Check out this link:

http://www.eaa1000.av.org/technicl/instcal/instcal.htm

When I worked at an FBO, we were lucky enough to have an A&P school next door.  They had a case which included a calibrated altimeter, airspeed, & VSI, with a vacuum/pressure pump.  It would get T'ed into the pitot and static system and the pressure adjusted with the pump, then the readings compared.  Best bet is to see if you can find something similar, then you can reasonably guarantee the results.

http://www.barfieldinc.com/services-and-offers/ground-support-test-equipment

Or check around to see if a fellow builder has a new or recently calibrated altimeter that you can borrow for a day or two.  The syringe can provide the vacuum.

Patrick

-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net]
Sent: Thursday, September 24, 2015 8:57 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] OT: Calibrating altimeter and alt encoder

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