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  #1  
Old 03-29-2021, 06:37 PM
00Dan 00Dan is online now
 
Join Date: Nov 2016
Location: Atlanta
Posts: 129
Default Mechanical versus electronic tachometer

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 havenít flown it yet but when I did a ground run to test the installation I noticed the two didnít 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 havenít had a chance to test with an optical reader yet, thatís on the list. My question is more to the operating principle of each and if it biases towards one or the other.
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  #2  
Old 03-29-2021, 06:43 PM
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
Posts: 5,895
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Quote:
Originally Posted by 00Dan View Post
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.
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RV-7
Lord Kelvin:
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.
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  #3  
Old 03-29-2021, 06:45 PM
David Paule David Paule is offline
 
Join Date: Dec 2009
Location: Boulder, CO
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When I changed engines on my non-RV, I had the chance to use a digital tach, and my mechanical tach was accurate through its range. At that time it was about 40 years old, but I'd replaced the cable as part of the engine change.

Dave
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  #4  
Old 03-30-2021, 07:16 AM
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Jpm757 Jpm757 is offline
 
Join Date: May 2013
Location: Sherman, CT
Posts: 945
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See if you can borrow a tachometer check box. About the size of a pack of cigarettes, place on the dash and it displays engine RPM. If you have the prop dynamically balanced, then that equipment will also accurately display RPM.
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