What is the function of the ominous tachymeter scales, which are found on many watches on the bezel?
A guide is not to be found in any Rolex booklet. For this reason we wanted to get to the bottom of this with the help of Matthias (Prof. Rolex) from the R-L-X Forum, who has thankfully made this guide available:
In general, it is assumed that the bezel is a speedometer with which the speed of a car can be measured. While this is a possible application, however, the measurement accuracy is very rough because the required distance for this must be exactly 1km and the speed must be constant over the measuring time, certainly not a very practical environment for such a measurement!
As already noted on the bezel, units are measured per hour (units per hour). These can, of course, be the above-mentioned kilometers, but also any other dimensions, e.g., produced units or transported persons. In principle, everything can be measured, which can be specified in units per hour (UPH).
Rolex Cosmograph Daytona 16520
How does the scale work? To calculate the units per hour from the measured seconds for a unit, 3600 seconds (= 1 hour) must be divided by the measured seconds, and this creates the scale perfectly.
There were 40 seconds measured for a unit: U = 3600/40 = 90 units/hour. The number 90 corresponds to 40 seconds on the bezel.
The scale follows the mathematical function U = 3600 /x, where U is the calculated units per hour and x is the measurement in seconds. With the aid of the tachymeter scale, only the value on the bezel has to be read off. By the way, the function is a simple hyperbola which approaches zero asymptotically.
In an automotive plant, the time between two successive working steps is measured on the production line at 100 seconds. On the bezel it can be seen that 36 cars pass through these work steps per hour in this factory.
Finally, an example to illustrate the problems with the old Daytona bezels:
For a cable car with cabins for 6 persons each, the time interval between 2 cabins is measured at 15 seconds. The bezel of an old Daytona, for example, with the reference number 6263/6265, shows no value at 15 seconds – the measurement range is not on the bezel.
Rolex Cosmograph Daytona 6263
This is due to the fact that the Daytonas built up until 1990 only had a measurement range between 200 UPH (= 18 seconds) and 50 UPH – the numbers between 60 and 50 UPH stand for 60 to 75 seconds measurement time. This was probably a concession to use it as an auto speedometer, which, as mentioned above, is not very practical. Thus, since about 1990, the Daytona bezel was correspondingly changed to suit the use of the bezel for e.g., production measurements. Here the measuring range between 400 and 200 UPH is needed rather than the range between 60 and 50 UPH. With a current Daytona, the delivery capacity of the above-mentioned cable cars can be easily measured: With 15 seconds distance between the cabins, the bezel shows 240 cabins per hour, so 240 x 6 (persons/cabin) = 1440 persons/hour.
Pictures: ©Rolex/Jean-Daniel Meyer
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