JAMES EALY wrote:
The biggest problem with the cheapies is that they are not temperature
compensated, or VERY poorly so.
There are 2 types of temperature dependance with respect to electronic
altimeters.
The first is how temperature change affects the pressure sensor's ability to
convert pressure into voltage. This relationship should be absolutely constant
no matter what temperature the sensor is at, but in the real world this is not
the case. The current state of the art solid state sensors, however, do have
built in temperature compensation circuitry and do a pretty good job in removing
this effect. A large portion of the remaining temperature effect is also
cancelled out since the AGL altitude is computed by measuring absolute altitude
in the air, measuring absolute altitude at ground level, and subtracting the two
values.
The second is how temperature affects the pressure to altitude algorithm that
all pressure based altimeters use to calculate altitude. Altimeters measure
pressure, not altitude, so a model of the atmosphere is used which predicts how
pressure varies with altiude. This is an ideal model so it obviously cannot
match the real atmosphere at any time and any place on earth. The models are
also based on an ideal sea level air temperature of 59F. So, if you're not
flying at 59F, the model will be off. There are algorithms to compensate for
this temperature effect and that's why our FlightView software allows you to
enter the temperature of the day you were flying. It should be noted that this
error can be quite large (like 5%-10%) if you are flying on a very hot or very
cold day. It should also be noted that on hot days, the altimeter will actually
read low, so if you fly your non-temperature compensated altimeter on a summer
day and it says you were at 2000ft, you were probably higher.
I'd also like to relate the following observation. When we were developing the
RAM altimeters, we were flying them in 1.5m discus launch gliders and taking
them pretty high. The plots showed that we were getting to around 2000ft. We
initially didn't believe this so we measured out 2000ft at the flying field and
one of us held up a glider while the other observed it from 2000ft away. It was
definitely flyable at that distance and looked about right compared to what we
saw in the air.
It is also very important to understand how important the conditions are when
flying at altitude. Contrary to what many believe, the worst type of conditions
is a clear dark blue sky. The best is a clear sky with nice dense cumulus
clouds. If you get a cloud behind your glider, you can see it twice as high as
you can with blue sky behind it. Just don't fly out from in front of the cloud
or Poof! it will be gone. :-)
Randy
Soaring Circuits
RCSE-List facilities provided by Model Airplane News. Send "subscribe" and
"unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and unsubscribe
messages must be sent in text only format with MIME turned off. Email sent from web based email
such as Hotmail and AOL are generally NOT in text format
