How to use a chronograph and a tachymeter

The pushers

• The top pusher, at 2 o'clock, starts and stops the chronograph. Press it again to restart; the time adds on.
• The bottom pusher, at 4 o'clock, resets the hands to zero.
• The large central seconds hand is the chronograph seconds. The small seconds, if there is one, keeps running all the time in a subdial.

Timing something

1. Check every chronograph hand is at zero.
2. Press the top pusher to start.
3. Press it again to stop. Read the hours subdial, then the minutes subdial (usually counting to 30), then the central seconds hand.
4. With the chronograph stopped, press the bottom pusher to reset.

Rules that protect the movement

• Reset only when the chronograph is stopped. Pressing reset while it runs can damage a standard chronograph. A flyback chronograph is the exception: it is built to reset and restart in one press.
• Never press the pushers underwater unless the manual says you can. If they screw down, unscrew them first and screw them back before water.
• Leaving the chronograph running all day is not harmful, but on most movements it uses power and wears the chronograph parts. Stop and reset it when you are done.

Using the tachymeter

The tachymeter scale on the bezel or the edge of the dial turns the time taken over a known distance into a speed. Start the chronograph as you pass one marker and stop it at the next, one kilometre or one mile later. The chronograph seconds hand then points to your speed in kilometres or miles an hour.

The scale reads speed = 3600 / seconds. 45 seconds for a kilometre is 80 km/h; 30 seconds is 120 km/h. It only works for times between about 7 and 60 seconds. The same sum works for anything per hour: time one part off a production line and the scale shows how many an hour.

Try it

Pick the chronograph face on the sync screen and its pushers work, with the same rules: reset only works when it is stopped.