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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