On some of the analog watches or wristwatches from branch such as
Omega and Casio, there is a outer scale bezel that inscribed around the
rim and named tachymeter (also known as tachymetre or tacheometer). If
you flip through the watch’s user guide or manual, you will not likely
to find any explanation or definition on what is tachymeter or how to
use the tachymeter functionality. Tachymeter is actually a tool used to
compute speed based on travel time over a fixed distance travelled (like
one mile or one kilometer). Thus, the tachymeter bezel or dial is a
logarithmic scale (actually it’s not, as spacings between the marks on
the tachymeter dial are proportional to 1/t where t is the elapsed time)
that simplifies the computation of speed of an object, by computing the
following function:

Tachymeter Dial = 3600 / Elapsed Time In Seconds

Depending on on the range of the scale printed
on the bezel, tachymetre timer normally works and valid for all elapsed
times from about 7.2 seconds to 60 seconds, thus only able to measure
speed greater than 60 miles per hour or 60 kilometers per hour.

To use tachymeter, simply start the chronograph or stopwatch when the
object measured passing the starting line. When the object reaches the
next mile or kilometer marker, stop the chronograph or stopwatch. The
point on the tachymeter scale that adjacent to the second hand of the
watch indicating the speed (in miles per hour or kilometers per hour) of
object traveling between the two points.

For example, we want to measure the average speed of a vehicle is
moving. Start the chronometer or stopwatch function when the car passes
the starting line, and stop the timer after the vehicle traveled exactly
1 mile or 1 kilometer. Then look at where the chronometer hand (the
second hand of the watch) is pointing to, and get the reading or value
of the corresponding number at the tachymeter bezel or dial. Let’s say
the stopwatch stop at 3 o’clock position, meaning 15 seconds had elapsed
for the car to travel 1 mile or 1 kilometer. At 3 o’clock position, the
tachymetre value shows that number of 240, which means the average
speed of the vehicle was 240 mph or km/h.

Due to the limitation and constraint on the tachymeter scale
mentioned above which makes tachymetre measurements works and valid only
for certain range of speed, so in order to calculate and measure slower
speeds or higher speed, user can decrease or increase the unit of
measurement (e.g. change to half-miles or half-kilometers, or ten miles
or ten kilometres). However, simple calculation is needed in order to
get the correct average speed by using this technique. It’s also true if
the length of the distance does not exceed a unit of mile or kilometer,
or other unit which the speed will be based on.

For example, if we want to measure how fast the 200m athlete is
running. Start the chronometer when the race starts, and stop the
stopwatch when athlete crosses the finishing line. Let’s say the athlete
used 20 seconds to finish 200m running, so according to tachymeter, the
speed of the athlete is 180 km/h. However, the athlete only ran one
fifth (1/5) of a kilometer, so we should divide 180 with 5 or 1/5 of
180, which mean the actual speed of the runner is 36 km per hour.

The above example can be used for object or thing that moving too
slowly too, as when the object travels 1 mile or 1 kilometer, the
duration would have exceed 60 seconds, the maximum limit of tachymeter.
So the possible workaround is to measure the time taken for the object
to move a shorter distance (such as 100m) and divide the tachymeter
value with 10 (as 100m is 1/10 of a kilometer).

What if the object travels too fast, such as a rocket or plane. In
this case, user can increase the units of the distance covered by the
object to measure a longer period of time so that the duration is
greater than at least 7.2 seconds (typically the lower limit). We can
measure the time taken by the object to cover 10 miles, let’s say it
took 30 seconds, so the tachymeter will tell us the the speed is 120
miles per hour. However, 10 miles have been traveled, so that actual
speed is 120 times 10, which is 1200 mph.

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