As we outlined when we took a look at different types of bezel scales found on watches, a tachymeter is a scale on chronograph watches measuring speed and distance.
It is found on the rehaut of the watch (just inside the dial from the bezel) or can be inscribed on the bezel itself. Tachymeter comes from the Ancient Greek words for swift and measure, or rapid measurer.
In this article you will learn how to use the scale in different scenarios:
- What Is The Tachymeter Scale?
- How To Measure Speed With A Tachymeter
- How To Measure Distance Using a Tachymeter
- Tachymeter Limitations
- Using Digital Tachymeters
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The scale allows for the conversion of elapsed time in seconds per unit to the speed in units per hour. The seconds tracked can be anywhere from approximately 7 seconds through to 60 seconds. For measurements over 60 seconds, you’ll need to get a little creative on the math!
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The Tachymeter Scale
Looking clockwise around the dial or bezel, you will see the tachymeter scale starting anywhere from 400 to 1000 units. As you go around the dial, there is more elapsed time between the next marking, taking you to the finish at 60 units directly at 12 o’clock.
There is the central chronograph seconds hand which follows around the dial and the scale. When the chronograph is stopped the hand lines up with the marks on the tachymeter scale.
The equation T=3600/t is used to calculate most of equations; T is the tachymeter scale value while “t” is the elapsed time. The 3600 equals the number of seconds in an hour. This calculation converts elapsed time into speed.
How To Measure Speed With A Tachymeter
To figure out the speed of an object, you’ll first need to know the distance between two points. Keep in mind that the points are designed for 1 single unit, which can either be a mile or kilometer. If the distance is shorter or longer, there is more math involved!
Steps for measuring speed using the tachymeter on a chronograph:
- Know the set distance between two points
- As the object passes point one, start the chronograph
- When the object passes point two, stop it
- Look at where the chronograph seconds hand falls on the minute track
- Line up the hand with the tachymeter scale and this is your speed
Using the above steps, let’s take a look at an example. You’re watching a race and you know the distance between two signs is one mile. As the car you’re following passes the first sign, you start your chronograph.
When it hits the second sign, you stop it. Looking down at the chronograph, you see the hand falls at 35 on the minute track. Extending this to the tachymeter scale, it lines up with about the 103 mark. This means that the car was traveling at an average speed of 103mph between the signs.
This same calculation can be used for kilometers per hour. Instead of a mile between two points, you would know that it is a kilometer. Therefore, the final speed would be read in kph.
Using A Tachymeter to Mesure Speed In Different Distances
For distances shorter than 1 mile or kilometer there is more math involved.
If you were watching a car make a trip between two points that were 1/4 of a mile apart, you would look at 1/4 of the tachymeter measurement.
You can also use the same method to track something that is over 60 seconds in time. This time you’re watching a cyclist on a 1 mile race and you want to know how fast your friend is traveling.
You used your chronograph to start and stop when the cyclist passes the start line and finish line. You note that it took 1 minute 40 seconds, or 100 seconds total.
This is over the 60 second mark on the tachymeter scale, but you notice that if you take half of the seconds, 50, it falls within the tachymeter scale. This then lines up with the 72 tachymeter mark but you know that you then need to divide that by 2, giving you an average speed of 36 mph.
The same method can also be used to figure out how much time it takes to produce something in an hour. Say you want to know how many ties you can tie in an hour. You time that it takes 45 seconds to tie a tie start to finish. Looking across to the tachymeter scale, this lines up with 80, meaning you can tie an average of 80 ties in an hour.
How To Measure Distance Using a Tachymeter
Now that you know how to measure speed using your chronograph, measuring distance is quite similar. Instead of knowing the distance between two points, you need to know your traveling speed and make sure that it is constant. Your speed should also be above 60 of your preferred unit.
Steps for measuring distance using your chronograph and tachymeter:
- Know your constant traveling speed.
- Start your chronograph when you pass a certain point.
- Stop the time when you have traveled the speed lining up with the tachymeter scale.
- When the seconds hand is at your speed, it means you have traveled one mile.
Let’s take a look at an example of how to measure distance. You know you’re riding in a car going 80 miles per hour. You pass a house and start your chronograph. When the seconds hand reaches 80 along the tachymeter scale, you know you’ve traveled a mile. This means that every 45 seconds you’re traveling one mile. The same method works when using kilometers per hour.
Limitations Of A Tachymeter
There are few limitations when it comes to measuring using a tachymeter scale. We’ve already covered how to calculate distance when the time is over a minute, but what if the event is high speed? Most tachymeters start around the 5 to 7 second mark. If the event falls between 0 and 5 seconds, there are no marks on the scale to line up with the speed.
Similarly, very rarely will people need to measure a normal event where something is traveling faster than 500 miles per hour. It is also quite difficult to fit the numerals on the scale and start/stop the chronograph so quickly due to human reaction speed limitations.
Most chronographs feature a sweeping seconds hand which means that the chronograph hand glides between each mark. There are some watches that feature a deadbeat hand instead, so it jumps from mark to mark.
This makes it difficult to measure decimals of seconds, such as if something takes 50.5 seconds which then lines up with 71 on the tachymeter scale. These deadbeat hands only read the whole numbers. Very few chronographs feature this type but if you’re looking at buying a chronograph it is something to take into consideration.
Using Digital Tachymeters
Furthermore, digital tachymeters and chronographs are less expensive and more accurate than the classic mechanical version. The tachymeter scale displays on the digital screen after you start and then stop the timer.
However, finding and using the tachymeter may become buried in the other functions on the digital watch. The digital chronograph also gives a more sporty feel, so if you were looking for a multi-purpose watch you can wear with suit a digital version may not be sophisticated enough where as it may be when you’re dressed casually on a race track.
Overall, the tachymeter is both a functional scale and a bit of style. Once you learn how to master the different measurements, the scale is no longer something intimidating. In fact, it can be something to take advantage of and use.