Barometric altimeterBarometric altitude gauge is a measuring device which is mostly used in aviation as a pilot-navigation gauge. It is often called barometric altimeter. It is used for determining relative and absolute flight altitude.That means it determines altitude in relation to a certain reference point (for example level of the airport), or in relation to a sea level - for absolute altitude. Generally, it is a barometer-aneroid with a scale graded not for pressure but for altitude.So how does it work? It is known that atmospheric pressure drops at altitude gain. Let's say, if we know pressure at ground surface and measure it at flight altitude then we can get their difference that will indicate distance between the two points, ie flight altitude.Pressure values at airfield or sea levels should be acquired from ground services through radio communication, and then entered into the device manually. It is extremely important to do that right as this affects reading accuracy of the meter.Besides, atmospheric pressure at sea level changes between 950 and 1050 millibars which depends on weather conditions. Even on a stable day atmospheric pressure "floats" ±1 millibar due to air temperature. This actually corresponds to altitude change of ±8 meters. That means that the meter has to be recalibrated from time to time.So why won't pilots just use radio altimeter which works according to the principle of a radio pilot. Then they would know altitude above the current point ...
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