Originally Posted by 00Dan
I have two ways to read RPM in my plane currently. I have a traditional cable driven mechanical tachometer, and I recently installed an EDM 800 that reads RPM with a Hall effect sensor on one of my magnetos. I haven’t flown it yet but when I did a ground run to test the installation I noticed the two didn’t quite agree.
Should I trust the electronic reading more than the mechanical, assuming the mechanical may be inaccurate due to cable wear, or are there other factors at play? I haven’t had a chance to test with an optical reader yet, that’s on the list. My question is more to the operating principle of each and if it biases towards one or the other.
Unless the electronic is 2x or 1/2x off, yes trust the electronic. It's not a voting machine.
Edit: I should add that a cable should make no difference with a mechanical tach. Many have a permanent magnet rotating inside an aluminum cup. The eddy currents created result in drag and a mechanical torque. That mechanical torque is resisted by a spiral spring and can be adjusted. Reference
for other types.
ďI often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.Ē
Last edited by BillL : 03-29-2021 at 06:56 PM.