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How does a tachymeter work?

Geplaatst op 17-07-2023 door Frank Andringa

Have you ever wondered how a tachymeter works? Or what it actually is? Or what those numbers on the bezel around the face of your watch actually mean? Not at all strange, because it's not something you use every day.

Tachymeter for decoration?

Is it for decoration, those numbers on the bezel of your watch? No definitely not. They can be there for diving or to measure speed. In the latter case, they are a tachymeter.

Whereas a diving watch has the digits 50, 40, 30, 20 and 10 on the ring around the glass (diving ring), a tachymeter usually starts around 400. Incidentally, not at 12 as we are used to, but between 1 and 2. Slowly, those numbers decrease to 60, which is above the 12. Why such a strange count? We are happy to explain that to you.

How does the tachymeter work?

A tachymeter is designed to measure speed. And actually, it works quite simply. Most importantly, you need to have a distance of exactly 1,000 metres with a start and end point. The moment a vehicle passes the start point, you turn on your watch's chronograph. Then, when the vehicle passes the end point after 1,000 metres, you stop the stopwatch.

Also read: 'How to set the second hand of a chronograph watch'

And now?

So at this point you know how fast the vehicle has driven over a distance of 1,000 metres. With this data, you can do some very complicated calculations on how fast the vehicle has driven. But the tachymeter makes this unnecessary. This is because the second hand of your watch points to a point on the tachymeter. This is the average speed the vehicle drove over a the kilometre you measured.

Citizen watch with tachymeter
Citizen watch with Tachymeter


Measuring speed, an example:

In forty seconds, the measured vehicle travels exactly one kilometre. The second hand of your watch not only points to the 8, it also points to a number on your tachymeter: 90. So the vehicle drove an average of 90 kilometres per hour.

Is that right?

Let's do the math. If you travelled 60 kilometres in an hour, you drove an average of one kilometre every minute, since an hour consists of 60 minutes. If you have driven 90 kilometres in an hour, how many seconds does one kilometre take you? Divide 60 (minutes) by 90 (kilometres). That is 0.66666666667, or 2/3. 2/3 of a minute is 40 seconds and that says the vehicle was travelling at an average of 90 kilometres per hour.

Tip: 'Never change the date of a watch between 10 and 4 o'clock'

Disadvantage of a tachymeter

A tachymeter has a big disadvantage. Basically, you can only measure speeds from 60 kilometres per hour. Some watch brands have found a solution to this. For instance, they show 55 and 50 kilometres per hour on the right-hand side of the 12. But in doing so, you use more than 60 seconds.

After reading this, are you looking for a watch with a tachymeter? You can find our watches with a tachymeter here.

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