ALTIMETERS

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A pressure altimeter (also called barometric altimeter) is the altimeter found in most aircraft.

In it, an aneroid barometer measures the atmospheric pressure from a static port outside the aircraft.

Air pressure decreases with an increase of altitude—approximately 100 hectopascals per 800 meters or one inch of mercury per 1000 feet near sea level.

The altimeter is calibrated to show the pressure directly as an altitude above mean sea level,

in accordance with a mathematical model defined by the International Standard Atmosphere (ISA).

Older aircraft used a simple aneroid barometer where the needle made less than one revolution around the face from zero to full scale.

Modern aircraft use a ,,sensitive altimeter,, which has a primary needle that makes multiple revolutions,

and one or more secondary needles that show the number of revolutions, similar to a clock face.

In other words, each needle points to a different digit of the current altitude measurement.Diagram showing the internal components of the sensitive aircraft altimeter.

On a sensitive altimeter, the sea level reference pressure can be adjusted by a setting knob.

The reference pressure, in inches of mercury in Canada and the US and hectopascals (previously millibars) elsewhere, is displayed in the Kollsman window, visible at the right side of the aircraft altimeter shown here.

This is necessary, since sea level reference atmospheric pressure varies with temperature and the movement of pressure systems in the atmosphere.

In aviation terminology, the regional or local air pressure at mean sea level (MSL) is called the QNH or ,,altimeter setting,, and the pressure which will calibrate the altimeter to show the height above ground at a given airfield is called the QFE of the field.

Pressure indicating window in milibars :

 

 

 

 

 

In attach below you can see, how complicated is altimeter systems :

 

 

Two pictures below show you difference between 980hPa and 1013.25hPa pressure in Kollsmans window.

First .. pressure in window is set to 980hPa .. altimeter redings is " 0 " feet.

 

Second .. pressure in window is set to 1013.25hPa .. altimeter redings is "930" feet. ( In this window pressure we calibrate altimeters ).

 

The figure below, let me show you what they look like two altimeters altitude the same value, but each has a different scale of the tender pressure. One of the Anglo-Saxon value in / Hg and the other one mbar. Both devices are then displayed differently even tens of thousands of feet range. On the left hand is small, then on the right is thin and long, still finished with a triangular symbol ..

  

 

 

 

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