Tachymeter

A tachymeter or tachymeter scale is a scale that can mostly be found around the rim of an analog watch. It is a converter of elapsed time (in seconds per unit) to speed (in units per hour).

The speed can, depending on the scale, be calculated in (kilo)meters per hour, miles per hour (all kinds of miles including statute miles and nautical miles), and knots per hour. Most watches with a tachymeter have a chronograph too.

To use it, you need to follow these steps:

– Start the chronograph as you pass a known distance marker, typically 1 kilometer or 1 mile
– Stop the chronograph when you pass the next distance marker
– The scale will show a number corresponding to the speed you’re traveling in units per hour (km/h or mph)

For example, if it takes you 30 seconds to cover 1 kilometer, the chronograph hand will point to “120” on the tachymeter scale, meaning you’re traveling at 120 kilometers per hour.

A tachymeter only works for speeds within a range of about 60 to 500 units per hour, as the markings become compressed for shorter times. It’s a simple, effective tool for measuring speed over a fixed distance.