June 27th, 2009

Understanding the tachymeter function on an analog watch

On some of the analog watches or wristwatches from brands such as Rolex or Omega, there is a outer scale bezel that is named tachymeter . This tachymeter is simply a classic tool used to compute speed based on travel time over a fixed distance travelled (like one mile).

These tachymeter’s are not designed to measure times over 60 seconds.  Depending  on the range of the scale printed on the bezel, tachymetre timer normally works accurately for all elapsed times from about 7.2 seconds to 60 seconds, thus these originally were designed to measure speeds greater than 60 miles per hour.

To activate your tachymeter function, simply start the chronograph when the subject measured is 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 is adjacent to the second hand of the watch will display the speed (in miles per hour or kilometers per hour) of the object traveling between the two points.

For example, we want to discover the average speed of a particular Harley Davidson Roadster. We start the chronometer function when the motorcycle runs the red stoplight, and stop the timer after the motorcycle travels exactly 1 mile or 5,280 feet and gets pulled over. Then glance at where the chronometer hand (the second hand of the watch) is pointing to, and get the reading of the corresponding number at the tachymeter dial. Let’s say the stopwatch stops at the 9 o’clock position, meaning 45 seconds had elapsed for the motorcycle to travel 1 mile or 1 kilometer. At the 9 o’clock position, the tachymetre value shows the number 80.  This means the average speed of the motorcycle was 80 mph.

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 faster speed, user can decrease or increase the unit of measurement (e.g. change to half-miles, or ten miles). However, basic elementary math is required to get the correct average speed by using this method.

For example, we want to measure how fast the senior adult is going through the crosswalk we drive through daily. Start the chronometer when the green walk light starts, and stop the stopwatch when the elderly person gets to the other side. Let’s say the old person took 10 seconds to finish walking 53 feet. According to the tachymeter, the speed of the senior is 350 miles/hr. However, the elderly person only went one hundreth (1/100) of a mile, so we should divide 350 with 100 or 1/100 of 350, which means the actual speed of the pedestrian is 3.5 miles per hour (go speed racer!!).

If your speedometer ever quits on your car while traveling down the interstate you can also use the tachymeter function of your watch to acquire the speed.  All interstates have mile markers.  Start your chronometer function at one mile marker and stop it at the next.  If the watch only made it to the 40 second mark, you need to slow down.  You could get a ticket for going 90mph on US interstates.

What if the object travels too fast, such as a bullet or space shuttle. In this case we 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 to be accurate). We just measure the time taken by the object to cover 10 miles, let’s say it took 15 seconds, so the tachymeter will tell us the the speed is 240 miles per hour. In reality, 10 miles have been traversed, so the correct speed is 240 x 10, which equals 2400 mph. Since the speed of sound is 768 mph, your subject is over Mach 3.I just don’t feel that tachymeter’s were ever designed to accurately measure speed faster than sound.

After reading this tutorial you should now know how to use your tachymeter (commonly misspeled as tachymetre or tacheometer).  Of course premium established vintage watch shops typically carry a large selection of watches with Tachymeters like this used watch shop in Miami, FL.

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