Flytec 3040/TT34 * - Manual (Page 6)

Operation manual Flytec 3040 Balloon 5
4. The Altimeter
4.1 How does an altimeter work?
A barometric altimeter calculates altitude (elevation) from the actual air pressure of the
atmosphere at a given location. Air pressure decreases with increasing elevation,
however, since air is compressible, the pressure change is exponential not linear.
Altimeters designed for aviation use the CINA (Commision International de Navigation
Aérienne) formula to derive altitude from air pressure. In this calculation the CINA–
atmosphere is used where standard atmospheric pressure at sea level is 1013.25 hPa
(Hecto-pascal) at a temperature of 15°C. Temperature also decreases with increasing
altitude and must also be considered in the altitude calculation. A constant temperature
decrease of 0.65°C per 100m ascent is also assumed in the CINA equation. Because of
these assumptions with respect to pressure and temperature a barometric aviation
altimeter only indicates the actual altitude when the weather conditions correspond to the
standard atmosphere and lapse rate. In reality the atmosphere rarely corresponds to the
CINA standards.
The weight of the atmosphere and its corresponding pressure, are appreciably affected by
air temperature. If the temperature of the atmosphere deviates from standard atmosphere,
the altitude computed with the international formula is not correct. Altitudes will be shown
lower than actual in the summer and higher than actual in the winter. A deviation of 1°C
per 1000m will result in approximately a 4m error in altitude. For example, if a pilot sets
his altimeter on a warm summer day where the air temperature is 16ºC warmer than
standard atmosphere and then changes altitude 2000m, his altimeter will show 2 x 4m (per
1000m) x 16ºC = 128m lower than actual!
To further complicate matters, the air pressure over a given location changes will change
as weather systems move across the area. In order to compensate for pressure changes
induced by changes in the weather an altimeter must be adjusted prior to each flight. This
can be done by setting the altimeter to a know elevation (e.g., Launch). Another method
of setting an altimeter is to enter the current QNH pressure value. The QNH is the
barometric pressure at a measuring station reduced to sea-level. If an altimeter was set to
the QNH at a measuring station (regardless of elevation) and then brought to sea-level it
would read zero. The QNH value is constantly updated and can be obtained from flight
service stations and can be requested from airfields over an aeronautical radio. Keep in
mind that the atmospheric pressure can change up to five millibars over the course of a
day, such as with the passage of a cold front, corresponding to a change in elevation of
more than 40 meters.
4.2 Altimeter Displays
The 3040 is equipped with three Altimeter dispays:
ALT1 Absolute altimeter in feet
QNH QNH pressure in hPa or ingHg
ALT3 Absolute alimeter in meter