Flytec Windwatch Pro - Manual (Page 18)

18 Altimeter
Altimeter
The Windwatch PRO has a state-of-the-art pressure sensor that, in addition to providing precise
barometric pressure readings, provides a fully developed and highly accurate altimeter. The
altimeter is fully temperature compensated so that the displayed altitude does not change when
the temperature of the WWP changes.
General Information about Altimeters
Barometric altimeters calculate altitude based on air pressure. Air pressure decreases as
altitude increases. Due to the fact that air is compressible, the pressure decrease is not linear,
but
exponential. The calculation for deriving altitude from air pressure is based on an
international formula (Commision International de Navigation Aérienne) and assumes a
standard atmosphere with idealized characteristics. The CINA standard atmosphere assumes
that the pressure at sea level is 1013.25 hPa (Hektopascal) at 15°C. A continuous decrease in
temperature at the rate of 0.65 °C per 100 meters of ascent is also assumed. Consequently, a
barometric aviation altimeter only displays the correct altitude if weather conditions are
consistent with the standard atmosphere.
In practice, these conditions are more likely to be an exception! The WWP incorporates a Flytec
innovation that significantly improves the altitude accuracy by correcting for air column
temperatures. If this feature is used, the correction is applied to the CINA altitude formula,
making the altitude readings on the WWP more accurate than standard altimeters. The
correction need not be used, in which case the altitude readings will coincide with aircraft
altimeters.
Atmospheric pressure also changes due to prevailing weather conditions. Depending on the
weather, the air pressure at sea level (QNH) may vary from 950 to 1050 millibars (mb). Even on
a stable day, temperature-induced differences in air pressure of
± 1 mb may occur, equivalent to
a height difference of ± 8 meters. During rapid weather changes, such as the passage of a cold
front, the air pressure may change up to 5 mb within one day, corresponding to an altitude
change up to 40 meters. Consequently, an altimeter must be calibrated prior to each use. This
means the altimeter must be set to a known benchmark altitude (e.g., airport, launch site, topo
map, elevation marker, etc.). An alternate method for setting an altimeter is to enter the current
QNH pressure reported by local aeronautical weather reports or reliable meteorological
institutions.
The Windwatch PRO features another Flytec innovation (patented), called AutoSet. When this
feature is enabled, the instrument automatically factors out changes in the altitude reading that
were caused by changes in the weather. For more information on this feature see the AutoSet
heading later in this section.