1- First of all in the closed hangar carefully cover your static port(s) with a piece of tape and also put the cover on the pitot tube.
2- Start your Dynon(s) -> Hold buttons 7 and 8 for a few seconds to enter in the SETUP MENU -> HARDWARE CALIBRATION -> ADAHRS CALIBRATION -> Zero Airspeed Calibration...
I've just did it yesterday and it works fine. The Dynon tape now indicates a steady "---" instead of a bouncing value around 20 knots. My major concern was that this will also affect the upper range of speeds by shifting down the entire speed scale, but not. I’ve checked this issue in flight and the Dynon indicated airspeed is (almost) the same with the analog gauge indication.
Read here http://dynonavionics.com/cgi-bin/yabb2/YaBB.pl?num=1468169485/7#7 the Dynon support answer for this same subject.
I also have to check the static and the pitot system for leakage and to do the check of the ASI instruments for accuracy, but before doing so I have to build a water manometer.
Regards,
Alex.