How to use a tachymeter

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Understanding how to use a tachymeter is a challenge because there are so many

Although smart phones seem to be able to do everything including tell time, a lot of us still like to wear wristwatches. The digital watch is very popular due to its versatility and price, but many people still prefer precision time instruments that are now called Ďanalogí watches because they have sweeping hands on their faces instead of digital displays.

These watches often have a number of extras and dials. One of the more sophisticated features on both menís and womenís watches thatís extremely handy but requires a little practice and a bit of math to understand and use is the tachymeter. The tachymeter is a numbered scale on a rim or bezel around the face of the watch that can be used to measure travel time or distance.

The numbers on the tachymeter scale usually range from 500-700 (the first number which is generally around the 1 on the clock face) and decrease as you move clockwise, with 60 being the number over the 12. These numbers are calculated by the formula 3600/elapsed time in seconds, where 3600 is the number of seconds in an hour. If this sounds complicated already, letís try a simple example.

The tachymeter scale is most useful for computing the average speed of something over a fixed distance, like a carís lap time at a race track. Pick a fixed distance for the car to travel in under 60 seconds Ė most tracks have mile markers. When the car reaches the starting line or first mile mark, start the watchís stopwatch (or start timing when the second hand crosses the 12 Ė a little more difficult). When the car travels one mile, stop the stopwatch and look at the number on the tachometer where the second hand stopped. If the hand is at the 35 second mark, the number above it is 100, meaning the car was traveling an average of 100 miles an hour. If the hand is at the 20 second mark, there may be no number between the 160 and 200 on the tachymeter, but since it is halfway between them, that means the car was traveling at about 180 miles an hour.

Obviously, the scale is not precise but itís a pretty close for estimates. Also, itís limited to times between about 7 seconds (where the first number is) and 60 seconds.

To measure distance traveled, you need to know the rate of speed at which youíre traveling. Letís say youíre traveling at 70 miles an hour. Start the stopwatch and note where youíre at. Stop the stopwatch when it hits 70 on the tachymeter and youíve traveled one mile. Since the scale starts at 60, this only works at speeds great than 60 miles an hour. For speeds slower than that, multiply the speed by 2 (or more) to get the number higher than 60. For example, if youíre traveling 35 miles an hour, multiply by 2 to get 70. Now when stopwatch hand hits 70, youíre traveled a half mile (one mile divided by 2.)  If youíre traveling 20 miles an hour, multiply by 3 to get 60 and when you reach 60 youíve gone one-third of a mile (one mile divided by 3).

With a little math, using a tachymeter can be a fun, old-school way of measuring speed and distance traveled.

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