A tachymeter is a scale that is often found on the bezel or dial of a wristwatch. It is used to measure speed over a known distance, making it a popular feature in sports and racing watches. The tachymeter scale is typically calibrated in kilometers or miles per hour, and is used in conjunction with the chronograph function of the watch.
To use a tachymeter, the user starts the chronograph function as they pass a fixed point, such as a mile marker or a lap line. The user then stops the chronograph function as they pass the next fixed point. The tachymeter scale on the watch can then be used to determine the speed at which they were traveling over the known distance. For example, if a user covers a distance of one kilometer in 30 seconds, they can use the tachymeter scale on their watch to determine their speed in kilometers per hour. If the tachymeter scale shows 120, it means that they were traveling at a speed of 120 kilometers per hour.
For most watches with integrated Tachymeter, it is assumed that the to be measured process has a duration between 15 and 75 seconds. As a result, the scale is divided in such a way that the fastest process is indicated at 3 o'clock at the 15 second mark, while slower processes can still be read at, for example, 2 o'clock. This slightly twisted scale division might seem confusing at first, as one is initially oriented to the usual 12 o'clock position, but it is necessary to avoid significant measuring errors which would occur if the measuring times were to short.
Tachymeters were first introduced in the early 20th century and have since become a popular feature in many types of watches, including chronographs, sports watches, and even dress watches. They are highly valued by watch collectors and enthusiasts for their practicality and aesthetic appeal and found a wide range of use in many fields from aviation to racing.
Tachymeters can be found in a wide range of watch styles and designs, from classic vintage models to modern sporty models. Some watches have tachymeters that are incorporated into the dial of the watch, while others have a separate bezel that is used for the tachymeter function.