On 9/24/2015 11:12 AM, ARGOLDMAN wrote:

Not sure that I understand completely, however the encoded altitude has nothing to do with your local setting.

Yep. I set the Dynon to 29.92. Then the displayed altitude should match what the encoder sends out.

Problem is how to accurately (preferably to within 10 feet) calibrate the Dynon. Obviously using nearby METAR reported pressures was not successful. And I'm beginning to think that the instruments that reports pressure to METARs may not be all that accurate -- varies from airport to airport.

 

all encoders are calibrated to the same calibrations so that when ATC or whoever reads the reply all of the aircraft that they are looking at have the same altitude bias independent of errors in setting the kollsman window in the various aircraft.

 

Since you have gone into the encoder, there is a possibility that the alterations that you have done may yield a constant error for ATC which is worse than not having an altitude report at all. You might want to have your encoder checked with proper instruments.

 

Something that comes to mind is that since the encoder and the altimeter are both run from the static source, that there is a blockage, kink, water etc that may be influencing the readings.

 

Good luck in your quest for vertical reading stability

 

Rich

 

In a message dated 9/24/2015 8:58:23 A.M. Central Daylight Time, flyrotary@lancaironline.net writes:

Off topic, except it's in a 13B Van's RV-3 ;-)

Again, after flying home from Sun'n'Fun, ATC saw me several hundred feet
below what I saw on my altimeter.

Last week I finally got around to removing all the screws that holds the
fuselage top over the instruments.

I hooked LEDs across the data lines that run from the altitude encoder
to the transponder (gray code).
(note that 1 is actually 0 volts -- active low).

I checked METARs at nearby airports -- CTY, GNV... and I know my
elevation exactly.
That matched what my Dynon D10A and steam altimeter showed.

I then spent several hours adjusting the high and low pots on the alt
encoder until the codes changed within 10 to 20 feet of the 50 foot
points when applying vacuum to the static system with a syringe. Can't
get better that that, I though.

Alas, after replacing the fuselage top and multitude of screws,
yesterday I hopped over to Cross City (CTY) a mere 15 miles away.  I had
to set the altimeter to 0.03 or 0.04 below what their METAR reported in
order to get altimeter to show their field elevation -- 42 feet.
Basically shows 30 to 50 feet too high altitude if I set the Dynon to
the reported pressure. All that work for nothing!

Now I'm beginning to suspect that the pressures reported by the METARs
are not all that precise.
Checking METARS right now at nearby airports, they range from 29.98 to
30.02.

Any suggestions on how to obtain an accurate air pressure reference?

Finn


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