X-Virus-Scanned: clean according to Sophos on Logan.com From: "Patrick" Received: from mout.perfora.net ([74.208.4.194] verified) by logan.com (CommuniGate Pro SMTP 6.1.5) with ESMTPS id 8014877 for flyrotary@lancaironline.net; Thu, 24 Sep 2015 10:45:01 -0400 Received-SPF: none receiver=logan.com; client-ip=74.208.4.194; envelope-from=patrick@hoffmann1.net Received: from D8USMCHMJ375WC ([173.161.59.81]) by mrelay.perfora.net (mreueus002) with ESMTPSA (Nemesis) id 0M5fUk-1acjez1aDg-00xeIc for ; Thu, 24 Sep 2015 16:44:25 +0200 To: "'Rotary motors in aircraft'" References: In-Reply-To: Subject: RE: [FlyRotary] OT: Calibrating altimeter and alt encoder Date: Thu, 24 Sep 2015 09:44:23 -0500 Message-ID: <076801d0f6d7$820bd740$862385c0$@hoffmann1.net> MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0769_01D0F6AD.9935CF40" X-Mailer: Microsoft Outlook 15.0 Thread-Index: AQKFPNPg43OScL9DbRjmgmlbR/9G2ZzjLvOg Content-Language: en-us X-Provags-ID: V03:K0:OSM/+KjwlHApZpmqlxJFvSydpEhfY46mhCHTdoHKN2wxtq2Uigk 7q6Fr4MZ/drq/S5Yds9JVwYNDr9gX+mRyCVXyWy515YEYwWA4Lj4nO+9Z2vEH/yK3KyVeEN BdH54i4utt2Dzaoxl3T4i3omlLYv3TAVCnwF3gZFMlR3vYhghlI1uLPcS9EQ+baU0hUVF3V AJhvZO/96LZ2HkOQv9hJQ== X-UI-Out-Filterresults: notjunk:1;V01:K0:oI1PyJKvya4=:uH7NaLDPXroBb95karWCiS uT9dbpix/iG1y6Zk7RfWHeodDHBrcy1/fs9ihlHF84WX8YdEU2Bh4V+wt7CO674MQvCxDBsMY PQGHnI6sxZ6ZdE923gTMtpg4ix+JgTDJxNIEiLGCTRseFiSox2Tih8xWYitu5DfAr//9RUkbF Kxnvzv89Mx2fP/zvossSxOMdRMc9/hpQFhhKxAHKlEndmsbarZouzgjv8Yxz14OY7c3OlBImP B20dhdm60LT5/uxeK3ahK3tefXMX0eo2YYN3qjM9ycOdCTkgUdmq8G/9+UgyLxFROMcJLCiIi jx3ZKbBHLkT1d9gYgDNY+XV6w1Xr2qcs0UdE/vWmDfXkIy59Kip3vgA9rgkPy4zeh72w+I6rb PBJzUNM/w3UnEwkdRwEDYrjyRwoHiPYH27pqQE9WsE3nTSmq7Vcn4BlGtNKtviGIP6T9b2Izq 4S1y7zjd9uLJsoG3vvwzyTrJf3d4nUcKxW+lddgh2mLZsSVU3LxM0Z84cFm+34ZpftBpnjgI4 b0nN84gd8ai32v9p/jcP1FJAMA/jR/au3e3syh+L5ieoQpfLXCU6XKenoHmWB/TkJMDSLEaHY 6wq6e6rgNw8vfZOCRwty1awdB7MTon4+4mQyC4egBV6WoUNDZwcowGkKhnAODKoCBphapa5PN tJCPL3sUSkAd1nF44WuNO6cWrgHdDoGbUw3pKNLqnTqZpaWdnfrl728rlN+8aRHmXxG54eLVs r1F8QNdQ57AkfkM8 This is a multipart message in MIME format. ------=_NextPart_000_0769_01D0F6AD.9935CF40 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Check out this link: http://www.eaa1000.av.org/technicl/instcal/instcal.htm =20 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-equipm= ent =20 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. =20 Patrick=20 =20 -----Original Message----- From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net]=20 Sent: Thursday, September 24, 2015 8:57 AM To: Rotary motors in aircraft Subject: [FlyRotary] OT: Calibrating altimeter and alt encoder =20 Off topic, except it's in a 13B Van's RV-3 ;-) =20 Again, after flying home from Sun'n'Fun, ATC saw me several hundred feet = below what I saw on my altimeter. =20 Last week I finally got around to removing all the screws that holds the = fuselage top over the instruments. =20 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). =20 I checked METARs at nearby airports -- CTY, GNV... and I know my = elevation exactly. That matched what my Dynon D10A and steam altimeter showed. =20 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. =20 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.=20 Basically shows 30 to 50 feet too high altitude if I set the Dynon to = the reported pressure. All that work for nothing! =20 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. =20 Any suggestions on how to obtain an accurate air pressure reference? =20 Finn ------=_NextPart_000_0769_01D0F6AD.9935CF40 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable

Check out this link:

http://ww= w.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. =C2=A0They had a case which included a calibrated altimeter, = airspeed, & VSI, with a vacuum/pressure pump.=C2=A0 It would get = T'ed into the pitot and static system and the pressure adjusted with the = pump, then the readings compared.=C2=A0 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-suppor= t-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.=C2=A0 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.=C2=A0 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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