Tachometer

A 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.^[ref]

If the readout detects $p$ pulses per revolution and counts $N$ pulses during a gate of duration $T$, its average speed estimate isHere $\omega$ 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.