Kestrel 4200 (HVAC) - Manual (Page 7)

Kestrel ® 4200 Pocket Air Flow ® Tracker 13
the altitude readings tend to fluctuate by a few feet.) After obtaining a current altitude from the ALTITUDE
screen, move to the BARO screen and enter this value as your reference altitude by following the same
procedure. Both readings will now be accurate.
Starting with a known altitude for your location
You can obtain your altitude from a topographical map or local landmark. Google Earth
is an excellent free program that provides the exact altitude for any given address:
www.earth.google.com/. Set this value as your reference altitude on the BARO screen
to determine your barometric pressure: Press the button to enter the reference
setting mode. Press the button to increase the reference altitude or the button to
decrease the reference altitude. You will notice that the barometric pressure will change with changes in
the reference altitude. Press the button to exit the adjustment mode. Again, allow the Kestrel Meter to
stabilize, then enter the value from the BARO screen as your reference pressure on the ALTITUDE screen by
following the same procedure. Both readings are now accurate.
When reviewing stored data, remember that changes in pressure AND changes in location/altitude will
affect the stored values. When tracking pressure changes relative to weather, set the reference altitude on
the BARO screen and keep the Kestrel Meter in one location. Your graph history will now show trends in
barometric pressure. Your altitude as shown on the ALTITUDE screen will change as the weather changes,
but you can ignore this screen for this purpose.
If you are planning a day hike would like to track your altitude, you’ll need to enter the correct reference
pressure on the ALTITUDE screen as described above in “starting with the known barometric pressure.”
You can now track the altitude changes as you hike. In this instance, you should ignore the values on the
BARO screen, since the pressure changes will be due to changes in elevation far more than to changes in
the weather.
In general, changes in barometric pressure associated with weather changes are small over the course of
one day, but they will affect the accuracy of the altimeter over time. This is why aircraft reset their altimeters
at every airfield by entering the field’s “altimeter setting” or reference pressure. Accordingly, if accurate
altitude readings are your primary interest, you should reset the reference pressure on your Kestrel Meter
regularly. If you encounter an elevation landmark, you can adjust the reference pressure until the altitude
matches the landmark elevation. This will correct the altitude for any pressure changes due to the weather.
(Or, you can obtain an updated reference pressure from the sources described above.)
Some final notes — If you wish to know the actual or station pressure for your location (such as for engine
tuning), simply set the reference altitude on the BARO screen to “0”. In this case, the Kestrel Meter will
not make any adjustment and will display the measured value. (Engine tuning and ballistics software
sometimes refer to atmospheric or station pressure as “absolute pressure.” These applications are
concerned with the actual air density, as opposed to pressure gradients relating to weather, so barometric
pressure is less useful.)
Also, the above discussion applies to ALL pressure altimeters, including one you may have in a watch or
other device, but not to GPS altimeters, which use satellite triangulation to determine altitude. Note that
with present GPS technology, pressure altimeters remain more accurate for measuring altitude change.
This is why airplanes still rely on pressure altimeters, not GPS.
Finally, the DENSITY ALTITUDE screen is calculated from the absolute values of station pressure,
relative humidity and temperature, and is not affected by the reference values entered in the BARO and
ALTITUDE screens.