= Tachometer
A \b[tachometer] measures rotational speed, commonly displayed in revolutions per minute. A non-contact optical tachometer illuminates a reflective mark on a shaft or rotor and detects its passage as a pulse. HIOKI's fan-speed experiment uses reflective tape and counts reflected-light pulses.<Measure Fan Speed Rotation by HIOKI>[application description, reflective tape and pulse counting]{ref}
If the readout detects $p$ pulses per revolution and counts $N$ pulses during a gate of duration $T$, its average speed estimate is
$$
r=\frac{60N}{pT}\quad\text{revolutions per minute},\qquad
|\omega|=\frac{2\pi N}{pT}.
$$
Here $\omega$ is the <angular velocity> magnitude in radians per second. For an original example, $p=2$, $N=120$ and $T=0.5\,\mathrm{s}$ give $r=7200$ revolutions per minute. Mistakenly configuring one pulse per revolution would double the displayed value without changing the rotor.
The counting step corresponds to $60/(pT)$ revolutions per minute per pulse: $60$ in this example. Increasing gate time improves that counting resolution but averages over a longer interval, potentially hiding speed changes. At low speeds, measuring the time between successive marks can be more useful than waiting for many counts. A <rotary encoder> provides a related pulse-based measurement and can also encode angular position or direction.
Missed marks, extra reflections and an incorrect pulses-per-revolution setting are distinct from uncertainty in the timing clock. A single repeated mark normally measures speed magnitude; it does not establish the direction of <rotation>. A convincing check specifies the mark pattern, counts per revolution and timing convention rather than treating every electrical pulse as a complete turn.
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