Photograph the watch against a reference clock, wait a known interval, and photograph it again. The difference divided by the interval is the rate. It gives rate only, and only to about a second per day.
How to measure watch rate without a timing machine
issue ISSUE 01checked 2026-09-16responsible Escapement Papers (a named bench, not a person)
What you need A camera or phone; a reference clock you trust; a fixed position; a note of the interval; patience for at least 24 hours.
tape card: what the method can and cannot give
| Quantity | Available | Method |
|---|---|---|
| Rate | Yes, about 1 s/d | Two photos over a known interval |
| Amplitude | No | Needs a timing machine |
| Beat error | No | Needs a timing machine |
| Position error | Yes, roughly | Repeat in each position |
Steps
- Set the watch to the reference clock and photograph both together. The photo is the record of the starting offset.
- Wait at least 24 hours, in one position, undisturbed. Longer is better; a week halves the error in the answer.
- Photograph both again, in the same framing, at the same time of day.
- Read the offset in each photo and subtract. The difference, divided by the number of days, is the rate.
- Repeat in a second position. The difference between the two is the position error, roughly.
What the method cannot tell you
| Reading | Why the camera cannot get it | Consequence |
|---|---|---|
| Amplitude | It requires timing a fraction of one beat | A tired movement looks fine |
| Beat error | It requires resolving two beats | A fork fault is invisible |
| Short-term stability | The interval is too long | You see the mean, not the spread |
| Rate to better than 1 s/d | Reading error in the photos | Small errors hide in the noise |
Why the mean rate is still worth having
A camera measurement over a week gives a mean rate that is more representative of how the watch is actually worn than a ten-second reading on a machine, because it includes the real positions and the real temperature. It is a worse instrument and a better experiment, and the two answer different questions.
The bench holds that a watch owner who records a camera rate over a week before sending a watch for service will usually get a better conversation with the watchmaker, because the complaint comes with a number attached.
References
- Ref 1Time and Frequency Division, NISTsupports the reference timekeeping the comparison depends on
- Ref 2Frequency standards and clocks: a tutorial introduction, NBS Technical Note 616, NISTsupports the measurement framework for clock and oscillator stability
- Ref 3NIST Time and Frequency Metrology Groupsupports the standards work that reference timekeeping rests on
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