An aneroid barometer is an instrument used to measure atmospheric pressure. The name “ANEROID” itself means ‘without water’ , that gives clear indication that there is no involvement of liquid.
A metal box partially exhausted of air (vaccum) will undergo changes of shape as the outside pressure varies. These small movements of the box are transmitted to a pointer displayed on the face of the aneroid barometer by a system of levers.
Construction & Working:
Its construction includes a thin metal chamber which is sealed but having partial vacuum inside. This chamber is fitted with certain levers and spring mechanism to which a needle is connected , which shows the barometer readings in milibars or hectopascals projected on a graduated circular card. The thin metal chamber has elastic properties , when atmospheric pressure increases this chamber gets compressed , when atmospheric pressure decreases , the chamber gets expanded , this inward and out ward movement of the chamber acts on the needle mechanically and the needle then registers a reading on the graduated card. The accuracy of the reading depends upon the size of the metal chamber , if the chamber is large , accuracy will be better.
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Height above sea level
With the increase in the altitude, the air pressure reduces. Hence, the reading is then adjusted to an equivalent sea-level pressure for purposes of reporting. Aneroid barometers have a mechanical adjustment that allows the equivalent sea level pressure to be read directly and without further adjustment if the instrument is not moved to a different altitude.
The elasticity of the barometer capsule varies depending on temperature. A bimetallic plate is used for temperature compensation. Once pressure distort an elastic body, it doesn’t completely return to its original shape even after the pressure is relieved. Due to this characteristics called hysteresis, because of the changes in vacuum chamber’s elasticity over a period of time, index error occurs and error is accounted as perits prefix given i.e. + or -. It can be found on certificate issued after callibration of the barometer.