Timegrapher readings explained: rate, beat error, amplitude and BPH

How does a watch timing machine work?

A mechanical watch keeps time with a balance wheel that swings back and forth at a fixed rate, and each swing makes the escapement tick. The machine picks up those ticks with a microphone and times them very precisely.

Witschi, a Swiss maker of watch test equipment, explains that each tick is really a cluster of small sounds. The first is very sharp in time, so it is used for rate and beat error. The third and loudest is used to work out amplitude. Witschi also notes that if a watch ticks very quietly, or there is strong background noise, the gain has to be adjusted to pick out the tick reliably. That is why quiet surroundings matter.

What does the rate number mean?

Rate tells you how many seconds a day the watch would gain or lose if it kept running exactly as it is now.

• Positive (for example +5 s/d): the watch is running fast.
• Negative (for example -10 s/d): the watch is running slow.

Witschi describes how it is worked out: the measured time between ticks is compared with what it should be for that movement, the difference is averaged over the measuring time, and the result is converted to seconds per 24 hours.

What is a good rate? Beyond the Dial says a well regulated watch should be within a few seconds a day, and that watchmakers usually prefer a watch to gain slightly rather than lose. As a benchmark, the Swiss chronometer testing institute COSC certifies watches against an average daily rate of -4 to +6 seconds a day.

Why does the rate change when you turn the watch?

Beyond the Dial explains that the rate can change depending on whether the watch is dial up, dial down or vertical, and that this positional variation is normal, especially in older watches. It gives a nice example: a watch with the crown at 3 o’clock, worn on the left wrist, spends a lot of time crown down. So one reading in one position is only part of the picture.

What is beat error?

Each swing of the balance one way makes a “tick” and the swing back makes a “tock”. Ideally the two are perfectly even. Beat error is the difference, in milliseconds (ms).

Beyond the Dial suggests that a very small beat error has little effect on timekeeping, that more than about 0.4 ms could point to a poorly regulated watch, and that more than about 0.8 ms could point to a problem with the balance. A higher beat error also makes a watch harder to regulate and tends to increase the difference between positions.

What is amplitude, and what is a healthy figure?

Amplitude is how far the balance wheel turns on each swing, in degrees. Witschi puts typical values for today’s popular wristwatches at about 260 to 310 degrees, and notes that amplitude drops gradually as the oils age.

It varies by movement: Beyond the Dial notes that many Swiss movements should show over 300 degrees when running well, while a modern Seiko may be as low as 230 degrees from the factory. Its key advice is simple: low amplitude usually means the watch needs a service.

Lift angle: the setting behind amplitude

To calculate amplitude, the machine measures the time between the first and third sounds of each tick and needs the movement’s lift angle, a fixed figure set by how the movement is built. Witschi says it is about 51 degrees for most standard movements. A wrong setting gives a wrong amplitude, so use the figure for your movement where you can find it.

BPH: beats per hour

BPH tells the machine what “on time” sounds like. Witschi lists common values:

Tips for a better timegrapher reading

1. Find a quiet room. Background noise makes ticks harder to pick out.
2. Wind the watch first. Beyond the Dial notes amplitude is higher at full wind than after hours of running, so compare readings at the same state of wind.
3. Keep the watch still and in one position for the whole reading.
4. Check more than one position and compare.

Questions people ask

Where Watch Timegrapher helps, and where it does not

Watch Timegrapher is our Android app that turns your phone into a simple watch timing tool. Rest your watch on the phone in a quiet room and it listens to the ticks with the phone’s microphone. Every tick is timed and matched to its beat, so it gives a rate in seconds per day with a plus or minus band, finds the beat rate (BPH), and draws a live timegrapher trace. You can save and share reading cards, and recent readings stay on your phone. The sound is analysed on your phone and never uploaded. A one-time Pro unlock adds beat error, longer listening, a guided six position test, a watch log and calibration for your phone. It does not measure amplitude. A contact microphone can help it hear the tick.

What it is good for: a quick check of whether a watch is running fast or slow, comparing positions, and keeping an eye on a watch between services.

What it will not do: a phone microphone in a home is far less controlled than a bench instrument with a proper watch holder, so treat the numbers as a practical estimate, without bench precision. It cannot regulate or repair a watch. If your watch is losing or gaining a lot, stops, or you want an amplitude check, take it to a qualified watchmaker.

Questions or bugs: Email us.

Sources

1. Test and Measurement Technology for Mechanical Watches (2010), hosted by NAWCCWitschi Electronic Ltd · mb.nawcc.org
2. Collector Guide: Interpreting Timegrapher ResultsBeyond the Dial · beyondthedial.com
3. COSCWikipedia · en.wikipedia.org