A measuring instrument for iPhone
Stillspin Strobe Tachometer
Flash at a spinning part until it stands still and read how fast it turns — then see every other rate that same freeze could mean, instead of one number that hides the rest.
Stillspin is distributed on the App Store for iPhone, with the strobe free and one optional purchase that opens the camera and microphone routes.
On the bench
A strobe you sweep, or a speed you type
The reading, the flash rate, the marks per turn and the display timing stay pinned at the top of the screen while you sweep, with hold-to-repeat steps of 5%, 0.5% and 0.05%. Tap the reading to type a speed in RPM or Hz; if the torch or the screen cannot flash that fast, the app says where it tops out. With the screen as the lamp, everything else on the display goes dark until you tap to stop.
Every rate the freeze could mean
A spinning object also stands still at half its speed, a third of it, and at multiples of its blade count. After a lock the app lists those rates in one aligned column, each tagged ×2 or ÷3 against the most likely one, with a confidence bar and a short line on what it assumes, followed by the procedure that settles which one is yours.
Turntable speed at the exact rate
Pick 33⅓, 45 or 78 on the strobe screen and the flash is held at exactly that speed — a hundred thirds of a revolution a minute, never a rounded 33.33. Time the creep of a marker and the error comes out in percent and in cents of pitch. When the timing was too short to separate the error from 33⅓, the app says so instead of printing a figure.
Film the freeze
Once a freeze is held, the video button records eight seconds of it, switching the flash on for the clip if it was off, with a countdown and a Cancel button. The rate, the flash frequency and how many rates the freeze could mean are written into the clip, above the band that social apps cover with captions. No sound is recorded and nothing is uploaded; the clip goes wherever you send it from the share sheet.
A first freeze in under a minute
After the safety notice and a short report of what this iPhone can flash, a card on the strobe screen follows what you are doing: where to point the light, when to sweep, when to lock, and what the number you locked means.
Two more instruments, in the unlock
The camera reads how often a mark's brightness repeats, with a live status, signal bar and labelled spectrum, and names the rates folded down by the frame rate. The microphone reads the shaft rate of a machine you cannot see. Compare lines them up and calls a whole-number disagreement a symmetry setting rather than a fault. A camera turntable screen keeps reading while you turn the adjuster, with a centre-zero deviation meter and a fixed-scale trace.
It says what it has not verified
Every measuring screen carries three lists: what the method can do, what it cannot, and what nobody has measured yet. The screen ceiling is arithmetic from the refresh rate the iPhone reports; Apple publishes no switching rate for the torch; no reading has been compared against a reference tachometer.
Readings stay on the iPhone
Saved readings live in one file inside the app. Camera and microphone frames are analysed as they arrive and never kept; the freeze clip is the one file made from the camera, and it is deleted when you leave that screen.
What the turntable readings will not tell you
Both of them — the strobe check on the free strobe screen and the camera screen in the unlock — report platter speed against 33⅓, 45 and 78, as a percentage and in cents of pitch, and nothing beyond that.
It is not a wow and flutter meter. Wow and flutter is defined against a test record — a 3150 Hz tone under IEC 60386 and DIN 45507, or 3000 Hz under NAB and JIS — and is read out of how that tone's frequency wavers. Nothing an iPhone on a platter measures is that quantity, so the app does not print a number and call it one.
It cannot see the wow most listeners actually hear. A spindle hole punched slightly off centre makes the groove speed rise and fall once every revolution, about 0.55 times a second at 33⅓. A phone sitting on the platter turns with the record, so that error is invisible to it, and it is cured by re-centring the record rather than by adjusting the deck.
It is not a rumble meter. Rumble under DIN 45539 sits sixty to eighty decibels below the signal in the 2 to 100 Hz band and needs a rumble test record and a calibrated chain. How an iPhone microphone responds below about 50 Hz is not published and differs between models, so the measurement is left out.
What it asks for
The camera, for the optical method, the turntable screen and recording a freeze. The microphone, for the sound method only. Adding a clip to your photo library, only if you choose Save Video in the share sheet. iOS asks before each, and declining one leaves the other methods working.
Price
The download is free, and the strobe is complete in it: the sweep, typed rates, the lock, the whole candidate ladder, the procedure, refine-by-drift, the turntable strobe check, the emitter check and the freeze clip. One in-app purchase, paid once, adds the optical instrument, the acoustic instrument, Compare and the camera turntable screen. Readings you have already saved stay readable whichever method took them.