The sirometer is a resonating reed-style tachometer, a device first developed in the 1950s. It can be used on any engine that vibrates. It works by contact. Resonant vibration of the reed indicates when a given rpm has
been reached. It has a range from 800–50,000 rpm & 14 - 750 Hertz

Operation

Place the sirometer tightly on the outside case of the running motor.
Turn the top part of the sirometer which extends a piece of wire, when the wire is in resonance with the vibrations of the machine, it can be seen to vibrate. When the wire reaches this resonance point, the rpm or frequency can be read. The upper part of the scale shows rpm and the lower part cycles per second, Hertz.
Reduction gears show a misreading.
Gas motors give the rpm of the crankshaft but other fast running shafts may influence the reading.
If the top part of the sirometer seems heavy after time, put a drop of thin oil into the brass wire loop after it has been turned.