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 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
the 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.

Updated:
22 May 2017 11:04 PM

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