RPM Measurement Online — Strobe, Sound and Camera

Because a screen flash lasts a whole frame — 13 ms on a 75 Hz monitor — and nothing can shorten it. In that time the object turns 148°, so the mark smears into an arc almost a third of the circle long and the screen looks permanently lit. This is not a fault but the limit of the method: a screen honestly works up to roughly 600 rpm. Above that use the Sound tab, which is built for exactly that range.

The refresh rate is held by a crystal, so the flash itself is stable. The limit is elsewhere: flashes can only land on whole frames. The tool searches not just "every k-th frame" but also flashes once every few turns, so it hits the target within 0.5% across almost the whole range — where a plain frame search missed by tens of percent.

The gyroscope: it reads angular rate directly, with nothing in between, and on a turntable it resolves thousandths of a revolution plus unevenness and drift. The catch is that the phone must lie on the object itself, and the sensor saturates around 300 rpm.

Because under 50 Hz the exact mark count for 45 rpm is 133⅓, and a third of a mark cannot be drawn. A 133-mark disc stands still at 45.11 rpm — an error of +0.25%. Classic discs carry the same error but say nothing about it. Choose the screen as the source and the flash rate is matched to a whole mark count, so the error disappears entirely.

The stroboscopic effect needs light that flickers with the mains. Incandescent and neon lamps flicker at twice the mains frequency — 100 or 120 times a second. Most LED bulbs rectify the current and shine almost steadily, so the marks never freeze. Check a specific bulb with the "Measure light flicker" mode, and use this screen in a dark room instead of a lamp.

A mark also freezes when the flash lands on every second turn: a strobe cannot tell a frequency from half of it. The rule is simple — having found a freeze, double the speed. If the mark freezes again and there is only one, the true speed is twice as high; if two ghost marks appear, you had it right the first time.

With the "Measure light flicker" mode. First calibrate against an ordinary incandescent mains lamp — it flickers exactly 100 times a second on 50 Hz mains, and its bands give your camera's row rate. After that, bands from any source convert to hertz. Shoot a monitor with another device's camera: a phone cannot see its own screen.

Unevenness is the spread of speed around the average — wobble within a turn, which is how a worn idler, a bent spindle or a rubbing belt shows up. Drift is the slow change of the average from the start of the measurement to the end: usually a stretched belt, thickened grease or a motor warming up. They are separate numbers because they are separate repairs.