pitot static question

[Paul 5r4]

Top of the morning guys! Recently went to the uncontrolled airport where I keep the RV. I set the altimeter to the field elevation of 75 ft as always which yielded a pressure of 30.58. Later when flying I decided to tune in the Pensacola, FL ATIS to see what setting they were reporting. Pensacola is about 25 miles or so east of my home base. The altimeter setting from ATIS was 30.48 so I reset mine. Just to get a little more information I contacted Pensacola apph for a mode c altitude check. With 30.48 set in the window the controller reported me at 2000 which was my readings exactly, (on both the dynon and steam gauge altimeter). After landing back at the home field, both of my altimeter were showing an altitude of about 5 feet about sea level. I had a look in one of the FBO's cessnas to see what it's altimeter setting was for the field elevation of 75 ft. With the cessna's pressure set to 30.48 it was indicating correctly at 75 ft. Soooo, mine is off somehow. I have only one common system/line for my pitot and static system. It is VFR certified. This is something new because before it always matched. Do I need to look into the static side of the system... what could be the problem? It's always hangerd so no water in the lines. I'm thinking maybe a bug... would that cause an error like this? Both my altimeters match perfectly but for some reason they're both off about 200 feet. Thanks in advance for your input!

[lr172]

I would say your static system is fine. Your system is allowing the altimeters to operate properly. Blockage would impede the altimeters from changing readings freely. Further, you confirmed that your altimeters are matching your encoder, providing a cross-check, assuming you are using a blind encoder and not a feed from the Dynon. It is possible that you have a leak in the static system, as this could give some variance in reading in flight, as you will be given cabin based pressure. In theory this could be adjusting for your error when not in motion.

It is possible that your field did not have a true 30.48 pressure and was different from Pensecola's. It was also possible the Pensecola's altimeter had not been updated recently for changes. I believe that ATC does it's conversions from Pressure altitude based upon the reported altimeter, not actual. I fly from an uncontrolled field and set my alt based upon field elevation. I often get an altimeter reading from a nearby tower that is different. I also would not assume that the Cessna you observed was accurate. Better to check against a barometer.

Larry