# Total station (clock)

A tachymeter or correct tachometer (from ancient Greek ταχύς tachýs "fast" and μέτρον métron " measure , measure") is a scale on dials or bezels of stopwatches or wristwatches with a stopwatch function ( chronograph ) in the watch industry . A tachymeter is used to estimate the speed with which an object is moving or a process takes place with a fixed known route length (depending on the scale, e.g. 1 km) using the pointer.

Typical tachymeter scale on a wristwatch (orange-colored numbers on the outside of the dial)
Slide rule bezel and tachymeter (numbers just outside the innermost thin white ring on the dial)

## functionality

The tachymeter scale shows the reciprocal of the part of the hour that has elapsed since the second hand pointed to 12. For example, 30s is 120 because 1/120 of the hour has passed. The order of magnitude of the tachymeter scale is 1 / h (read "per hour"), so when multiplied by any length you get the change in this quantity per time, for example km / h (kilometers per hour) or liters / h (liters per hour) . It relieves the user of dividing the measured variable (distance, volume, etc.) by the measured time in seconds and then multiplying it out with 3600 seconds / h (to arrive at the unit per hour ).

With many total stations (like those in the two photos above) it is assumed that the process to be measured takes at least 15 seconds, but no more than 75 seconds. For this reason, the scale division is applied in such a way that the fastest process is indicated at 3 o'clock (after 15 seconds), while slower processes are indicated at e.g. B. 2 o'clock (after 70 seconds) can be read. This slightly twisted scale division is confusing at first, as one initially orientates oneself to the usual 12 o'clock position. This rotation is due to the fact that a significant measurement error can occur if the measurement times are too short. If a process with z. B. measured only 3 seconds in length and the measurement error is 1 second, results in an inaccuracy of about 33%. With a measurement time of 15 seconds and a measurement error of 1 second, the inaccuracy is only approx. 6.7%.

## Application examples

Speed ​​of a car: The speed of a vehicle can easily be determined by measuring the time for a known distance (e.g. 1 km) on the tachymeter scale. If the distance is covered in 30 seconds, the tachymeter scale shows 120 (1 / h). The speed for a distance of 1 km is 120 km / h. Calculated exactly with units:

${\ displaystyle 120 \ cdot {1 \ over {\ rm {h}}} \ times 1 {\ rm {km}} = 120 {{\ rm {km}} \ over {\ rm {h}}}}$

Speed ​​of a ship: To determine the speed of a sailing boat, a buoy with a line of known length is thrown into the water while it is in motion. The time or the value on the tachymeter scale is stopped until the line tightens and the buoy is pulled along. This tachymeter value is multiplied by the length of the line and thus gives the speed of the boat. If the length of the line was measured in meters, the speed is obtained in meters per hour. If you want to know the speed in knots, i.e. nautical miles per hour, you have to convert accordingly. It is easier if the line has a length of z. B. 0.01 nautical mile (NM), i.e. approximately 18.5 meters. Then only 0.01 NM has to be multiplied by the total station value to get the speed in knots.

Speed ​​of a filling process: To z. B. To determine the water consumption of a lawn sprinkler, one determines the total station value for filling a container with 1 liter of water. The total station value corresponds to the water consumption in liters per hour.

Printed total station for heart rate measurement

Pulse beat: A nurse's watch counts a fixed number of pulse beats. Counting begins when the second hand passes the 12 o'clock or 6 o'clock position. The patient's pulse can be read off directly on the total station scale. For the nurse's watch in the illustration, exactly 20 heartbeats are counted. A required counting time of 20 seconds corresponds to a pulse of 60 / min. If the required counting time is reduced to 15 seconds, the pulse is 80 / min.

## scale

The tachymeter scale is often engraved or printed on the bezel of wristwatches, especially if the watch is optionally available with a rotating bezel to determine the time. One should not confuse a tachymeter scale with a slide rule bezel. The latter is provided with a logarithmic graduation like a calculating disc rule . The tachymeter scale does not have to be rotatable if you start counting when the second hand has reached the 12 o'clock position. Furthermore, it is also not absolutely necessary to use a stopwatch function.