**A tachymeter is one of the most common additional functions of wristwatches. Especially sporty chronographs are often equipped with it. How it works and how to use it correctly, we have summarized for our readers in this article.**

**Tachymeter Scale**

After the** ****logarithmic scale** is calibrated based on a **kilometer** or a **mile**, to measure the average speed of your car or motorcycle, for example, you need a section of that length. At the beginning, for example when passing a kilometer mark, you usually **start**** **the **chronograph** by pressing the top pusher. When you pass the end point, i.e. the next "milestone", stop the chronograph again using the same pusher. The tip of the central **c****hronograph** hand now points directly to the **average speed** achieved on the **tachymeter scale.**Reset the chronograph to zero using the lower pusher. It is already ready for another measurement.

**The Historical Use of Tachymeter Scale**

Historically, chronographs with a tachymeter scale were extremely important in certain areas - after all, many decades ago it was difficult to just pull out your smartphone and do the calculations in an app.

In 1964, the Swiss watch manufacturer Heuer brought out the Carrera model, the first chronograph specially developed for timing car races. The model was made famous by the five-time Formula 1 world champion and Heuer partner Juan Manuel Fangio at the Carrera Panamericana in Mexico. Furthermore in **athletics****,** for example at the Olympic Games, it was used to be taken in the classic way with chronographs, which often had a tachymeter scale. The Swiss watch manufacturer Omega has been the official timekeeper at *almost* all Olympic Games since 1932.

Historically, chronographs with a tachymeter scale were also important in aviation. At the beginning of the **Second World War** for example, German manufacturers such as Tutima or Hanhart had to develop and produce robust **pilot's chronographs with a tachymeter scale for the fighter pilots of the German Air Force. **Here, too, the purpose of the application was to measure the average speed. Another important tool here were maps, which is why the measurements were often carried out by co-pilots.

**How to Measure Speed Using Tachymeter**

A look at the history of the tachymeter quickly reveals that measuring speeds has a long tradition. The origin of the word lies in ancient Greek, where the term "tachymeter" is composed of the Greek words "ταχύςtachýs" for fast and "μέτρονmétron" for measure or scale.

The tachymeter is usually located directly on the bezel or the outer dial of wristwatches. The tachymeter scale with its speed information is therefore easy to recognize and cannot be overlooked. An additional stop function is a prerequisite for measuring speeds. Although you could also wait until the second hand has made a full revolution when measuring the speed, this is much more convenient with the stop function. It is precisely for this reason that the tachymeter scale can be found on almost all chronographs.

So, simply put, a tachymeter scale is an additional inscription on the watch (on the dial or bezel) that is given in units per hour. To do this, you have to measure the time between two units and use this to calculate how many units come out per hour.

The scale is arranged as follows: In order to be able to calculate the units per hour based on the measured seconds, 3600 seconds (which corresponds to one hour) are divided by the measured seconds. For 40 seconds you get (3600/40) so 90 units. As a result, the number 90 on the tachymeter is in the place of the 40 seconds. For 30 seconds you get (3600/30) 120 units, so the 30 second marker is 120. So the label follows the mathematical function U=3600/x where x is the seconds measured and U is the units.

* Example*: Let's imagine we run a distance of 200 m in 50 seconds. In this case, the tachymeter scale would read 72. So, the speed would be:

__72 x 1 hour x 0.2 km = 14.4 km/h__

**Other Measurement with This Scale**

Another example would be measuring the number of pieces of a product produced in a factory. Imagine a worker on an assembly line measuring the time between two consecutive products. Then, with the help of the tachymeter, it can be read how many pieces per hour are produced in this factory. For example, if the time between the first and the next product is 30 seconds, then it can be said that 120 pieces are produced per hour.

**Why Does the Scale only Go to 500?**

The scales on the common wristwatches usually only go up to 400 or 500 units per hour. The simple reason for this is that the measurement would also be too imprecise. Because in the range of more than 500 units, a small deviation of a tenth of a second already results in a very large deviation in the units.

**What Watch Lovers should know about the Accuracy of Tachymeter Measurements**

If you want to use your chronograph with a tachymeter scale to determine speeds, you should keep an eye on the achievable accuracy and the reading logic on which the scale is based. What is read on the tachymeter ring is actually the reciprocal of that portion of an hour that has elapsed since the second hand left the twelve o'clock position. The 120 found at six o'clock therefore means that exactly 1/120th of an hour has passed when the second hand arrives there. So the unit read on the tachymeter ring is "per hour" and it can be multiplied by any lengths or units to give the relative change in that parameter per unit of time.

In addition to speeds given in kilometers per hour, it was also possible to measure flow rates in liters per hour. The user of the tachymeter thus saves dividing the measured variable by the measured time specified in seconds and then multiplying by 3,600 seconds per hour, which ultimately results in the measured units per hour. The fact that the tachymeter ring usually does not begin at the top center at twelve o'clock, but only at a certain distance to the right side, is due to the fact that it is assumed that the measurement to be carried out will take at least 15 seconds, because shorter measurements due to the human reaction time would be too imprecise anyway and be subject to significant measurement errors.

If the measurement error is one second, this would mean an inaccuracy of almost seven percent for a measurement lasting 15 seconds. The method is therefore not suitable for exact laboratory measurements, but it does deliver useful results in everyday life.

**REFERENCES**

**Brunner, G.L. (2016). How the Tachymeter Scale Works.**

**ChronoNautix. (2020). Tachymeter Scale on Clocks – Function and Calculation Examples.**

**Citizen. How to Use the Tachymeter. Retrieved from **https://www.citizenwatch-global.com/support/exterior/tachymeter.html

**Roos, H. (2021). Tachymeter. Retrieved from **https://www.horando.de/uhrenmagazin/en/tachymeter/