How to use a tachymètre

The one bezel to rule them all according to some, the most useless decoration to others, it is indeed the tachymètre (aka the tachymeterTachymeter A tachymeter is a feature found on some chronograph watches, it is a scale that is usually printed on the bezel, sometimes on the dial. It allows the wearer to measure the speed of an object over a known distance by timing the object's movement with the chronograph's second hand. The wearer can read the speed of the object on the tachymeter scale. [Learn More]).

It can be used to measure either (average) speed or distance.

Found on most chronographChronograph A chronograph complication is a feature in a watch that allows the wearer to measure elapsed time in addition to telling the time. It works by having a separate set of gears and levers, called the chronograph mechanism, which is activated by pressing a button or a pusher. The chronograph mechanism starts and stops the chronograph's second hand, which is usually located on the watch's dial, separate from the regular watch hands. The elapsed time is usually displayed on a sub-dial or a register on the watch's dial. [Learn More] watches, it is easy to forget how to use this handy little tool. It can be used to measure either (average) speed or distance. The most fun, is measuring speed, which is what we’ll have a closer look at. We shall take the Winter Olympics as an example on how you can actually use the tachymètre on your watch. We’ll be using the Omega Speedmaster as an example, Omega being the official timekeeper of the Pyeongchang Winter Olympics and all that..

A quick note: easiest things to time are longer than 7 seconds (as the scale only starts after 7 seconds on most tachymètres) and less or equal to 1 minute. Anything other than that will require extra (basic) math.

MEASURING AVERAGE SPEED:

Let’s take the example of a new world record set by Wu Dajing who won China’s first Olympic gold medal in the men’s 500 meters Short Track Speed Skating. Everywhere you read, you got the following information: “a world record time of 39.584 seconds”

But how fast did he go? How to use the tachymeter to get to the answer?

The method below shows how you can very quickly see what his average speed was, as he was flying over the ice on essentially two razor blades strapped to his feet. Note that you can do this ‘live’ as it is happening, or also after the facts.

• Live measuring:
  • As the race starts, also start your chrono;
  • As the racer crosses the finish line, stop your chrono;
  • Now look at where your second hand is pointing on your tachymètre;
  • In our case, you can see it stopped at 39,584 seconds and it is pointing at the number 91 on the Tachymètre (see pic above);
  • Now don’t jump the gun thinking he’s going 91 km/h… we still need to divide this by two considering he’s only doing the 500 meters, and not the full 1km;
  • Dividing 91 by two, equals 45.5 km/h average speed! There you go, now you know 😉
• Post-race measuring:
  • Skip the above 2 steps, and just look directly at the number on your tachymètre next to 39 seconds. You’ll see it is at around the 91 mark;
  • Dividing 91 by two, equals 45.5 km/h average speed yet again 😉

Once you start using this to time average speeds, it can get addicting though.. user be warned.

Little bonus knowledge

Speed doesn’t have to be kilometers per hour of course, it can also be miles per hour or even… strap changes per hour.

Say it takes you 25 seconds to change your watch strap on a watch. How many strap changes can you do on average in one hour (if you were to take no breaks)? Just read the number on your tachymètre next to 25 seconds… the answer is 145 strap changes in one hour!