How to measure speed with a sports watch?

　　I often see watch enthusiasts asking how to measure speed with a sports watch. Here, I have put together methods for measuring speed with a sports watch to help everyone out.

　　Methods for measuring speed with a sports watch

　　Generally, watches with a stopwatch function have sequentially decreasing numbers such as 500/400/350 engraved on the outer bezel. These represent a physical quantity which is speed, in kilometres per hour. This bezel is called a tachymeter.

　　How to use it: taking a vehicle travelling on a motorway as an example, since distance marker posts are usually erected along motorways; start the stopwatch when passing the starting point, with the stopwatch hand beginning timing from 12. When travelling 1 kilometre, stop the timer, and the number pointed to by the hand at that moment is the average speed over the preceding kilometre;

　　For instance, if there is a 60 at the 12 position, since it took one minute to travel 1 kilometres, the speed is 60 kilometres per hour.

　　How are the positions corresponding to the numbers on the bezel calculated? It is actually very simple. To know which position the number 500 should be engraved at, a simple calculation is required; that is, calculating how long it takes in seconds to travel 1 kilometre at a speed of 500 kilometres per hour.

　　1 hour = 3600 seconds, so 500 kilometre takes 3600 seconds to complete. According to the principle of proportionality, the time needed for 1 kilometres is 3600/500 = 7.2 seconds, so 500 must be engraved at the 7.2-second mark on the dial.

　　Similarly, 400 should be engraved at the 3600/400 = 9 second mark, while 100 is engraved at the 36 second mark.

　　When the speed is below 60 kilometres per hour, the tachymeter cannot be used directly unless a low-speed scale is added.

　　Common knowledge for sports watch maintenance

　　(1) When wearing a watch, perspiration on your hand can corrode the case. All-steel cases have better corrosion resistance due to their nickel-chromium alloy composition, whereas base-metal cases are copper and corrode easily when exposed to sweat for long periods. You should regularly wipe off sweat with a soft cloth or use a plastic watch protector pad to prevent sweat corrosion.

　　(2) Do not open the case back casually, to prevent dust from entering the movement and affecting the normal operation of the watch.

　　(3) Do not place the watch in a wardrobe containing mothballs, to prevent the watch oil from deteriorating.

　　(4) Do not place the watch on radios or televisions, to avoid magnetisation.

　　(5) For watches stored long-term without being worn, wind them regularly once a month to prevent the components from remaining stationary for extended periods, thereby ensuring the movement's running performance.