Additional Information; Altimeter; How Does An Altimeter Work - Brauniger IQ-BASIC-GPS Operation Manual

Operation Manual IQ-BASIC-GPS
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9

Additional Information

9.1

Altimeter

9.1.1

How does an altimeter work?

An altimeter is really a barograph because it doesn't directly measure altitude, but air pressure.
The barometric altimeter calculates altitude from the prevailing air pressure of the atmosphere.
However, air pressure decreases with increasing altitude.
Why does pressure change with height?
Air pressure at any given point on the earth is created by the weight of air in the atmosphere above it.
As air can be compressed, the pressure drop is not linear, but indeed exponential. For this reason
air pressure is decreasing in altitude – one has less air above the head!
In practice, however, it is not as simple as that because of the many other factors that influence air
pressure. Therefore, air pressure is also depending on temperature and of course, on weather
conditions. On a stable day, temperature induced barometric variations of 1mbar may occur, which
corresponds to an altitude change of approx. ± 10 . Depending on weather conditions, air pressure
at sea level (QNH) may vary from 950 mbar to 1050 mbar. In order to eliminate this influence of
weather, the altimeter has to be calibrated at certain intervals. This means that the altimeter has
to be set to a known altitude and that it must then show this value.
The basis for altitude calculation in aviation is an international formula which defines a standard
atmosphere.
In the CINA- Standard atmosphere the basic pressure on sea level is 1013,25 hPa (Hektopascal) at a
temperature of 15°C. Furthermore it defines a continuous temperature decrease at increasing
height of 0,65°C per 100m ascent.
Therefore is binding: a barometric aviation altimeter displays the precise altitude only if weather
conditions are in exact accordance to the standard atmosphere. In practice, such compliance is
more likely to be the exception!
Air weight and pressure are strongly influenced by air temperature. If temperature deviates
from standard atmosphere, the display of altitude calculated as per the international formula
is no longer correct. The altimeter displays during summer, when temperatures are higher, indeed
altitude parameters which are too low, and during wintertime it is exactly the contrary!
Flying at lower temperatures is effectively done at lower altitude, and at higher temperatures
flight altitude is higher than the altimeter displays! The deviation of 1 °C per 1000 height meters
induces approx. 4 m error. (This empirical formula is valid for up to 4000m!)
If you are flying during summer through 2000 height meters in an air mass being too warm
by 16 °C compared to standard atmosphere, the altimeter will then display 2 x 4 x 16 = 128m
difference in altitude below real height! Based upon the internationally determined altitude calculation
with standard values, this display error caused by air temperature shall not be rectified by the
instrument.
Air pressure changes in relation to weather conditions. In order to compensate for display
variations, an altimeter always needs to be gauged. This means that the altimeter has to
be set precisely before take-off for any flight to a well-known altitude value. The IQ-BASIC-GPS
sets altitude after energising automatically to GPS-altitude. However, it may also be subject
to variations.
Caution: the atmospheric pressure may change during the timeline of one day up to five Hektopascal
(for ex. cold front). As a result this is after all the equivalent of more than 40 meters height difference.
There is another possibility to gauge the altimeter which is to enter the current QNH pressure value.
The QNH (Question Normal Height) applied in aviation matches the current local air pressure, as it
would be at sea level, so that the altimeter would indeed display 0m.
Due to this process the local pressure data recorded by the various measurement stations is area-wide
comparable, irrespective of the geographical height.
The QNH-value is subject to be continuously updated and may be read in the flight weather report,
or required by radio from airfields, or by enquiry on the Internet.
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