Tachymeter

A tachymeter is a scale, printed on the bezel or on the outer flange of a chronograph dial, that turns an elapsed time into an average speed. Start the chronograph as you pass the beginning of one unit of distance, stop it at the end of that unit, and read the number the seconds hand points to. That number is your average speed in units per hour. Nothing inside the watch is involved: the scale is a printed nomogram, and the chronograph is just the clock that feeds it.

How the scale is built

It is a single calculation drawn as an unevenly spaced scale: speed equals 3600 divided by the elapsed time in seconds. There are 3600 seconds in an hour, so covering a mile in 45 seconds means 3600 divided by 45, that is 80 miles per hour, and 80 is printed opposite the 45 second mark.

Because the numbers rise steeply as the times get short, the scale is crowded at one end and open at the other. A standard single-revolution tachymeter runs from 60 seconds, marked 60, down to about 7.2 seconds, marked 500, though many bezels stop at 400 or 300 where the numerals would otherwise become unreadable. Anything slower than a minute cannot be read, because the chronograph seconds hand has gone round again.

That compression is also where the accuracy goes. At the slow end the scale is forgiving: half a second of reading error at 45 seconds moves the answer by about one unit. At the fast end it is not: the same half second at 7.2 seconds moves it by more than thirty. The mechanical resolution is finer than your thumb, since a chronograph hand on a 28,800 vibration per hour movement steps in eighths of a second, so the error you actually get is human, and it is worst exactly where the numbers are largest.

Base and other markings

Some bezels are engraved with a base, most familiarly the Speedmaster's "TACHYMETRE BASE 1000". That is the maker stating the unit the scale assumes, 1000 meters in that case, so a kilometer timed on the chronograph reads directly in kilometers per hour. The arithmetic behind a base 1000 scale and an unmarked one is identical.

Spiral or snail tachymeters, printed on the dial rather than the bezel, wind inward through two or three revolutions and so extend the scale below 60 units per hour, at the price of a busy dial and a scale you must count revolutions to read. Omega claims the Speedmaster of 1957 as the first chronograph with the tachymeter moved from the dial to the bezel, a change that freed the dial and made the scale legible on the wrist. The Carrera has carried the scale in both places across its generations.

In practice

The unit is up to you, provided it is consistent. Time yourself between two motorway markers a kilometer apart and the scale reads kilometers per hour; time a measured mile and it reads miles per hour.

The same scale works for anything measured per hour. Time one item coming off a production line and read the units-per-hour figure directly, which is how tachymeters were sold to foremen as often as to drivers.

Two cautions belong to the chronograph rather than the scale. Do not reset a running chronograph unless the watch is documented as a flyback, and do not touch the pushers underwater unless the manual explicitly permits it, which it almost never does. Both points, and the mechanisms behind them, are covered in our guide to chronograph watches.

Common misconceptions

A tachymeter measures a rate, not a distance. It cannot tell you how far away something is, and it does not measure instantaneous speed, only the average over the unit you timed. Slow down halfway through the mile and the scale will never know.

It is not a tachometer either, despite the near-identical name. A tachometer is an instrument measuring rotational speed, the dial in a car that reads engine revolutions per minute. The two words are unrelated in use, and the confusion is common enough in listings to be worth checking.

It is also not one of the other scales that appear on similar dials. A telemeter measures distance from the delay between a flash and a bang, and a pulsometer is graduated to convert a count of heartbeats into beats per minute. A slide rule bezel, as on the Navitimer, is a different tool again: two logarithmic scales that multiply and divide anything, no chronograph required. All of them look alike at a glance and do completely different jobs.

Finally, a tachymeter printed on a watch with no chronograph is decoration. You can still use it by starting your measurement as the running seconds hand passes 60, but you cannot stop the hand, so the reading has to be taken on the fly. If you are weighing up whether the complication earns its place, what is a chronograph for takes the question head on.