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A tachymeter is one of the most common additional functions found on wristwatches. Sporty chronographs in particular are often fitted with one. In this article, we explain how it works and how to use it correctly.
Tachymeter or Tachometer?
The term tachymeter is a combination of the two ancient Greek words tachýs and métron, meaning fast and scale or measure, respectively. In practice, the term refers to two fundamentally different things. In geodesy, a tachymeter is an optical or electronic instrument used to determine horizontal directions and vertical angles. Unlike theodolites, tachymeters also allow surveyors to determine slope distances, that is, distances to specific target points measured at an incline.
The instrument owes its name to the fact that these measurements can be taken relatively quickly and easily. A tachymeter on a watch, by contrast, is a function for measuring speed, which, strictly speaking, ought to be called a tachometer. The fact that, in practice, it isn't referred to by the same term as, say, the speed-measuring device in a vehicle is certainly due in part to tradition.
On the other hand, it also points to the fact that there is a considerable difference between the two. The speedometer in a vehicle continuously measures the speed based on the distance traveled and displays the result directly. The tachymeter scale on a watch, however, works somewhat differently.
More Estimate Than Measurement – How It Works in Practice
First of all, it should be noted that a watch's tachymeter ring is more a tool for estimating speed than a precise measuring instrument. In addition, anyone who wants to determine their approximate speed needs to be able to identify or read off fixed, known distances.
In practice, this is mostly done using kilometer markers or kilometer signs alongside roads, railroad tracks, or racetracks. The measurement works like this: the chronograph seconds hand is started from the 12 o'clock position when passing a kilometer marker and stopped again after one kilometer. The speed can then be read off the tachymeter scale at the point where the hand has stopped.
Pressing the pusher at exactly the moment you pass the kilometer marker requires a certain amount of concentration, so for safety reasons the driver should delegate this task to the front-seat passenger or someone else on board rather than doing it themselves.
It's easy to understand why motorsport-minded fans of retro-styled watches are drawn to this method of measuring speed. Yet even in a modern car with a speedometer, there can be situations in which this method comes in handy. The reason is simple: the speedometer fitted as standard in the dashboard shows the current speed, but this often varies considerably while driving, for example in city traffic or on winding roads where you frequently have to brake and accelerate again.
The resulting overall average speed can differ significantly from this, but for a rally crew or participants in longer races, for example, it is a crucial piece of information that matters far more to the final outcome of the race than the speed driven at any given moment along the way.
What Watch Enthusiasts Should Know About the Accuracy of Tachymeter Readings
If you want to use your chronograph with a tachymeter scale to determine speeds, you should keep in mind the achievable accuracy and the logic behind reading the scale. What you read on the tachymeter ring is actually the reciprocal of the fraction of an hour that has elapsed since the seconds hand left the 12 o'clock position. The 120 found at 6 o'clock therefore means that exactly 1/120 of an hour has passed when the seconds hand gets there. The unit read on the tachymeter ring is thus "per hour," and it can be multiplied by any length or unit to give the corresponding change in that parameter per unit of time.
Besides speeds in kilometers per hour, it could also be used to measure flow rates in liters per hour, for example. The tachymeter thus saves you from dividing your measured quantity by the elapsed time in seconds and then multiplying by 3,600 seconds per hour, which ultimately gives you the measured units per hour. The fact that the tachymeter ring usually doesn't start at the top center at 12 o'clock, but some distance to the right of it, is because it is assumed that each measurement will take at least 15 seconds; shorter measurements would be too imprecise anyway due to human reaction time and would be subject to significant measurement errors. This position of the scale may seem confusing at first glance, but anyone who uses it regularly usually gets used to it quickly.
If the measurement error is one second, this would mean an inaccuracy of just under seven percent for a 15-second measurement. The method is therefore unsuitable for precise laboratory measurements, but in everyday life it delivers perfectly usable results. The tachymeter scale is usually positioned around the edge of the dial or on the bezel. Models featuring the ring can be found from all the major brands, such as Tag Heuer, Rolex, Omega, Breitling, or Hublot, and many others.
