Assessing the Gandhi Watch

Last time I showed you a practice watch I'd obtained, a refurbished HMT movement-based wristwatch with a picture of Gandhi on the face. I alluded to something called a timegrapher that is used to assess a movement's wellness, and today it's time to see what it says about my new watch. 

If you're not familiar with how a watch movement works, the following is a very brief summary of what makes it tick.

A watch's mainspring is a coiled band of metal that is wound around a post and the tighter it's wound the more energy it contains. That energy tensions a series of reduction gears that end with what's called a pallet. The pallet rocks back and forth in unison with the oscillation of a spring-mounted wheel called the balance. With each oscillation the pallet pushes the balance a little and gives it a bit of energy. So: mainspring stores energy, the gears and pallet transfer energy, and the balance dissipates energy via friction as it oscillates.

The balance wheel is basically a torus of metal that oscillates around the axis through its center and perpendicular to the plane of the torus. It is designed to oscillate at an exact frequency (18000 beats per hour in the case of the Gandhi watch). This causes the energy to be released from the mainspring and the reduction gears to turn at a fixed rate, thereby driving the watch hands around the face.

Now that you completely understand how a watch works (ha ha) we can continue. 

A perfect watch keeps perfect time; it has a balance wheel that oscillates at a constant frequency regardless of how the watch is oriented. Of course, no watch is perfect. The goal of servicing a watch is to find the imperfections and try to minimize them. Toward that end there's a diagnostic device called the Timegrapher:

A timegrapher includes a stand (left) that holds the watch to be assessed. It has a very sensitive microphone in it so that it can hear the workings of the watch and send that information to the display unit (center). 

This morning I put the Gandhi watch on the stand (in the face-up orientation) and put my timegrapher to work. Here are the results and a little about what they say about the watch

This will need a little decoding, so number by number here's what it says.

1. Daily time error. It's slow by about a minute a day, which is just okay. A few seconds per day would be better. 
2. Balance amplitude. The balance is turning through 133 degrees, which is far too small. A healthy watch turns through well over 200 degrees.
3. Beat Error. This is a measure of the asymmetry in how the balance is being fed energy. A decent beat error is under 1ms, so this is much too large. One consequence of this error is that the watch won't start running easily, and that's what I observed--it would not start until half wound. It also means the watch may perform quite differently in different orientations.
   
4. Alternating display of lift angle and beats per minute.  For this watch the lift angle has been set to 52 degrees and the unit has detected that the watch should be operating in the 18000bph mode.
5. The daily time error during the previous sampling period of 12 seconds. 
   
6. The timegrapher provides a visual summary of items 1 and 3 with sloping lines. The greater the slope the larger the error, and the amount of beat error is indicated by parallel lines some distance apart. A watch with zero beat error would show only one line.

Next I put the watch stem-up on the microphone stand 

The watch is now only 19s slow per day, but not much else has changed. (7. happens to be showing the beat rate in this image.)

Two things stand out, the terrible amplitude and large beat error. As a first guess this watch needs a good cleaning and oiling to improve the amplitude. Disassembling the watch may reveal other problems.

Next time, the Gandhi watch in pieces.

And after that, creating a watch for astrophotography.