Before the era of the digital tach's like the one whitetiger uses, I had a digital strobe light at work..
I also have a reflective stripe type as it is small and easy to carry.
Unlike the current strobes which require reflective strips etc, my unit illuminated a big area and you could change the rate of the strobe flashes until the rotating object you were pointing at appeared to stop.
We used it because we could freeze the rotating components and study if there were contacts that should not happen.
I mention this type of device because there may be app's for modern cameras which would do the same thing.
I just tried this one for my android phone: "
RPM Meter Strobe Light"
Strobe
This app might just meet your needs.
Dave
p.s my kids like to play with the strobe at Halloween. They would get in a very dark area and turn the strobe to a low flash rate and then theys would try and walk around. It was very disorienting.
If someone was susceptible to epilepsy or seizures I would avoid!
I used the old strobe when I bought a Samurai Cutter. They wanted it to be run around 400 strokes per minute. In contrast to measuring rotating objects, the strobe light will freeze reciprocating motion.
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It is amazing you can now get a free app for your phone which will do the same job as this old unit which costs many thousand of $
Dave