Reviving an old thread...
I was thinking about tach accuracy in our aged airplanes, and using the trusty old hand-held laser model airplane tach to verify accuracy, but really don't care for the "ya gotta futz with it" nature of that gadget.
Came across a number of self-contained, panel mounted, inductive digital tachometers with hour counter, countdown timer, programmable for type of engine and number of cylinders, and cheap enough to experiment with.
Here's an example:
https://www.amazon.com/Tachometer-Ge...8CXKV7HF&psc=1
It runs on an internal CR2450 battery, has a 5' lead with two wires: one wraps on an ignition (spark plug) wire for signal, the other to engine ground.
It can be programmed to start operating when the engine is running, track hours of operation, max RPM, and programmable countdown timer (set for 25 hrs and when it reaches zero schedule an oil change!)
I think this will be useful for seeing if your tach is accurate, and when running in a new engine, tracking some operational values.
Since it is not wired to the plane it is a portable device like a USB charger or a Garmin aera GPS, and could be considered a reference instrument.
Mount to the panel near the mechanical tach with dbl-stick tape.
Here's the user's guide from the OEM, who sells this thing for ~$55.
http://www.enmco.com/pdf/PT14-Engine-Monitor.pdf
The rebranded unit seen on Amazon is ~$15.
I'm going to get one and test it on my lawncopter and then try it on the Tri-Pacer.
Cheers,
GG