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Complications: The Tachymeter Explained

Anyone who’s either owned or held a chronograph in their lives have more than likely noticed a series of etchings surrounding the watch bezel. To be more specific, numbers (often beginning at the 12 o’clock position) and labeled in ascending order counterclockwise around the bezel are often present on any halfway-decent chronograph. Now, with a bit of common sense and intuition, you may have figured this one out on your own. That is to say, given the chronograph’s inherent purpose as a stopwatch for the racing world, these numbers have something to do with timing, distance, and speed. And as luck would have it, you’d be right.

It’s here where the tachymeter was born, purposed to decipher both the speed and time it takes to travel a specific distance at a constant speed. In short, a tachymeter is a means of converting elapsed time into speed (miles or km per hour). For reference, two of the most common watches in production today hosting this feature happen to be the Omega Speedmaster chronograph and the Rolex Cosmograph Daytona. So, how exactly does a tachometer work? It’s actually quite straightforward once you understand the methodology behind the tool.

The Basics

Three Different Kinds of Tachymeters

Before pressing forward, however, we felt it would be beneficial to start with a brief introduction of the three types of tachymeters that have since evolved since their initial introduction in the late nineteenth century: Circumferential on a fixed bezel, Snail-Type, and Circumferential on a rotating bezel to be exact.

Circumferential on a Fixed Bezel

Our first type of tachymeter is not surprisingly the most commonly-found iteration around today. It’s here where, as you might have guessed, a watch’s tachymeter scales are located in fixed positions around the perimeter of either the watch’s bezel or dial. These can also come in the form of either a fixed outer or inner bezel as well. Again, more often than not, you’ll find that majority of chronographs on the market feature this sort of tachymeter setup.

Snail Type

As the name might suggest, snail-type tachymeters are best used to measure slower speeds (i.e. events that occur slower than 60 cycles per hour). What results instead is a tachymeter design that sort of spirals inward on the center of the dial allowing for a wider range of speeds since the scale it’s usable for multiple rotations of the second hand. Typically, this is present via three colored rings measuring times from 0 to 60, 61 to 120 and 121 to 180.

Circumferential on a Rotating Bezel

The rarest of the three, rotating bezel tachometers offer something fixed bezel iterations cannot: that is, measuring average speeds for more than one single base distance. These came in response to the need to calculate average speed over longer distances. Here, drivers could easily set the rotary bezel to the position of the minute hand, note the current distance, and check the position of that same hand 60 units of distance later and determine an average speed.

Measuring Speed

Simple and Straightforward

Since most of the tachymeters out there are featured on fixed bezels, measuring speed is a straightforward task. The process? Simply time yourself – or the driver – over the measured distance and read the speed off of the tachymeter (the numbered scale). To use this tool, let’s take a second to images you’re watching a Formula 1 race and wish to calculate the speed of a car as it circles the track. You know, for instance, that the track 1 mile long. So, you start the timer as soon as the driver starts the lap and then stop it immediately once they finish the lap. How long did it take for the driver to make the lap? 25 seconds? 40 seconds? If it was 25 seconds you’ll notice that syncs up with 145 – the same goes for 40 seconds that syncs up with 90 – equating to the driver’s constant speed: either 145mph or 90mph since the tachymeter measures the frequency of something occurring within one hour.

Now for those who wish to see the equation for themselves, we’ve outlined below how a tachymeter converts the amount of time it takes for an event to happen and the number of times that event will take place over the course of one hour.

Measuring Speed

T = 3600/t

Here, T is the scale value you’d find on the chronograph; t is the time it takes for a specific event to occur, and 3600 represents the number of seconds in an hour. So, using the example from above, 90 (T) is equal to 3600 (# of seconds in an hour) divided by 40 (t).

Measuring Distance

Precise Measurements

Much like you can calculate speed when you understand the distance involved, calculating distance is just as straightforward when you have a speed calculated. Here, speed must be held constant in order to gauge an accurate measurement of distance (which is measured in time taken to cover a certain area). For example, the tachymeter scale is rotated to align perfectly with the chronograph’s second hand. So, when the second-hand reaches the number on that scale where you know speed is indicated, one unit of distance (be it miles or kilometers) has been traversed. Much like you can calculate speed when you understand the distance involved, calculating distance is just as straightforward.

For instance, if you’re traveling at 80 mph, then the distance traveled during the time it takes the seconds hand to sweep around to 80 (45 seconds) is how long it takes you to travel 1 mile. From here, you can calculate an exact ETA to your destination – barring changing traffic patterns, etc. Additionally, your watch’s tachymeter can be used to calculate other events as well where a per-hour set could be useful. Specifically, assembling products in a manufacturing facility, packaging goods at a customer fulfillment center, or even determining the writing speed of a reporter could all be determined using a tachymeter.