I have gone out a bought a 'DPT' for them that do not know what they are. It's 'DIGITAL PHOTO TACHOMETER'.
Been looking for one for some time now, but just got around to buying one.
As with most things of this nature, we first need to know if it is accurate. So a quick test against a known 100% good one will give us that answer. Off to play with M97. She will know, 'SHE' is always right ;-) @ 1000 RPM on M97, I get 1000 RPM on the DPT, on to 2000, again, DPT reads 2000. @ 3K, DPT reads 3k and so on up to 6,000 RPM and DPT reads 6,000. Cannot go higher than that. M97 only revs to 6,500 RPM. So I think the DPT is good for go. (Could be way off at 20,000, but don't tell him).
I have done this sort of testing before. AC 16V PSU, connected to LED's to get a strobe effect. DISCO time again Folks. Black and White disc. Disc should stop at 3,600 then again at 7,200. at which point you take a voltage reading. lets us say 3.7V at 7,200RPM = 12/3.7*7200=22918rpm. You get the idea.
Most motors you have the reading printed on them. MABUCHI, please take note, and others.
Now onto the DPT, AC reading in RED.
So, there you have it. Some not far out, others way off.