Tachometer accuracy

[rturbett]

My mechanical tach was placarded as being 100rpm off, so we had it overhauled at last annual.  I checked it with an optical rpm (device placed on the dash), and now see it has improved, but is still off by 80 rpm. 

I'm curious on the accuracy of the digital tachs- do they get their information the same way, but display it nicer?

Thanks,

Rob 

[toto]

I’m not an expert on this by any means, but I replaced the old mechanical tach with a digital tach driven from the P-lead, and my understanding is that the digital tach counts impulses from the magneto as the engine turns. 

Comparing my displayed RPM against this audio-based app is always very very close (but I would expect the audio app to be less accurate).

https://apps.apple.com/us/app/engine-rpm/id340401811

[Vance Harral]

Almost all "digital" tachometers sense RPM from magneto impulses, though there are a few outliers that still use a mechanical cable.  Sensing RPM from magneto impulses is very accurate, assuming the magneto is functioning properly.  Most are designed to work by connecting to one, conventional, mechanical mag (not electronic ignition), and involve some sort of electrical interface between the tach and the magneto that down-converts the high voltage impulses at the mag down to something the electronic instrument can tolerate.  This interface might be as simple as resistors, or as complex as a standalone electronic converter box.  That interface is an additional point of failure in the system, and I've dealt with some problems on that in a twin I fly for a flight school.  But it tends to be reasonably reliable compared with a mechanical cable.

If you're considering a tachometer with digital guts, give some thought to what kind of display you want, and whether the display is itself more "digital" vs. (perhaps artificially) "analog".  I've found that the displays that look cool in pictures are often a distraction in actual use.  For example, Mitchell makes a tach with electronic guts that still drives an analog needle on a traditional gauge:

Note that this unit has a mechanical tach time display that you can read regardless of whether or not the master is on.  I think it's a great solution, but it's not very flashy.

The worst option, in my opinion, is a tach that only displays numbers, with precision down to 10 RPM:

The numbers on these units often "blip" back and forth regularly, in ways that aren't meaningful, and cause pilots to spend too much energy trying to achieve an exact RPM indication.  Even when you tell yourself that it doesn't matter whether the indicated RPM is 2230 or 2200, it's hard to avoid noticing it.

The 1990s-era EI design isn't much better.  Its ring of LED indicators has no scale and is essentially useless, and the numeric display is difficult to read in sunlight:

Note that this unit displays down to 1 RPM precision (if configured that way), which is completely meaningless.  Note also the "tach time" button, which works OK, but requires the master to be on to get the tach time and it can't show more than 4 digits at a time.  We have one of these on a flight school airplane I fly.  I hate it.

Modern engine monitors are a little better, but still tend to lack scale on their arc indicators.  But the displayed RPM values tend to bounce around less.  Not sure why, but I suspect the computer inside the box applies a smoothing function to the calculated/displayed RPM:

Just my $0.02 here, everyone has their own preferences.

 

[rturbett]

Thank you for the time and effort you put into educating me! This is exactly the information I was looking for!!!

[Jetpilot86]

Just put in an Insight G4. There is about a 75RPM split between it and the OEM. Probably going to get that app.