...."you can measure the number of revolutions and frequency of each running engine or operating machine with great accuracy.
With the TREYSIT Sirometer you do not need to access the often hard accessible shafts or turning parts, because it analyses the the number of revolutions or frequency through the vibration of the housing and shows it on two scales."
This works on every engine I own. Simple and precise. It works by placing it on a engine housing and turning the dial. The wire with the yellow circle will vibrate. You turn the dial until the wire is stationary. When the wire is still, it gives you both the rpms and the resonance frequency of the engine.