Watch Escapement Analyser - eTimer
Stability and accuracy...
There has been some speculation
concerning the possible accuracy, and the long and short term stability, of this software
based watch timer system. In particular, promoters of a PIC chip timer are
often heard to say that the Windows sytem clock is inaccurate. Of course it is,
everybody knows that. This program does not make use of the Windows sytem clock.
reference clock is the multi-media wave sample clock. This is divided from a
stable crystal oscillator which has a consistent and predicatble rate.
In fact its performance far exceeds
that of any mechanical watch. It also outperforms most mechanical watch timers.
To support this claim I have arranged a simulated watch tick
from my watch-timer calibrator. It is externally clocked by one of the workshop standards; a Datum System 2000 GPS disciplined rubidium source.
For our purposes it is a perfect watch. The simulator produces an acoustic tick from a piezo sounder which is gripped
in a watch stand microphone. This is fed to a very cheap USB audio adapter (sound card). Below is shown the waveform,
and paper tape display, of the simulation.
We see a clear waveform of the unlocking, impulse
and drop, together with a nice straight line on the simulated paper.
Here is a long term graph plot of the same test.
There are three traces enabled. The red one is the instantaneous rate.
Superimposed on the same graph in pale green is the average rate. Below these in green is the total deviation.
This is a measure of how far the standard watch and the program have drifted apart during the 1.7 hour test. There is a steady rate error
of 6 ms (yes, milli-seconds) per day. This could easily be trimmed out if desired; it only amounts to 1 second in 167 days. But the acid test is the straightness of the deviation curve.
The test system was subject to normal workshop temperature variations, and no special preparation
was undertaken. And remember, this is a very cheap USB dongle sound
adapter. There is likely to be something better in your computer.
This watch timer system has been under development
for fifteen years. It is worthy of serious consideration.