pressure from the 



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