Manometer and Flight Test Required
Hi Ron!
The manometer for checking a mechanical ASI is a great idea, however you need to be aware of a couple of things. First, it's possible to damage any ASI or the transducers in your Dynon if you over-pressurize them. Also, on the Dynon website, there is a white paper that details the proper way to perform a pitot/static check without damaging the instruments. In all likelihood, the transducers in the EFIS are spot on. If your ASI was certified (bench checked) at some point, instrument error may be recorded in a log book or on a form 8130.
Chances are pretty good, that it's a combination of static source pressure, installation and mechanical error that's producing the difference in readings. If you look at Kevin Horton's website (he's a regular poster here on VAF), he has an excellent discussion regarding static source pressure error.
It takes proper flight test, data reduction and ground testing to get a pitot/static system fully dialed in and to account for differences such as the one you are seeing in airspeed. Do know that both of your airspeed indications are accurate
ratio displays insofar as displayed stall speed (regardless of the accuracy of the actual airspeed number), is always displayed stall speed (for a given gross weight) and any ratio based on that number (Vref, ONSPEED, etc.) is going to be fairly reliable. Another thing to note is that different portions of the speed envelope may be effected differently by error--i.e., it may be more pronounced at slow speed/high AOA.
Also note that the 24-month "pitot/static" check you have performed at an avionics shop does not check for installation or static source pressure (indicated airspeed) error although it's a great way to check for leaks in your system.
Proper pitot/static checking and getting a static system properly "dialed in" can be a bit of work, especially when mixing mechanical with electronic sensors. I've been through this integrated the back-up mechanical ASI display with my EFIS in my RV-4.
Also, 43-13 has some good, basic techniques using surgical tubing as a pressure source that can be helpful diagnosing basic pitot/static problems. Here's a good link on how to build an inexpensive manometer if you choose to go that route:
http://www.iflyez.com/manometer.shtml I built one and have found it to be quite accurate. I modified mine and use a small blood pressure bulb with a relief valve for pressure input so I can make very fine changes and "hold" the pressure with the valve to leak check the pitot system. It can be used with an EFIS if you follow the Dynon guidance and don't over pressurize, but as I said, I think you'll find the "instrument" error in an electronic transducer to be negligible.
There is lots of great discussion on this board about airspeed accuracy! You might want to search the forums for discussion on "airspeed calibration, static source pressure error, GPS speed runs, static" and "pitot" as key words. If you've got a system that's built in accordance with Van's plans (including the static port configuration), you are likely already in the ball park and one of your indications (ASI or speed tape) is closer to ground truth, but you won't know until you test. If you are interested in discussing test techniques, drop me a PM or e-mail and I'd be happy to help you out.
Cheers,
Vac